The Basics of Swimming Pool Filters

Written on:April 17, 2022
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de4800The Basics of Swimming Pool Filters

Filter Terminology and Design

The Basics of Swimming Pool Filters. Swimming pools and spa are subject to constant contamination from foreign matter brought in by swimmers, wind and articles used in and around the pool. Filtration is the mechanical process of removing this insoluble matter from swimming pool and spa water. Water clarity is important for appearance, hygiene and safety. Filters are universal in pool systems, linked to the circulation system along with the motors and pumps.

As water pumps through the circulation system of a pool or spa, impurities are strained by a filter. The filter itself has no moving parts and is made of simple components.

In the glossary that follows, we begin our discussion of the three basic types of filters – sand, diatomaceous earth and cartridge by providing definitions for many of the terms you’ll encounter as we present a series of filter articles.

A guide to basic DE, sand and cartridge filter terminology


Anode A component, usually made of zinc or magnesium, that pre-vents electrolysis or galvanic action in steel filters.
Backwash The process of cleaning the filter medium and elements by reversing the flow of water through the filter – and thereby flushing accumulated debris out of the filter.
Bridging A phenomenon in DE filters in which the filter medium builds up between filter elements, limiting flow.
Cartridge filter A filter that uses replaceable paper or fabric-like cartridges as the filtration medium.
Diatomaceous earth (DE) A powder made of fossilized skeletons of tiny plankton, called diatoms, that serves as a filtration medium when it forms a cake on the filter element.
DE filter A filter that uses diatomaceous earth as the filtration medium.
Effluent line The plumbing line leading from the filter equipment to the pool or spa.
Filter area The filtering surface area through which water flows in the filter housing; measured in square feet.
Filter cycle The operating time between cleaning and backwash routines.
Filter medium The material used to filter debris from the water – usually sand, a cartridge or diatomaceous earth.
Influent line The plumbing line that leads from the pool or spa to the filter equipment. Also known as the suction line.
Multi-port valve A valve that permits the multi-directional control of the flow of water through a filter; it combines the function of two or more single valves.
Pressure differential The difference in pressure between the influent and effluent lines of a filter.
Pressure filter The most common type of filter; the water is forced through the filter by a pump mounted on the influent side of the filter.
Pressure gauge An instrument that measures the water pressure in influent and effluent lines. An increase or decrease in water pressure indicates either cleaning or backwashing operations or a plugged line.
Sand filter A filter that uses graded layers of sand as the filtration medium.
Septum The part of the filter element or grid on which the filter medium, usually DE, is deposited or caked.
Turnover rate The time required to circulate a volume of water equal to the capacity of a given pool or spa.
Vacuum filter A filter through which water is pulled by a pump positioned on the effluent side of the filter. Most vacuum filters use DE as the medium

Filter Types

There are three basic types of filters: Diatomaceous Earth (DE), sand, and cartridge.

DE3600-FilterDiatomaceous Earth (DE) Filters

This filter consists of a tank with a series of fabric covered grids, also called filter elements. The fabric is coated with a filtration substance media called DE, or diatomaceous earth. DE is the fine ,white powder found in large deposits in the ground. They act like filters by allowing water to filter through while leaving the impurities behind. They are able to filter out microscopic particles.

A filter must be properly sized to a pool or a spa’s circulation system. This size is determined by square footage of surface area of the filter media, which equals to the total square footage of the girds. A typical filter has eight girds that total 24 to 72 square feet. The girds are placed into tanks that are 2 to 5 feet high and about 2 feet in diameter. Without the filter girds, DE would turn into caked mass, when wet, making it impossible for water to flow through.

There are two basic types of DE filters: the vertical grid and the spin type.

Vertical Grid Filters

The grids in this type of filter are assembled vertically on a manifold. A holding wheel secures the grids to the manifold and retaining rod screws into the base of the tank to secure the assembly. Water enters the tank at the bottom and flows up and around the outside of the grids. It then flows down the stem of each grid, into the hollow manifold, and out of the filter.

Spin Filters

The spin filter is obsolete but still can be found on older pool systems. The girds are wheel shaped and lined up horizontally like a box of donuts. They operate in a similar manner to the vertical grid filter, but to clean them, a crank is turned to spin the grids. Although this is supposed to clean the grids, it is not very effective.

Sand-FilterSand Filters

Sand filters are 2 to 4 feet in diameter and look like large balls. Older models generally are housed in metal tanks. The sand in the filter strains out impurities as the water pushes its way through. The water enters the top or side of the filter through a multiport or piston backwash valve and sprays over the sand. The sharp edges of the grains catch the impurities. The water is pushed through the laterals and bottom manifold where it is then directed out of the filter. The individual drains of the drain manifold are called laterals. A drain pipe is located in the bottom of the tank for emptying out the water when necessary.



Cartridge-FilterCartridge Filters

The operation of a cartridge filter is similar to that of a DE filter except there is no DE involved. Water flows into the tank which houses one or more cylindrical cartridges of fine, pleated mesh material. The tight mesh of the fabric strains out impurities. Unlike the backwashing method used by DE and sand filters, when it is time to clean the cartridges they simply are removed and washed.

Makes and Models

Selecting a good filtration system is the key to healthy, clean and sparkling water in the pool or a spa. A filter fits into the equipment sequence after the pump – so whether you’re replacing an old filter or installing a filter on a newly built pool, you’ll need to match the filter to the pump and the size of the pool.

To properly size and select the filter for a pool or spa, you must first calculate the pool’s volume and capacity. Next, compute the pool/spa flow rate and the filter flow rate. Once you’ve done all that, and have taken some other factors into account, you’ll be ready to select the right filter for the system in question.

Sizing and Selection Techniques

Filter systems are required to meet Health Codes for turnover rates and filtering capabilities in gallons per square foot of filter area. For this certain criteria are to be calculated.

A. Calculate The Volume
The first step in finding the correct filter model is to figure out how much water has to be filtered. Here are some simple formulas and techniques to use when calculating the volume of a swimming pool:

For a rectangular pool, multiply the length by the width by the average depth. For a circular pool, multiply the radius by the radius by 3.14 (pi) by the average depth. Oval pools are actually rectangles with semicircles on the ends. These are not true ovals and require either combining the formulas for circular and rectangular pools, or using the using the grid technique.

Make a scale drawing of the pool on a piece of square-grid graph paper, with each square representing one square foot or any standard unit.

Then simply count up the number of squares, not missing those partially filled squares making complete ones, and that will give you a close estimate of the pool’s area in square feet. Multiply the area by the approximate average depth of the pool gives you, the volume in cubic feet.

B. Calculate The Capacity
Compute the capacity or the number of gallons of water that the pool will hold. To calculate the capacity, simply multiply the pool volume by 7.48 (the number of gallons of water contained in a cubic foot of volume).

As an example, suppose you have a rectangular pool that is 40 feet long and 20 feet wide, with an average depth of 5 feet. Plug these numbers into the volume equation and multiply: 40 x 20 x 5 = 4,000 cubic feet. Now plug the volume into the capacity equation and multiply: 4,000 x 7.48 = 29,920 gallons. Simple.

C. Calculate The Flow Rate
The flow rate is the volume of water flowing past a given point during a specific period of time, measured in gallons per minute (gpm) or gallons per hour (gph). To calculate the flow rate, divide the capacity of the pool by the turnover rate, which is the time required to circulate a body of water equal to the capacity of the pool.

