Can a Filter Be Too Oversized?

The simple answer that most pool pros will give the perplexed pool owner is a straight “no.”  That answer usually allays their worries due to its simplicity but we here at Inyo like to dig a little deeper.  So here we are to give the reasoning behind the “Go Big or Go Home” logic of pool filters.

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Flow Rates

Take a look at your current filter’s tag (if it is still readable, of course) and you will note the jumble of numbers and ratings that adorn it. The list is likely to include under-pressure-detail-1maximum working pressure, filtration rate, filter media requirement and even clearance. All this information is important for the proper operation and installation of the filter. Without the aforementioned ratings, we would not know when to clean or replace the filter media. Also, knowing proper clearance helps us avoid plopping the filter in an area that makes it totally inaccessible to be serviced. All that info is need to know!

Amongst the rabble of numbers, do you see a minimum flow rate? No. The reason being is that there is no true “minimum flow rate” of a filter. The tank may take longer to fill up if we have a ½ HP pump on a 800 pound commercial sand tank but the filter will still scrub the water to the same standard.

Advantages to Over-sizing


The main advantage to over-sizing a filter is the reduction of cleaning throughout the pool season.  With the larger filter there is more media in which the dirt can disperse, allowing water to flow freely. The larger the cartridge, the more dirt it can trap. The more dirt it traps, the longer we can go in between backwashes or cartridge hose-downs. And what does that mean for the sometimes lazy homeowner, like myself? FREE TIME!

Even the most fervent of pool geeks does not enjoy backwashing a sand filter on a hot Saturday in July.  Oversizing is a no brainer for the person who is constantly monitoring their pressure gauge as it will lessen the likelihood of seeing DEFCON 1 type pressure readings. By upping your filter media, you increase the area in which the dirt and debris is held, allowing the filter to maintain optimal water pressure for longer.

Motor Longevity

Your motor’s one and only job is to pull water from your pool basin and send it through the filtering system, it is a simple but a difficult job. This job is made harder when the motor  must fight built-up back pressure from an overwhelmed filter causing issues for both pieces of equipment.

For example let us say our pump has an output of 100 gallons per minute (GPM) but our pool filter can realistically handle only 50 GPM; the overflow of 50 GPM is stressing your system in two ways, overworking the filter which will likely lead to a premature failure, and secondly, the motor is overworking to as it attempts to push against the growing back pressure. The resulting struggle between the filter’s filtering capacity and the pump’s flow rate will eventually lead to a costly repair bill after one or both breaks.

If we alter the flow rates and make the filter 125 GPM and the pump 100 GPM, the filter can easily handle the pump’s flow rate. Also, the extra buffer in the filter’s flow capacity compensates for dirty filter media which will lessen the actual filter rate between cleanings. With a properly sized filter the pool is quickly and effortlessly cycled through the system while never testing the limits of your equipment. This makes for a happy and long lasting filtering system.

Changing Filter Media

The increase in filter size also means the sand, grids or cartridges are under less stress per square foot. A reduction in workload both from the flow rate and dirt increases its lifespan. The adage “less stress, longer life” applies to filter media and pool owner alike. The rigors of high flow rate, high dirt load and repetitiveness of cleaning can put a filter through the wringer.

The Dangers of Under-sizing

What happens when you overfill a water balloon? It bursts. Now imagine a water balloon large enough to hold 300 pounds of sand doing the same thing. Indeed, what a mess. That particular scenario is unlikely to happen to a filter’s tank but pipe bursts or other plumbing malfunctions may occur. These issues arise from back pressure created by a filter that cannot keep up with the pump.

Also, the need to clean your filter becomes a constant. When a filter gets dirty its flow rate capacity lessens, creating a rise in back pressure. This rise is seen in the pressure gauge and is the first sign that cleaning is necessary. If cleaning does not fix the issue, it may be time to replace your sand or cartridge.

19 thoughts on “Can a Filter Be Too Oversized?

  1. Could your filter be so oversized that the pump can no longer effectively backwash it? Smaller parallel filters might be better than one oversized one.

      1. Hi, in your article above, you said “there is no true “minimum flow rate” of a filter. However, there IS a minimum flow rate for backwashing (per manufacturer specs) and this number goes up as the filter gets bigger. So it does seem quite possible to have a pump/filter combination that will filter OK but not backwash properly. (In fact I think that is one issue I have with my current system, which is why I am googling for articles about pump and filter size 😉 )

        1. I just called into Pentair Commercial support yesterday and one of the things I was told is that you can use pretty much the largest filter you want for D.E. or cartridge but with sand, you can’t oversize it for your pump because it won’t be able to properly backwash. Filtering isn’t a problem. Backwashing is. Also for maximum efficiency you want to use larger 3″ plumbing with an IntellifloXF (If you are using Pentair)

  2. I opened up my pool today. I have a one year old DE filter. After running the system on “filtration” for an hour, I noticed that the skimmers dont seem to be sucking in as much water and the return are not pushing much water into the pool. When I switch the system to “circulate” the flow of water is perfect. What should I do to help the system filter the water as it should? The pump is working fine.

    1. If you opened your pool after months of being dormant, Did you do a healthy backwash and rinse treatment? Also, if you are opening the pool, do you have an algae problem? If so, that is another source of low pressure.

