• Pressure increase in a cleaned cartridge filter
  • The real deal on pump and filter sizing
  • Explaining service factors and uprated motors
  • GFCI trip thresholds
  • Pump squirting upon starting

I cleaned my cartridge filter this week but the pressure went back up after a few days. What’s the problem?

Scott P. Gleason:

A few things could be contributing to the quick pressure build-up.

For cartridges over two to three years in service, it is most likely time for replacement. Over time, pleated filter media accumulates particulate between its fibers. This cannot be removed even with the best of cleanings. As more and more of these particles become permanently trapped, the filter’s dirt-holding capacity is reduced while its efficiency is increased. The result is a quicker increase in the filter housing pressure and shorter cycle times between cleanings.

For cartridges less than two to three years old, there could be a build-up of calcium or minerals on the surface of the pleated filter media. This is most likely blinding off a good portion of the media’s open area. As particulate becomes trapped in the remaining open areas, the pressure quickly rises in the filter housing due to flow restriction. Typically, this build-up can be resolved with a good, standard cleaning of the filter cartridge and then an acid wash. Always follow the manufacturer’s recommended procedure for performing an acid wash to avoid permanent damage.

Another possibility for a cartridge less than two to three years old is that it has removed a non-water-soluble substance from the water, which has adhered to the media; or a bio-film has developed on the surface of the filter media. Unfortunately, in both of these cases, the substance cannot be removed, so a new filter cartridge will be needed, along with steps to remove the non-water soluble substance most likely still present in the water, so the new cartridge will not experience the same fate.

B. Champlin/T. Isberg/W. Kinard/ R. Raub/R. Thibault (Horizon):

If the pressure raises rapidly after cleaning, then the cartridge has either not been thoroughly cleaned or there is still a large amount of contaminants in the water.

After cleaning, there may be some contaminants left in the fabric of the cartridge filter. It could be body oil, facial cosmetics, shampoo, chemical residue or any number of substances brought in from outside the pool or spa. The condition of the fill water or addition of chemicals could also be contributing to an increase of total dissolved solids or calcium content of the water. All of these things can become impacted in the filter media and are very difficult to clean with plain water. Sometimes this residual matter can be hard to see in the cartridge — your indication is the pressure reading of the filter compared to the baseline reading that you recorded.

After washing the cartridge down, try soaking it overnight in a solution of trisodium phosphate (TSP) or other filter degreaser. This should remove most of the oil-based substances from the media. If, after reinstallation, the system pressure still does not return to the baseline, there may still be a calcium substance in the media, which will require a soak in an acid-based solution. Be sure you have first cleaned the cartridge in a degreaser solution to remove any oils that might be coating possible calcium contaminants. You might also test the water at this time for total dissolved solids, since a high level of TDS will increase the frequency of filter cleanings.

What’s with these huge pumps? Over the past year I have noticed new 3/4-, 1-, even 1 1/2-horsepower pumps on 36-square-foot DE filters! The filter is usually trashed — crushed grids, cracked tank, broken tank bands. I’ve been in the business for a long time and have never put in a 36-square-foot with a 1 1/2-horsepower pump. What is the real deal on filter and pump sizing? Is it just the gallons per minute, or does filter pressure matter? And how does 25-plus psi day in and day out affect the lifespan of the filter?

John Bent:

Filters should be sized based on gallons per minute after the pump has been sized for the pool. The pump flow rate should not exceed the maximum flow rate recommended by the filter manufacturer. Higher-than-normal filter pressure can force particulate into the media, making the filter difficult to clean, thus shortening its life.

Jeff Sanchez:

There were some schools of thought that one could use a larger-size pump and then valve it down to achieve the correct flow rates. However, with energy costs being high, the preferable method is to size the pump correctly from the start. To operate most efficiently you maximize flow (gpm) at the lowest possible system pressure (psi).

B. Aubrey/R. Denkewicz/S. Petty (Hayward):

Filters should be matched to the pump based on the design flow rate. It is usually better to select a larger filter than is needed, as it should prolong the time between cleanings.

Properly matching the pump and filter is usually easier for new construction, given that the equipment pad is a clean slate. The challenge for existing installations where a pump and filter are mismatched is breaking the understandable consumer expectation that you must replace like-for-like. Service techs and retailers will need to continue to educate the consumer on the benefits of right-sizing pumps and filters, such as possible reduced energy consumption and prolonged equipment life.

Rodney McCall:

A change in pressure occurs when the pump turns on and expands the tank. When the pump turns off and it contracts to the original shape, the pressure changes again. These changes cause mechanical stress. This is normal. The materials are flexible. However, like a copper wire, if you keep bending it back and forth it will eventually break. This expansion and contraction can decrease the life of the filter tank and O-rings. The higher the operating pressure, the greater the expansion and contraction. To maximize the life of the filter vessel, it is best to design a system with low operating pressure.

The following chart can be used as a guide, but it is always best to check manufacturers’ recommendations when sizing your pump and filter.

Filter and Pump sizing
GPM per sq. ft. of filter area
Cartridge Filter.375.5

To figure out the square footage of a sand filter, use this formula: (radius2 x 3.14)/144.

If you have a 36-inch diameter filter, the radius is 18 inches.  18 x 18 x 3.14 / 144 = 7.06.  That filter would perform best with 105 to 141 gpm; 7.06 x 15 = 105 and 7.06 x 20 = 141.