Let’s say you want to set up the pool to have an eight-hour turnover rate. Your success in achieving that will depend to a large extent on which filter you eventually install. Therefore, you must first determine the desired flow rate – one that will facilitate the turnover you’re seeking.

The equation to find the flow rate for the 29,920-gallon pool noted above (no matter what its shape) is: 29,920 -. 8 = 3,740 gph. To calculate the flow rate per minute, divide the flow rate per hour by 60. In this case, 3,740 @ 60 = 62.3. That is the rate at which you want the filter to work.

Stating this situation another way, the calculations for your 29,920-gallon pool show that you require a flow rate of 62.3 gpm to filter the pool’s capacity in eight hours. Therefore, your goal is to determine which model of filter will filter 62.3 gallons of water per minute – resulting in a complete turnover every eight hours in the pool in question.

D. Calculate The Filter Flow Rate
To achieve that goal, you have to find a filter whose filter flow rate matches the desired flow rate for the pool. The filter flow rate is defined as the amount of water filtered over a given period of time – expressed in gallons per minute.

To determine the filter flow rate, multiply the filter area by the filter rate. The filter area is the filtering surface area through which water flows in the filter housing – measured in square feet. The filter rate is the number of U.S. gallons of water that flows through one square foot of effective filter medium per minute during the operation of the circulation system.

If you didn’t follow that, don’t worry – all you have to do is get both figures from the filter manufacturers. In our example, let’s say you have a filter area of 5 square feet and a filter rate of 12.5 gpm. (These numbers are based on a popular model of sand filter.) If you multiply 5 x 12.5 you have a filter flow rate of 62.5 gpm pretty darn close to our desired flow rate of 62.3!

Now you’ll need to consult filter manufacturers to find a model with the necessary filter area and filter rate. Once these numbers are provided by respective manufacturers for various models, you can calculate the filter flow rate to see which particular filter will match the need.

E. Decide Which Type Of Filter You Prefer
What type of filter do you prefer: sand, diatomaceous earth or cartridge? Pressure or vacuum filters? From your poolside experience, you probably already have a good idea of the type you like. You may even have narrowed it down to a preferred manufacturer which will make it even easier to perform your calculations and select a filter that will do the job.

The mathematical relationships between filter area, filter rate and filter flow rate will remain the same for any type of filter. Never hesitate to contact manufacturers or their dealers to ask their professional advice in this area.

F. Make Sure To Oversize The Filter
When selecting the right-sized filter, it is important to keep in mind that as the filter removes debris from the water, the filter medium will become more and more clogged, and the filter will require an ever greater flow to clean an equal amount of water.

It is necessary, therefore, to select a filter that is larger than indicated by our calculations of flow requirements. This is especially true for commercial pools and for backwashing.

Using our example, you could either select a model with a larger filter area or a model with an increased filter rate. To allow for debris buildup and backwashing, you might select a model with a filter area of 5 square feet and a filter rate of 20 gpm. That gives us a filter flow rate of 100 gpm – well beyond our desired rate of 62.3 Alternatively, you could choose a filter with 4 square feet of filter area and a filter rate of 25 gpm per square foot.

G. Limit The Filter Rate And Adjust The Filter Area
In many cities and counties through-out the country, filter rates on pools are regulated. The National Sanitation Foundation sets maximum filter rates in order to ensure effective filtration – since the faster the water passes through the medium, the less effectively it is cleaned.

Filter-rate ceilings are imposed most often for commercial facilities. You may have to compensate by selecting a model with a larger filter area. By doing so, you can achieve the same flow rate without exceeding the maximum filter rate.

H. Select The Correct Filter
Taking all the above calculations and factors into consideration, you’re now ready to select the proper filter for a particular pool.

Backwash Valves

This process is used to clean DE and the sand filters. The water is send backwards through the filter, flushing the debris into a waste line or a sewer line. A backwash valve on the filter reverses the flow of the water. There are two types of backwash valves: the piston and the rotary (multiport is a variation of it).

Piston Valve

Water enters this type valve and is directed to the filter in the normal operation. The water is then filtered through the DE or the sand and return to the pool. When the handle of the piston is raised into the backwash position, the piston disks force the water into the filter tank through the outlet port. Thus the backward flow of water through the filter, flushes debris and dirt out of the tank and out of the valve inlet port. Once inside the valve again, the waste water is directed to the waste port.

Never change the piston position when operating the pump. This puts too much pressure in the pump, motor, and the valve O-rings and may result in leaks. The piston type backwash valve usually is located on the side of the filter tank.

Rotary Valve

The rotary backwash valve is used exclusively on vertical DE filters. The water direction is changed by rotating an internal rotor located below the filter tank. A rotor gasket seal or O-rings prevent the water from leaking. A retainer ring holds the valve body to the underside of the tank with bolts that pass through the bottom of the tank.

To backwash, rotate the rotor 90 degrees. Water enters through the middle and up the inside of the grids. The DE and dirt is washed off the grids as the water flows from inside the grids to the outside. The water is then flushed back through the rotor and directed to the opening marked “backwash”. Do not rotate the rotor while the pump is running for leaks may occur.

Backwash-valveMultiport Valve

The multiport backwash valve is used on sand filters and looks like a rotary valve when taken apart. Sometimes mounted on the side but usually mounted on top of the filter tank, this valve offers more than one choice for water flow direction. After the pump backwashes, clean water rinses out the pipes before returning to normal circulation. This prevents debris from returning to the pool after a backwash has been done.

Backwash Hoses

Regardless of the valve used, if the backwash discharge port is not plumbed directly to a drain or sewer line, a hose has to be attached to direct the dirty water onto the lawn or into the street (if permissible). The normal hose usually is 1 1/2 to 2 inches in diameter and made out of inexpensive, collapsible plastic. Backwash hoses are available in various lengths up to 200 feet. The pool vacuum hose can be turned into backwash hose by using a hose clamp.

Pressure Gauges and Air Relief Valves

Most filters are fitted with a pressure gauge, mounted on top of the filter. Sometimes the gauge is mounted on the multiport valve. These gauges read 0 to 60 psi and are useful in several ways. When the pressure goes beyond 10 pound over the normal operating pressure it is time to clean the filter.

The pressure gauges spot the operating problems in the system. If the pressure is lower than normal, it indicates that there is obstruction in the water that is coming into the filter; and when the pressure reads high ,then the filter is dirty or there is some obstruction in the flow of water after the filter. When the pressure fluctuates while the pump is operating , the pool or the spa might be low in water or have some obstruction at the skimmer. This shows that pressure gauge is an important instrument on the filter.

For commercial pools or spa ,some codes require a pressure gauge be mounted on the incoming pipe and on the outgoing pipe to compare the difference. The health department inspector checks the cleanliness of the filter system by comparing the two pressure readings. Normally the differential should be 2 to 4 psi.

An air relief valve is mounted on the T fitting along with the pressure gauge. When a system first starts up, or when it has lost its prime, there is lots of air in the filter. An air relief valve allows air to escape from the filter, when its plug is loosened. Thus air relief valve is important if the filter is to work to its full filter square footage.

Sight Glasses

Sight Glasses are the clear section of the pipe, that are normally installed on the backwash line coming out of the backwash valve. They can be installed anywhere in a line of pipe where you want to see the effectiveness of cleaning action. When installed on the backwash line and when the water is going directly into the drain, the sight glass helps to see the dirty water becoming clean and time to stop the backwash.

Repair and Maintenance

Repair and Maintenance of filters consists of installation, plumbing, cleaning and troubleshooting.