      When you put the filter on recirculate, it is bypassing the filter media. Because the filter’s pressure normalizes, you probably need to clean the filter.

  3. Haha everyone is an engineer lol. Common sense you restrict pump flow you harm the windings by creating to much heat. I e premature failure. High school physics

  4. Hi Matthew,
    I just found your page while looking for info around filters. I don’t want to hurt your feelings, but you really need to read a bit more about pump hydraulics before writing an article like this. In your response to Bernie you again confuse/mix power draw of the pump with power consumption per turnover of pool water. You also confuse the centrifugal pool pump with a positive displacement pump. Centrifugal pumps consume less power when the flow is restricted – thus the motor has to work less when the filter is plugged. To add another layer of confusion … the efficiency of the motor changes with motor load. But the absolute pumping job (turning over pool water) can be performed more efficient at lower velocity – although it will take longer to achieve same turnover (hydraulic work).

    1. I took your points to the Perry Mason of all motor questions over at Century. he is the guy that even the motor engineers defer too, so this is what he has to say:

      As is said last week, I don’t have a problem with the statements below, but there is more to the story. Work happens when something is moved. In the case of a pump/motor, less water will be moved if there is a restriction on either side of the pump. Less work means less power, and a motor that is not working as hard. Within a range of restriction, this could theoretically increase motor life. The “oversized” filter will have less resistance than an “undersized” one. I would definitely not recommend adding a restriction on either side of the pump as a way to save money. On an existing pool, it would be best to start with the pool size in gallons, and the plumbing size, and follow established guidelines for equipment, including the pump/motor and filter. The variable speed motor/pump is the “wonder solution” that enables a user to tailor a schedule to meet filtering needs and save money. It will still be an experimental process since each pool is unique.

      From my reading that, I gather that throttling the flow is not a good/safe idea for the motor or other equipment. We do research these articles before we post them, as well as having years of experience with the products ourselves.

  5. Just tested the theory about throttling the pump with restriction. When a heavy restriction was imposed cutting the flow to roughly half I checked the load on the motor. To my surprise it dropped from ~1400 watts to ~1350. I always thought it went up. So the amp load is down but with a huge loss in efficiency. This is at least for a centrifugal pump.

    1. Take into consideration the work being done over the 1350 watts in which efficiency has been reduced dramatically so most of the wattage is being waste as heat. On the other hand you still need to use the equivalent of hours of filtration per flow in the system therefore doubling the time the system should be on. If there was no energy being waste as heat it will be the ideal system. But taking into consideration is a single speed pump there is definitely waste.
      In real world words is like saying will you like 10 apples for a dollar or 5 apples for 90 cents. You save 10 cents but only get 5 apples.

  6. Your statement on pumps is entirely wrong regarding a restriction in discharge makes the pump work harder. As the discharge is restricted (throttled) less water is actually being moved. The less water being moved the less energy is being consumed, lower amps and thus lower wattage consumption. It would cause no harm to a pool pump to throttle a 100gpm pump down to 50gpm unless it causes cavitation. If cavitation occurs then erosion to the impellers may occur. In fact it is often time beneficial to throttle the discharge of your pools pump in order to reduce circulation volume and improve media filtration and you will achieve lower energy consumption as well.

    Also pool pumps have a very low head potential (psi is limited) hence you will rarely if ever see any type of pressure relief valve on a residential pool filtration system and filter canisters will not grenade the pump end will cavitate and dead head at a much lower pressure than the canister rating.

    1. Hello Bernie, thank you for your comment we appreciate the spark in discussion on our blogs. Your detailed post caused me to go back to do a fact check, which led to a few phone calls and emails with manufacture engineers to figure out who was right. So let me go through your points as you mentioned them and provide my findings from my chats:

      1) “‘Throttling’ a pump can save electricity”, that statement is partly true but only if your definition of throttling includes dual and variable speed motors exclusively; a healthy single speed motor will draw the same amps no matter the load. If you throttle a single speed you are changing nothing but the water flow. The only way to affect your electric bill in the manner you mention is to be able to control the speed of the motor, a la the aforementioned dual and variable speed. To be honest, my blog is a tad misleading in that area, I will admit and soon correct.
      2) “Throttling” would cause no harm to the pool pump. This point is made but then a list of issues (cavitation, and impeller decay) is provided soon thereafter as symptoms of throttling done wrong. The process of throttling a pump’s discharge unless done by a professional fully versed in water dynamics could be harmful to your equipment. Most homeowners are not lucky enough to have this advantage so it is best to go the easier route of sizing a pump or motor impeller combo to the desired flow rate.
      3) Filters do have a kind of pressure relief valve but that is mainly for air that is trapped in the tank. I believe the pressure relief valve you are speaking of is to prevent excess water pressure and pool filters are not usually equipped with those. This is because if a filter system is properly sized, as we discuss in this blog, excess pressure should not be an issue. A filter can burst due to high pressure there are plenty of broken tank pictures to show that fact.

      Thank you for reading. If you have any quibbles with my findings, let me know.

    2. What you said would only be true if your pump is variable displacement and compensates for pressure reduction. I work with pumps in many different systems and a blocked or restricted pressure line has caused early pump failure or damage to pipes or equipment.

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