Pumps that use variable-flow technology, which is different from variable-speed, automatically adjust themselves to prevent the crushing of grids or over-pressurizing the system. Variable-flow pumps automatically sense the clean system pressure after backwashing. They can be programmed so that when the system reaches a certain pressure, the pump stops monitoring flow and starts monitoring pressure. The pump then decreases its rpm, thereby lowering the pressure so it doesn’t crush grids or elements, or over-pressurize a filter when it gets dirty. It can also send a signal when it’s time to clean the filter.


The short answer is that both gpm and filter pressure are relevant to matching pumps with filters.

The filter’s flow rating in gpm tells you how much water can safely move through it in a given amount of time. The NSF flow rating for DE filters is 1 to 2 gpm per square foot of filter area. So a 36-square-foot DE filter will handle up to a maximum 72 gpm. To try and move more than that through the filter with a large pump will mean that filtering efficiency will go down due to particles of dirt being driven into the media at higher velocities and impacting the media. Once particles start impacting into the media, the flow rating of the filter is essentially decreased, making the situation worse and creating the potential for a higher-than recommended pressure reading.

The pressure (psi) rating of the filter tells you how much sustained pressure the filter body is capable of handling on a consistent basis. To exceed that is to create a safety hazard that must be corrected immediately. 

Also, there is the matter of differential pressures across the media. If the media is impacted with dirt, oil and other contaminants due to excessive flow of water (high velocity), or if the flow of water exceeds the maximum rating of the media, there will be a much higher pressure reading on the upstream side of the media than on the downstream side of the media. The media, clean or dirty, is creating a block or dam in the water. The pressure difference, if excessive, could eventually result in the implosion or crushing of the DE grids, filter cartridges or collection laterals. If something other than the filter media, such as a complicated return plumbing system, is causing a high pressure reading, then the pressure in the filter will be relatively equal between the upstream and downstream sides of the media. This situation is not as much of a concern as long as the volume of the water moving through does not exceed the maximum flow rating of the media and the pressure of the water is not in excess of the maximum pressure rating of the filter.

So, you must look at the flow curves of the pumps, calculate the total dynamic head of the system (and static head, if applicable), and then use the pump that has an appropriate output at that amount of dynamic head. If you find that the filter is a limitation to the pump’s output, then downsize the pump or increase the size of the filter, or install a bypass valve to route some excess water around the filter. If there are other features such as a waterfall that require additional water flow, then consider using valves and controllers to provide adequate water to the feature while staying within flow limits of the filter, or just use a separate pump for the additional feature.

How do you explain something as complicated as service factors and uprated motors to a consumer who is replacing a dead motor?

John Bent: Because of confusion created by uprated motors and service factors, installers and consumers need to look at the total or maximum horsepower (nameplate hp x service factor = total horsepower) rating of replacement pumps and motors for the true horsepower rather than the nameplate horsepower.

Jeff Sanchez:

To explain full-rate versus up-rate, one should introduce two terms: Total Horsepower (Thp) and Service Factor (S.F.). Total horsepower is the maximum amount of horsepower that the motor can deliver without overloading, and can be derived by multiplying full horsepower by service factor. In the event that an “uprated motor” has a service factor of 1.0, the uprated horsepower equals total horsepower.

Rodney McCall:

Explain that pump curves are how we measure pump performance. Then open a catalog and take them to the page of the pump you’re replacing. Show them that the uprated 1-hp pump has the same pump curve as a full-rated ¾-hp pump. It’s the same motor and impeller, and therefore the same performance. So just make sure that you are comparing uprated to uprated or full rated to full rated, or talk about pump performance and help the customer see that the numbers aren’t important.

Title 20 requirements, which some states are adopting, have simplified this by requiring that pool pumps list “service factor” horsepower, which is the horsepower times the service factor. Service factor horsepower is listed in most manufacturers’ catalogs as SFHP; this can be used to compare like for like.

Also keep in mind that selling the customer the same kind of pump that they had before may not be the best, safest, most energy-efficient alternative. The market has historically oversized pool pumps, which is why service factors on pumps were adjusted in the first place. To correctly size the new pump for the whole system, make sure that the new pump will not exceed the safe flow rate of the drain cover, ensuring that the velocity in the branch line between the drains does not exceed 6 fps if one drain is blocked. Also match the pump to the filter, piping (especially if it is copper), and other parts of the system.

Educate the customer on energy-efficient alternatives like variable speed, or improving the safety of the system by upgrading to pumps that include a safety vacuum release system. After going through this analysis with the customer, they will be assured that you are looking out for their safety and best interests.

Richard Howell:

The easiest way to explain this to a consumer is an uprated motor has a lower service factor, meaning more total horsepower (sf x rated hp = total hp) compared to a full-rate motor with a higher service factor. In today’s economy, an energy-efficient motor that is a full-rate motor will save you money.

How did the trip thresholds on GFCI’s get changed a few years ago? Even between the different manufacturers, the trip-points are different. Is there an actual calibration that’s done by the manufacturers to make sure that these are tripping at 5-6 milliamps and not 2?


Most manufacturers of pumps for permanent pools are building product per UL 1081. The requirements for GFCI products are dictated per UL 943, and their use in pool and spa installations is typically driven by the National Electric Code and/or particular state/local requirements. Pool equipment manufacturers, therefore, are usually not involved in the specific design parameters for GFCIs (e.g. trip threshold).

Why do pumps always squirt a little water out of the lid when the pump starts and stops?


The occurrence is not uncommon in high back-pressure installations, particularly with pumps that have non-pressure testable lids or covers. This can be exacerbated by pressure spikes (or “water hammer”) that can occur when valves, including some used in in-floor cleaning systems, are rapidly opened or closed.