Installation of Pool and Spa Filters

Installing a pool filter is a fairly straightforward process, but it always requires care and the faithful following of a few basic rules. The text below covers those basics, but before you start cutting PVC and lubricating O-rings, remember that this is a generic guide and that you should always consult manufacturers’ literature before working with any unfamiliar products.
Beyond that, once you survey the situation by inspecting the equipment pad, the plumbing design and the electrical system, you should be ready to go.

Sizing up the task at hand

When it comes to filters, some of the most fundamental considerations can make the biggest differences. Where the unit is located, how neatly it is incorporated into the overall circulation system and how it’s tied into the pool’s electrical circuitry are all very important.

Equipment pad: The equipment pad should be a flat, level slab of poured concrete, brick or concrete block. Never install a new filter on wood, because it can warp or decay beneath the filter’s footings and compromise the position of the unit. Three things to watch: First, filters should always be installed on a level surface; if things aren’t square, the unit may vibrate or operate to less than its optimum ability. Second, filters should be located as close to the pool as possible. Third, the filter should have adequate drainage and should be positioned to provide plenty of room for service access and maintenance.

Plumbing: The plumbing should be designed and installed with the shortest lines and the least number of fittings needed to achieve optimum water flow efficiency in the circulation system. If the filter is most conveniently installed away from the pool, however, increasing the pipe size between the filter and the pool will decrease the head resistance and compensate for a longer run. Inspect the plumbing so you can be prepared with the proper fittings and materials.

Electrical hookup: Although the filter is not directly connected to electrical power, the pump motor runs on electricity, which means that the filter must be grounded and bonded by a professional electrician. In addition, the electrical wiring and hookup of the motor must have been completed by a professional electrician in accordance with local and national electric codes.

Down and dirty

Now that the pad is ready for the new filter, it is especially important to refer to the filter manufacturer’s installation manual for specific instructions regarding the particular make and model of the filter you’re installing. Once you’ve finished your reading, the first step in the installation is to place the filter on the pad. You might consider bolting the unit to the pad, although relatively few filters are mounted in this fashion.

Next, connect the circulation plumbing to the filter. Every filter has two basic plumbing connections – the influent and the effluent lines. The influent line supplies water to the filter; the effluent line provides an outlet for water after it passes through the filter. A gate valve should be installed on both the influent and effluent lines. This will permit you to close the lines when it is necessary to service, remove or replace the filter.

The plumbing lines are connected directly to the filter’s multi-port valve either by hand-tightened union connections or, if appropriate, by bonding with an adhesive, such as a PVC cement. For threaded pipe connections, the application of PTFE tape to the threads before connecting the pipes is often recommended.

Be sure the O-rings on all valve fittings are clean and that each O-ring and O-ring groove is lubricated with silicon lubricant. Install O-rings in their grooves and tighten with the appropriate union collar. Before applying any adhesive, be sure all connecting points are clean and dry – and use the recommended primer before doping PVC components.

Allow an appropriate drying period before pressure testing or operating the equipment – and take into account the fact that temperature and humidity may affect the drying time of some adhesives.

Once the filter is installed on the pad, all of the plumbing connections are set and the unit is properly grounded, it is time to put the circulation system into full operation. The three main types of filter – sand, DE and cartridge – require different start-up procedures. As above, the following are general guidelines for your reference; manufacturers’ manuals will give you the specific help you need.

Sand filters: Most sand filters in use today are high-rate models in which water typically passes through a number of layers of sand and gravel that have been carefully placed in the filter tank.

The size of the sand particles used as the filter medium is very important for optimum efficiency. If the sand granules are too big, filtering efficiency is decreased; if the sand particles are too small, the filter will clog up quickly.

Check the specifications provided by the filter manufacturer, then fill the tank with layers of coarse, medium, and fine gravel followed by the silica sand layer on top as directed. The silica sand commonly used as the top layer has a diameter between 0.35 and 0.45 millimeters with a typical uniformity coefficient of 1.4. Also, plan to leave a space between the sand bed and the overdrain. This space is known as freeboard, and most manufacturers suggest it should amount to half the depth of the filter bed.

While most sand filters use several layers of material to clean the water, some models use only one or two filtering layers.

Flocculents are often used to improve the performance of sand filters. Most filter flocculents are alum based preparations that form a gelatinous layer on the top of the sand. As an alternative, diatomaceous earth can be used: Add one-half cup of DE for each three square feet of filter area after the unit has been filled with sand.

DE filters: These units filter water by passing it through a layer of diatomaceous earth that coats grids inside the filter tank. The DE is added as a precoat to the grids, attaching itself on the grid-covering mesh known as the septa. Common practice calls for adding two ounces of DE per square foot of filter area, with a typical variation of a half-cup either way depending on manufacturer specifications.

The DE should be mixed with water and fed into the filter as a “slurry,” or suspended mixture. After turning on the circulation system, add the slurry via the skimmer at as steady a rate as possible to permit even coating of the filter septa. Ideally, the DE will form a uniform “cake” between 1/16 and 1/8 of an inch thick.

DE can also be introduced to the filter by using a precoat pot, solution feeder or erosion feeder that is specifically designed for precoating.

Cartridge filter: Setup of cartridge filters is simple: Insert the filter cartridge per instructions and fire up the circulation system. You can also use a flocculating agent for cartridge filters.

With all types of filters, open the unit’s air release valve and turn on the pump. When a steady stream of water shoots out, close the valve. Manufacturers urge technicians and homeowners always to remember to open the air release valve when starting the filter, because air pressure in a filter can be very dangerous.

Safety first

Although we’re covering it last here, safety should be a primary consideration in filter replacement and repair – particularly with units outfitted with pressure-clamp assemblies, which, under certain conditions, can fly apart with tremendous force. In fact, flying parts from a “blown” filter are often the source of severe property damage and, unfortunately, the cause of severe, sometimes fatal, injuries.

Filter manufacturers often recommend tightening the clamp to specs using a torque wrench. Extra care in placing the assembly around the tank and tightening is critical, they say, with some recommending partial tightening of the clamp and then tapping the assembly into place around the perimeter of the tank.

Improper application of the clamp assembly may result in a poor seal that could cause the filter to blow. Alternatively, the uneven seal might slowly force the tank out of round over time and create serious problems in future servicing – and further increase the chance of blowout.

A final safety tip: Whenever any repairs are done on a filter or related components, cut off all the pool’s electrical circuits at the source!

Basic Diagnostics and Troubleshooting

A pool filter’s water-cleansing function is pretty straightforward, but small problems can become big service hassles if you let them get out of hand. Here’s a handy trouble shooting guide.

When a filtration system goes awry, the quality of the water can deteriorate quickly Poor filtration leads to increased chlorine demand and chemical costs.

The following outlines basic filter problems, defines probable causes and suggests solutions. For the most part, these discussions cut across all filter types – diatomaceous earth, sand and cartridge. Please note that these guidelines are for general information only; you should always consult manufacturer literature for specific recommendations and operating guidelines for each filter model.

Before you start, note this general rule: When checking an operating filter’s performance, tap the face or casing of the pressure gauge firmly to make sure the needle is not sticking. A gauge that fails to indicate a rise in pressure not only compromises your ability to monitor filter cycles, it can also be quite dangerous: Excessive pressure can result in cracking of the filter body or failure of the clamping device on the filter tank.

Symptom: Reduced flow of water through the filter

As dirt accumulates on the filter media, water flow is restricted – and pressure within the tank rises. When the pressure rises to a level specified by the manufacturer, it’s usually time for routine backwashing of a sand or DE filter, or simple cleaning of cartridge filter elements.

The ranges of operating pressures for filters vary widely, depending on type. A typical range for high-rate sand filters, for example, may be 10 to 15 pounds per square inch at the beginning of the filter cycle (that is, the period between routine backwashings or cleanings) on up to 25 to 30 psi when backwashing is required.

Conveniently, some filter systems – mainly large high-rate sand or diatomaceous earth filters on large commercial installations – have pressure gauges installed on both the influent and effluent lines. As the media becomes clogged with dirt, the influent pressure will become higher than the effluent reading. When the differential between readings reaches a specified level, it is time for backwashing.

Gradual pressure rises are normal in the course of the filter cycle, if the rise in pressure is relatively consistent, then normal backwashing or cleaning routines will suffice. If pressure begins to rise more rapidly than normal, however, it is time to take a close look at the elements of the filtration system.

Symptom: Short cycle between backwashes.

Most often, short filter cycles indicate excessive flow rate through the filter. This indicates in turn that the filter may be undersized or that the pump may be too powerful for the system. Install a properly sized system.

In other cases, short filter cycles indicate an unusual increase in the burden on the filter media caused by excessive dirt, debris, body oil, lotions, hair or algae. Very high bather loads, lead to overworked filters.

Now for more specialized problems:

With sand filters – and DE filters to a lesser extent – you can get into trouble when soda ash or coagulants are fed into the skimmer too fast. In these circumstances, the chemicals do not have sufficient opportunity to dissolve and, instead of passing through the bed in solution, clog the media and raise the pressure. It is best to introduce these agents more slowly or dissolve them before introducing them to the skimmer.

Note: Alum should never be used with a DE filter: It will only solidify the filter cake.

In addition, clogging of the septa in a DE filter – whether by rust, calcium build-up or soda ash – may increase the pressure and compromise effective filtration. If you suspect this problem, clean the filter elements and check for clogging of the fine nylon mesh covering the septa. Treating the septa to a light acid wash and hosing with a strong stream of water will usually relieve the problem.

Symptom: Inadequate filtering action.

All of the troubleshooting tips in this category refer to specific filter types.

In a sand filter, if the media has been charged improperly or in adequate quantities, channels may have formed in the sand and gravel bed, and may be allowing water to pass through unfiltered. Look for evidence of channeling or tunneling and recharge the filter if necessary.

Also in a sand filter, if the unit has not been backwashed consistently, mud balls may have formed on the surface of the sand bed, thereby limiting filtering action. Or, in extreme cases, the sand may have calcified and will no longer filter out dirt. Look for mud balls or evidence of calcification; backwash or remove the old sand and recharge the filter as necessary. Filter sand should normally last four to five years.

Moving to DE units, poor filtration often results from coagulation or solidification of the DE. If you observe hardening of the DE cake, remove and clean the elements as per the manufacturer’s instructions and recharge the filter with fresh DE.

If DE is fed to the unit by a slurry feeder, the unit may not be feeding enough DE into the filter to adequately coat the septa. Conversely, an inconsistent filter cake may result from feeding DE too quickly into a skimmer when recharging the system. The trick here is to observe DE introduction rates and adjust them as needed.

More can go wrong in coating a DE filter: If you have an over-rated DE filter or an under-rated pump, for instance, there may be inadequate pressure within the filter to properly coat the septa. Here, you need to check the manufacturer’s specs and replace equipment as needed.

Backwashing should be performed frequently enough and for adequate periods when it is performed, for media to be cleaned sufficiently.

It also pays to watch out for inadequate or plugged backwash lines that might not be allowing sufficient flow out of the filter during backwashing. If a portion of the backwash discharge is retained in the tank because of inadequate flow, the backwash line will clog over time with caked media. To address this possibility, check the lines for clogs and clear as necessary.

Last but not least, a specific tip for cartridge filters: Poor filtration without a rise in pressure may indicate torn or worn out cartridges that are simply allowing water to pass through without filtering. Replace these cartridges as needed.

Symptom: Low flow rates in the system.

A flowmeter is a handy diagnostic tool in evaluating filter performance. If you get a low reading with a flowmeter but a high reading on the pressure gauge, something is restricting the flow – most likely a blockage in the piping, or, possibly, under sizing of the entire piping system.

As a rule of thumb, the maximum flow rate through a 1-1/2-inch PVC pipe is 70 gallons per minute. Through 2-inch plumbing, it should be 110 gpm. These figures are based on a standard hydraulic specification of an eight feet-per-second maximum velocity.

A drop in the return flow may be traced to a clogged pump strainer basket or skimmer basket. The remedy here is simple: Clean the baskets.

If both flow and pressure readings are low, the pump may be undersized – or you may have a plugged pump impeller or lint trap. It is important to note that pump or motor trouble can lead directly to filtration problems.

Symptom: Sand or DE entering the pool.

The first suspect with sand- or DE clouded water is the backwash valve. If it is left in an intermediate position, media can flow back into the pool. If this obvious answer doesn’t suffice, you’ll need to look for solutions inside the tank.

With sand filters, broken laterals are a common culprit here – and replacement is the only solution.

In a DE filter, torn or worn out septa will let DE flow into the pool.

Symptom: Air pressure build-up.

Air present in the filter tank may allow some of the filter media to be forced into the pool and can otherwise compromise filtering action. In sand filters, it’s a prime suspect in channeling; in DE units, it may disrupt the filter cake.

Air pressure build-up in a filter is dangerous. If you have a problem with air in the tank, check for any hairline cracks or leaks in plumbing connections on the suction side of the pump. A low water level in the pool is another suspect: Air may enter through the skimmer.

It is always important to release any air present in the filter tank. Not only will the presence of air inhibit good filtration, it can also increase the danger of the filter tank suddenly cracking by rapidly increasing pressure within the tank.

Air is easily released by opening the pressure release valve and allowing the air to escape. When a steady stream of water comes out of the valve, then you have released all of the trapped air.

Multiport magic

If you are unable to perform immediate repairs on a sand or diatomaceous earth filter equipped with a multiport valve, the following tip may come in handy:

Manual feeding of chlorine or other sanitizers will maintain clean water for seven to 10 days, depending upon bather load and other such factors. Experts suggest you turn the multiport valve to the “recirculate” position, which will keep the skimmers working as well as the automatic chlorinator, if the pool is equipped with one. The water clarity may suffer a bit, but with sufficient sanitizer, it should not turn cloudy or green.

The nylon mesh on the septa can be repaired, depending on how large a hole or tear is present. Also check the points where the mesh is sewn to the frame of the grid: Even slight unraveling or separation will allow DE to enter the pool.

D.E. Filters Troubleshooting

When it’s time to work on a diatomaceous earth filter, here’s a guide to minimize the toil involved in tearing down a typical D.E. unit, cleaning its grids and then recharging it to keep a pool’s water sparkling clean.

Diatomaceous earth filters perform the same water cleaning function as sand and cartridge filters, doing their job with great effectiveness so long as too much dirt and debris haven’t built up to clog the system. At cleaning time, however, differences among these filter types become amply apparent.

Unlike a cartridge filter, which requires simple hosing off, soaking or routine replacement of cartridge elements, or a sand filter, which for the most part requires only periodic backwashing, a D.E. filter needs to be taken apart, cleaned and recharged at least once each year. And it’s a dirty job, no matter how much it helps ease the overall pool-maintenance task.

1. Release the pressure from the filter tank. – Before taking the unit apart – and with the pump turned off – open the filter’s air-release valve. Wait a few moments as air is drawn in and water flows out of the tank: When the hissing stops, you can get to work.

2. Mark the lid, unscrew the, clamp ring and remove it from the tank. – After marking the tank on the top and the bottom with a grease pencil, unscrew and remove the clamp ring. The marks will help you align the two sections when reassembling the filter

3. Remove the top of the tank and pull out the grids. – Using leverage if necessary – but being sure not to bend or otherwise distort the tank in the process – carefully remove the tank’s lid. Now pull the grid assembly from the tank, making certain you don’t break any of the manifold fittings or cut the grid fabric on any sharp edges in the vicinity. Note: Lifting the grids may require a good bit of muscle, so be sure to exercise care to avoid back injury.

4. Hose off the grids. – Using a pressure nozzle, make sure all of the old D.E. is thoroughly cleaned from the grid fabric. Dispose of the spent D.E. in accordance with local health department standards.

5. Inspect the manifold and grids. – If it becomes necessary to check the manifold and grids for suspected damage, carefully remove wing nuts with a pair of pliers, taking precautions not to strip the threads. Inspect the grids for small rips or holes. Note: you can temporarily patch most grids with PVC pipe adhesive.

6. Clean the filter tank. – Release the tank’s bottom drain and let the remaining water run out. Hose as much of the loose D.E. and debris out as possible. Note: A pair of channel locks or pipe wrench can help with sticky bottom drain.

7. Inspect the inside of the band / ring. – Use a screwdriver to free the band ring from the tank and inspect it for wear. Clean it thoroughly with water or, if grease or other lubricants have been used previously, clean it with soap and water. Put the band ring back, making sure that it is seated evenly and securely in place.

8. Inspect the 0 ring. – Check the ring for wear and replace it if necessary.

9. Put the grid assembly back in place. – Return cleaned grids and their assembly to the tank, making certain all grids are properly aligned and that the pipe fittings are securely in place as needed.

10. Place the lid back on the tank. – Return the lid to the tank, realigning it with the grease pencil marks. Put the ring clamp back on the tank, making sure it is seated properly all the way around the tank.

11. Tighten the bolt on the clamp assembly. – Using an open-end wrench if necessary, tighten the bolt on the clamp assembly, occasionally tapping the band ring to make certain it is seating itself properly. Follow the manufacturer’s recommendation with respect to how tight a fit you need.

12. Recharge the D.E. – Restart the pump, keeping the air-release valve open until a steady flow of water emerges. Place an appropriate quantity of D.E. in a bucket, then add water to make a slurry.  Note: Make the slurry thin enough so that it can be poured slowly into the skimmer and pour it only with the pump on. As an option, scoop cups full of dry D.E. into the skimmer, allowing it to mix with the water and circulate into the filter.

13. Watch the pressure gauge. – After adding the D.E. to the skimmer, mark the pressure reading down on the tank with a grease pencil. At subsequent stops, you will be able to tell at a glance how much the pressure in the tank has increased. Now all you have to do is hose down the area and head to your next assignment.

Cartridge filter maintenance

The basic design of cartridge filters makes them easy to service. Here’s a step-by-step guide to getting the job done correctly.

Servicing cartridge filters is simple. All there is to it is simply removing the cartridge elements, soaking them, hosing them off and putting them back in place. In fact, if there is any burden placed on service routines by cartridge filters, it’s because cleaning or replacement are the only options for cartridges.

Unlike sand or diatomaceous earth filters, cartridge filters cannot be cleaned by backwashing. This difference makes it important to stay on top of your filter maintenance routines with cartridge units.

As with all types of filter media, you can determine the need for service with cartridges by watching the pressure rise. When it is 8 to 10 psi above the baseline it is time to service the filters.

Because thorough cleaning typically requires overnight soaking, you also need to consider what to do without the equipment in the meantime. You can either leave the system off, or you can replace the elements, if you are a technician, at each cleaning and rotate elements from the filter tank to the soaking bucket. As always, consult manufacturer literature for specific maintenance procedures and take care to release air pressure from the system when restarting the circulation system.

1. Remove the lid – With the pump turned off and the pressure-release valve open, unscrew or otherwise loosen the clamp fitting and remove the tank’s ring-clamp assembly. To remove the lid or the upper half of the tank, apply leverage to the lip of the lid and carefully remove it, making sure not to force the lid out of round.

2. Remove the cartridges. – You can either remove the cartridges one at a time or all at once. To remove the cartridges one by one, unscrew the wing nuts and lift the cartridges out. If you need to pull the entire cartridge/manifold assembly, consult the manufacturer’s literature, typically, this method is used only if you suspect a leaking manifold and are removing the assembly for inspection.

3. Soak the cartridges. – Place the cartridges in a special cartridge-soaking solution or a solution of water and trisodium phosphate (TSP). Use approximately three cups of TSP in a 32-gallon drum or a water tight plastic trash barrel.

4. Rinse off the cartridges. – After soaking the elements for 1 to 3 days, rinse them off using high-pressure nozzle.

5. Inspect the cartridges. – Check the cartridges for damage such as large tears or holes that would compromise filtration. The cartridges should return to their original white or light gray color when properly cleaned.

6. Replace the cartridges. – Return the cartridges to their assembly, making sure they are carefully seated on the manifold fittings. Reapply the wing nut or fastening device. If you are returning the entire assembly to the tank, it may be more convenient to assemble the unit outside the tank. Follow manufacturer directions and make sure the manifold is properly connected to internal fittings.

7. Inspect the 0 ring and ringclamp assembly. – Inspect the 0 ring for wear – and replace it at the first sign of deterioration. Clean the ring clamp and lip of the lid if necessary.

8. Replace the lid – Firmly seat the lid by applying manual pressure. Leaning on the lid may help, but take care not to warp the tank or the lid as you press down. Carefully replace the clamp ring making sure that it is properly placed. Tighten the assembly to manufacturer specifications.

As with all filter systems, before you start the pump, make sure the air-release valve is open and wait for emergence of a stream of water.

Sand filters maintenance and troubleshooting

Sand filters clean pools gradually, removing dirt, debris and particulates as water passes through a deep bed of sharp sand. With the kind of proper charging and maintenance described in this article, these filters will deliver years of trouble-free service.

Backwashing is the regular service for high-rate sand filters. Unlike diatomaceous earth or cartridge filters – which must be opened for periodic cleaning and replacement of media – a sand filter can go almost indefinitely without needing fresh sand.

The basic physics of sand filtration are also different from rival filter varieties: High-rate sand filters clean water via a process known as depth filtration, meaning that dirt penetrates the sand bed and is captured in the tiny spaces between grains of sand. By contrast, dirt and particulates are captured on the surface of the media in both cartridge and DE filters.

The depth-filtration principle works just fine – unless, that is, the sand filter is not backwashed often enough. Without backwashing, dirt particles begin to accumulate on the surface of the sand bed and will result in short cycles, channeling, and poor overall filtration.

Conversely, if you backwash too often, you will also compromise filtration: When the sand bed is totally clean, some of the smaller particles of dirt will pass through unfiltered. As the bed begins to accumulate dirt, the filter begins to catch those smaller particles. In other words, getting ahead of yourself by cleaning the media too often will prevent a sand filter from doing its job.

Backwash-1When to Backwash

How do you know when it’s time to backwash?

One obvious cue is cloudy water: When the pool gets murky, a dirty filter is the prime suspect. Another, far better cue, however, can be found with the filter’s pressure gauge or gauges.

If the system has both inlet and outlet pressure gauges, you will note only minor pressure differentials -perhaps 3 psi – when the filter media is clean. As the sand bed begins to load up with dirt, that differential will begin to increase. In most high-rate sand filters, it’s time to backwash when the pressure differential reaches 18-20 psi.

If the system has an inlet pressure gauge only, you should backwash when the pressure increases by 8-10 psi from initial post-backwash readings. The best idea here is to mark the pressure gauge with a grease pencil right after a good backwashing – or to maintain a record of the running pressure on a route sheet as you monitor filter cycles.

These filter cycles can be affected by many different factors, ranging from heavy bather load and algae to wind-blown dirt and debris. In addition, a sand filter on a newly plastered pool will clog quickly with plaster dust, which often precipitates out of new plaster during start-up procedures.

This fine plaster particulate can easily clog a sand bed and greatly reduce the length of initial cycles. In fact, when first starting a sand filter on a fresh pool, it may be necessary to perform an extended backwash of two to three times the normal duration to rid the sand bed of the bothersome dust.

Routing the flow

Backwashing is a simple matter of reversing the flow through the filter by sending water up through the underdrain or laterals and diverting the outlet water to waste.

The procedure is simple: Turn off the pump to avoid damage to plumbing or valving, then turn the control valve to the backwash position and restart the system. Once the filter has been backwashed for the desired time, shut the system down and reset the valves. Don’t fire up the system right away: The sand bed needs time to settle back down into place.

When you restart the pump, a small burst of cloudy water may enter the pool. This is typically caused by a residue of backwash effluent present in the sand bed as a result of inadequate backwash time. Some valve arrangements dodge this problem by sending a small initial burst of water to waste before returning filter effluent to the pool.

Given a typical flow rate of 15-20 gpm per square foot of filter area, most manufacturers recommend backwashing for 2-3 minutes. As always, it’s a good idea to consult manufacturer service manuals for specific backwashing procedures.

Putting Sand to Bed

The beauty of a sand filter is that the sand itself acts as a permanent medium: Once it’s charged you will rarely – if ever – need to replace it. The keys to that longevity are steady backwashing routines, as described elsewhere in this article, and getting the system off to a good start with proper charging.

Here are the basic steps:

1. First, obtain sand of the proper size. Most sand filters use filter sand, which is typically .45-.55 millimeters in diameter. It is also commonly referred to as “pool grade #20 silica sand.” Note: some filters also use gravel in the filter bed. Consult manufacturer literature for proper gravel specifications.

2. Next, to keep sand out of the circulation system, cover vertically exposed plumbing or standpipes with some type of protective cap. On some models, you also will need to position the lateral assembly in the tank before adding the sand to ensure that internal plumbing seats properly when the valve at the top of the filter dome is reassembled.

3. Just before adding the sand, some manufacturers recommend filling the bottom of the tank with a few inches of water to act as a cushion for the sand as it is poured into the tank. This will help prevent damage to the lateral assembly.

4. Pour the sand gently through the top of the filter tank. Although different models require varying amounts of sand, a good rule of thumb is to leave 10-12 inches of freeboard – that is, the space between the top of the sand bed and bottom of the diffuser assembly. Adequate freeboard will prevent sand loss during backwashing. Conversely, if the sand bed is too shallow, the filter will work but will load with dirt quickly and require frequent backwashing. Service and installation manuals offer various tips to help you charge the filter. Some, for instance, recommend holding down the vertical standpipe to make sure it isn’t dislodged as the sand is being added.

5. Remove the protective caps and replace the top valve or dome on the top of the filter tank. Some assemblies are threaded into the top of the tank, while others are held in place by a clamp assembly. All assemblies use O-rings to create a good seal, with manufacturers typically recommending use of an appropriate O-ring lubricant to ensure a good seal.

During the backwashing cycle, the sand bed in a high-rate filter will actually rise up several inches in the tank as the water scrubs dirt from the sand.

Sand-FilterThis is what happens inside the sand filter:

1. This is a typical sand filter in the filtration mode or cycle. The flat surface of the sand bed indicates proper pump and filter sizing, which provides maximum filter efficiency without channeling.

2. As the backwash cycle begins, the sand bed rises evenly in the tank as a result of proper flow rate through the filter.

3. Within 2-3 seconds, the sand bed becomes semi-fluid. At this point, dirt and other solids break free from the media and are being discharged to waste.

4. Within 5-10 seconds, the sand bed is now totally fluid. Note the 6-7 inches of “freeboard” between the top of the sand and the bottom of the diffuser. If the pump and filter are properly sized, the sand will not rise too high in the tank.

5. Now in the “rinse” mode, the sand bed is beginning to re-settle.

6. The filter has now returned to the normal filtration mode. A critical factor in this simple operation is remembering to shut the pump down before changing the filter’s valves from one mode to another. Forgetting to turn off the flow could result in damage to the valve and/or the filter.

Filter Cleaning and Media Replacement

Keeping the filter clean is the simplest way to ensure the other components work up to their specifications and you end up with a clean pool. Let’s review the process for cleaning each type of filter.

DE filters

Backwashing a DE filter as a regular practice will backwash some dirt and some DE are flushed from the filter. The remainder drops off the grids and falls to the bottom of the filter in clumps. The manufacturers say that after backwashing 70 percent of the DE has been removed, so you need to replace that amount. If you add too little, the filter grids will quickly clog with dirt and the pressure will build right back up, even stopping the flow of water completely. If you add too much, you will get the same effect by jamming the tank with DE.

Backwashing cannot remove oils from the grids, which get there from body oil, oil in leaves, and suntan lotions.

Backwashing a DE filter is useful when the pool has been trashed by winds, mudslides, algae, or other heavy debris. You start to vacuum it and quickly the filter can’t hold any more dirt. To save a lot of time you backwash, add a little fresh DE, and get on with the job. You repeat this process until the big mess is cleaned up, then you break down the filter and clean it properly.

The other time you might backwash is when you’re vacuuming a normally dirty pool, but the filter hasn’t been cleaned in awhile and is just about full of dirt. You’re getting no suction because the filter is clogged. Backwash, add some fresh DE, finish cleaning the pool, and then do a breakdown and clean the filter.

When the water is going inside the grid and flowing outward, any debris in the water from the pool will clog the inside of the grids (or laterals on sand filters), rendering them useless. On a new pool startup where a lot of plaster dust or gunite debris might be in the water, don’t backwash. if you must, open the strainer pot and turn on the pump. Flood the pot with water from a hose and backwash as needed that way. Obviously, never vacuum a pool with the filter on backwash because the dirt and debris you vacuum will flow directly inside the grids (or laterals).

This get you to break down (or tear down) and cleaning of a DE filter. The Purex/Hydrotech style of vertical grid tank DE filter type is explained here. They are common in the field and if you can do these, you can do them all.

1. Turn off the pump and switch off the circuit breaker.

2. Open the tank drain and let the water run out. Remove the lid of the filter. On some filters, it is as easy as removing the clamping ring and applying light pressure under the lid with a screwdriver. Some filters make such a tight seal with the O-ring and lid and as the water is drained out, it sucks the lid on even tighter.

3. Remove the retainer’s wing nut and remove the retainer. Now gently remove the grids (elements). One design flaw of many grids lies in the fact that they are made like small aircraft wings-large, curving units-but they are set into the manifold on stubby little nipples. Applying a reasonable amount of force on the rather large wing part of the grid won’t hurt it, but the resulting torque on the flimsy nipple will snap it right off. Therefore, to remove the grids, wiggle them gently from side to side as you pull them straight up and out. Be prepared to hose out the tank while the grids are still in place (if the drain hole isn’t also clogged with DE) or patiently excavate the dirt and DE until you can free the grids.

4. Remove the retaining rod by unscrewing it from the base of the rotary valve. Sometimes it is corroded in place, so have pliers handy to grip the rod and unscrew it. A word of caution-the rod might be corroded enough that if you force it with your pliers, it snaps off at the bottom, leaving the threaded end in the rotary valve.

5. Reach in the tank and remove the manifold from the rim of the rotary valve.

6. Hose out the inside of the tank, the manifold, and the holding wheel. Hose off the grids and scrub them lightly to loosen the grime. If the grids are still dirty, soak them in a garbage can of water, trisodium phosphate (1 cup per 5 gallons of water), and muriatic acid (1 cup per 5 gallons). After 30 minutes, try scrubbing them clean again. Don’t use soap.

7. Inspect the manifold for chips or cracks. DE and dirt will go through such openings and back into your pool. Cracks can be glued. If chunks of plastic are missing, buy a new manifold. Particularly inspect the joint between the top and bottom halves of the manifold. These two parts are glued together, tend to separate. Replace the manifold as you took it out. Reinstall the center rod.

8. Carefully inspect the grids before putting them back inside. Look for worn or torn fabric, cracked necks on the nipples, or grids where the plastic frame has collapsed inside the fabric. Replace any several damaged grids. When you reinstall the grids, notice that inside each hole in the manifold is a small nipple and on the outside of each grid nipple is a small notch. By lining up the nipple and notch as you reinsert each grid, the grids will go back as intended. Now lay the retainer over the tops of the grids and spin it around until it finds its place holding down and separating the grids. Screw on the wing nut and washer that holds down the retainer holding wheel.

9. Get the lid back making sure the 0 ring on the tank is free of gouges and has not stretched. If it is loose, soak it for 15 minutes in ice water and it might shrink back to a good fit. If not, replace it. Apply tile soap as a lubricant to make it slide on easier (or silicone lube if you can afford it) to the inside of the lid around the edge that will meet the O-ring. Don’t use Vaseline or petroleum-based lubricants because these will corrode the O-ring material.

10. Now close the tank drain, turn the backwash valve to normal filtration, and turn on the pump. Let the tank fill with water. Turn off the pump and turn the valve to backwash. The water will drain out, sucking the lid down. Don’t be afraid to help it along by getting on top of the lid. Your weight will finish the job.

11. Replace the clamping ring, return the valve to normal filtration, and start the pump/motor. Open the air relief valve and purge the air until water spurts out the valve.

12. Never run a DE filter without DE, even for a short time. Dirt will clog the bare grids. Remember, it’s not the grids, but the DE that does the actual filtering. The label on the filter will tell you how much DE to add, or refer to the table on the bag of DE. It tells you how many pounds of DE to add per square foot of filter area.

DE is added to the system through the skimmer. Do not dump it in all at once. It will form in clumps at the first restricted area, like a plumbing of elbow or the inlet of the filter tank, or even crystallized it and make it hard. Sprinkle a little amount at a time, mixing it in the skimmer water with your hand. This will disperse it evenly in water. If the the unit is not assembled correctly, DE will flow back into the pool after passing through the unit, when you start up the unit. If the water has slight milky residue, which reduces with more flow of water then it is normal and there is nothing wrong in it.

Most of the pool have skimmers where you can add the DE. But if there is no skimmer than, make slurry of DE in a bucket turn on the pump and add it to the strainer pot, followed by clear water. This way the DE will coat the gird evenly. Cover the strainer and reprime the pump.

Sand Filters

Sand filters use specific size and quality of sand (#20). If the particle size is big then the filtration of smaller particles is not possible and if the sand size is too small then it will clog the laterals.

Sand filter need regular backwashing that are effective. When the tank is full of circulating water the sand is suspended in the tank. The sand is light enough to stay floating in the tank, but heavy enough that it does not flow out with the backwash water. This is why the multiport valve is located on top of sand filters, so as the backwash flows from the bottom toward the top the dirt flows up and out while the sand stays put. So backwashing is an effective way of cleaning a sand filter. Most rotary valves have the steps printed right on them, and they are very simple.

1. Turn off the pump. Rotate the valve to Backwash. Roll out your backwash hose or make sure the waste drain is open.

2. Turn on the pump and watch the outgoing water through the sight glass. it will appear clean, then dirty, then very dirty, then it will slowly clear. When it is reasonably clear, turn off the pump and rotate the valve to Rinse.

3. Turn the pump back on and run the rinse cycle for about 30 seconds to clear any dirt from the plumbing. Turn off the pump, rotate the valve back to Filter, and restart the pump for normal filtration.

When the filter gauge reads 10 psi more than when the filter is clean, it is usually time to backwash. A better clue is when dirt is returning to the pool or when vacuuming suction is poor.

When backwashing, be sure there is enough water in the pool to supply the volume that will end up down the drain. It is usually a good idea to add water to the pool or spa each time you backwash.

Sand under pressure and with the constant use of pool chemicals or dissolving pool plaster will calcify, clump, and become rock-like over time. Passages are created through or around these clumps, but less and less water is actually filtering through the sand and more is passing around it. This is called channeling. To correct or avoid this problem, regular teardown is the answer.

1. Turn off the pump. Disconnect the multiport valve plumbing by backing off the threaded union collars. Some valves are threaded into the body of the tank, others are bolted on. Remove the valve.

2. Some sand filters have a large basket just inside the tank. Remove this and clean it out. The sand is now exposed. Push a garden hose into the tank and flush the sand. As noted previously, it will float and suspend in the water. Bust up the clumps. As the water fills the tank, it will overflow, flushing out dirt and debris. Be careful not to hit the laterals on the bottom of the tank because they are fragile and break easily.

3. When the sand is completely free and suspended in the water, not clumped, turn off the water and replace the basket, multiport valve, and plumbing. Backwash briefly to remove any dirt that was dislodged by this process but not yet flushed out.

This teardown process also allows you to check to see if the regular backwashing has flushed out too much sand. You might need to add some fresh sand. Most sand filters need to be filled about two-thirds with sand and have one-third freeboard. Backwash after adding any new sand to remove dust and impurities from the new sand.

If channeling is a problem because of hard water or pool chemistry which speeds up calcification of the sand, introduce aluminum sulfate (alum) through the skimmer just like you would add DE to help prevent this problem. Use the amounts recommended on the bag.

Every few years you need to replace the sand completely because erosion from years of water passing over each grain makes them round instead of faceted and rough. Smooth sand does not catch and trap dirt as efficiently and it slowly erodes to a smaller size than the original #20 silica, allowing it to clog laterals and pass into the pool. Some manufacturers suggest adding a few inches of gravel over the laterals first. This keeps the sand separated from the laterals so the sand cannot clog them. To replace sand, or add sand to a new installation:

1. Open the filter as described previously. Remove the old sand by scooping it out.

2. Fill the bottom third of the tank with water to cushion the impact of the sand on the laterals.

3. Slowly pour the sand into the filter, being careful of the laterals. Fill sand to about two-thirds of the tank. Reassemble the filter parts and backwash to remove dust and impurities from the new sand, then filter as normal.

Cartridge filters

Cleaning a cartridge filter is perhaps easiest of all.

1.Turn off the pump. Remove the retaining band and lift the filter tank or lid from the base. Remove the cartridge.

2. Light debris can simply be hosed off, but examine inside the pleats of the cartridge. Dirt and oil have a way of accumulating between these pleats. Never acid wash a cartridge. Acid alone can cause organic material to harden in the web of the fabric, effectively making it impervious to water. Soak the cartridge in a garbage can of water with trisodium phosphate (1 cup per 5 gallons) and muriatic acid (1 cup per 5 gallons) for an hour. Remove the cartridge and scrub it clean in fresh water. Don’t use soap.

3. Reassemble the filter and resume normal circulation.

Replace cartridges when they won’t come clean, when the webbing of the fabric appears shiny and closed, or when the fabric has begun to deteriorate or tear.


Filters, do not have many repair or maintenance problems beyond cleaning as discussed. But however, the complaint is related to leaks of various kinds.

Backwash valves

Backwash valves leak in two ways: internally and externally. Internally, O-rings deteriorate and allow water and DE or dirt to pass into areas not intended.

Piston backwash valves: The valve has piston discs equipped with O-rings. As these wear out, water or dirt bypasses the intended direction. Similarly, the O-rings on the shaft, just under the handle, wear out from regular repeated use. If you suspect the disc O-rings or see water leaking from the top of the shaft, tear down the valve as follows:

1. Turn off the pump. Remove the screws on top of the valve cap. Pull the handle up as if you were going to backwash, but keep pulling straight up to remove the entire piston assembly. Replace the O-rings on each disc. They pull off like rubber bands and the new ones go on the same way. Apply silicone lube to the O-rings.

2. Remove the handle from the piston stem. It is held in place by setscrews or allen-head screws. This also allows you to slide the cap off the stem. Inside the cap, you will find two small O-rings. Pull these out with the tip of a screwdriver and replace them. Apply silicone lube.

3. Clean the stem and disc assembly and flush out the inside of the valve body. Grit or sand can create leaks or cause your new O-rings to wear out sooner than necessary. Reassemble the unit the same way you took it apart.

Rotary or multiport backwash valves: Rotary and multiport valves are similar in construction, A rotary valve is normally mounted under a vertical grid DE filter. As with piston-type units, these leak either externally or within the chambers of the unit itself. If water appears under the filter, use a flashlight to inspect underneath as carefully as possible. If you can see or feel a leak where the plumbing enters the valve openings, you can repair that without disassembling the entire filter. If the leak appears to be at the joint of the valve and filter tank, or if the problem is DE and dirt bypassing the normal flow and getting back into the pool, you will need to tear down the filter and valve.

Another typical symptom of an internal leak is drips coming from the backwash outlet even though the valve is turned completely to the normal filtration position. It employs a rotor seal that can compress or wear out. When the body gasket wears out and water bypasses the normal flow, some leakage gets to the backwash side and appears as a leak under the filter. If the backwash outlet is plumbed directly into a waste or sewer drain, this leak might not be visible.

Sometimes the problem is not in the pool or spa itself, but in some hidden area within the system plumbing. Such a hidden problem can also cause the system to lose prime overnight when the pump is off. The leak drains the water from the filter tank, then siphons the water out of the pump.

On start-up the next day, the pump has no prime. If the pump runs dry for several hours, overheats, loosens or melts the plumbing fittings, you will attribute the loss of prime to the damaged plumbing. You repair the plumbing and the same problem occurs the next day.

Have a sight glass on the backwash outflow line so you can see any leaks and/or have a shutoff gate valve on that line that stays closed when the valve is in the normal filtration position.

To tear down this type of valve, use the following procedure:

1. Cut the plumbing to isolate the filter and take the unit apart as described previously.

2 .Reach inside the bottom of the filter and remove the bolts that hold the compression ring with a nut driver. This ring holds the valve in place as well, so the valve will now fall away from the filter tank.

3. You now have the valve body with the rotor inside. Remove the handle on the underside of the valve by removing the bolt assembly that holds it on the rotor shaft and slide it off the shaft. Pull the rotor out of the body. Bronze rotors are very hard to remove and you might have to take the valve to a pump rebuilding shop.

4. Pull the old rotor seal gasket from the rotor with needle-nose pliers. Clean the rotor and inside the valve body. Put a new gasket on the rotor, being careful not to over-stretch the new gasket.

5. Lube the gasket with silicone lube and replace it in the valve body. On bronze rotors, each port has an O-ring instead of one body gasket seal as you will find on the plastic versions. Before reassembling the filter, replace the O-ring that sits between the tank and valve and the O-ring that seals the shaft as it passes through the valve body to the handle. Also replace the O-ring on the neck of the rotor. The grid manifold sits on this neck and the O-ring seals that joint, so to prevent dirt from bypassing the correct direction of flow, and lube all O-rings with silicone lube.

6. Reassemble the valve and tank the way you took it apart. Be sure the tank itself is clean and that the opening in the bottom shows no rust or cracks. If it does, you should clean it thoroughly and have the cracks welded. Replumb and restart the filter as described previously.

Lids and gauge assemblies

Lids on filters leak in two places: the O-ring that seals them to the tank and/or the pressure gauge air relief valve assembly. The lid O-ring can sometimes be removed, cleaned, turned over (or inside out), and reused. Try the cleanup/turnover method and if you still have leaks, then replace it.

Some filters will crack on the rim of either the lid or the tank where the O-ring is seated. Obviously, the problem in this case is not a bad O-ring, but a bad lid or tank. Inspect these stress areas carefully for hairline cracks that might be the source of the leak.

Air relief valves sometimes leak if they become dirty or they simply wear out. Some are fitted with an external spring that applies tension to create the seal, and when the spring goes, so does the watertight seal. Others have a small O-ring on the tip of the part that actually screws in to create the seal. Unscrew this type of valve all the way. The screw part will come out to reveal the O-ring on the tip that makes the seal, and you can easily replace that. Air relief valves themselves simply screw out of the T assembly. Apply PTFE tape or pipe dope to the new one and screw it back in place.

The pressure gauge also threads into the T assembly. If you have a leak there, unscrew the gauge, apply PTFE tape or pipe dope to the threads and screw it back into place. If the gauge doesn’t register or seems to register low, take it out and clean out the hole in the bottom of the gauge. Dirt or DE can clog this small hole, preventing water from getting into the gauge.

Remember, when removing an air relief valve or pressure gauge, you must secure the T with pliers or a wrench while removing the component. The T assembly can easily snap off the filter lid or come loose if you fail to hold it securely when removing or replacing a valve or gauge.

The T assembly itself can come loose and create a leak where the close nipple passes through the hole in the lid. In this case you must remove the lid and tighten the nut from the underside of the lid. Some makes of filters have a nipple welded to the lid, so you won’t have this problem unless you crack the weld.

Dirt passing back into pool

There are many ways dirt or DE gets through the filter and back into the pool. And the methods to make repairs. just as a summary, check the following:

◦Damaged grids, laterals, or cartridges
◦Backwash valves with bad gaskets or O-rings
◦Broken manifolds or retainers.

So when you feel a backwash valve getting hard to turn, do a teardown and lubrication before the leaks occur. Examine grids, laterals, cartridges, and manifolds carefully each time you break down a filter for cleaning, and always take your time when reassembling. Sloppy reassembly after cleaning is the cause of more leaks than anything else in filters.