When to Backwash a Pool Filter

When to Backwash a Pool Filter

When should I backwash my pool filter? Without a doubt, this question has to be one of our top five most frequently asked questions. Knowing when to clean your sand or diatomaceous earth (DE) filter is vital in keeping your pool water and equipment healthy. Identifying when to backwash your filter is simple, but there are different methods to go about it. In today’s blog we will discuss three common ways of determining when to backwash your pool filter:

  1. PSI
  2. Water Flow
  3. Water Clarity

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What Is Backwashing?

Unlike when you are sharing a drink with a friend, backwashing is A-OK to do and it is even encouraged. In pool terms, backwashing is the act of reversing the flow of water through your filter media, thereby dislodging trapped debris and flushing it out through your multiport valve waste line. For anyone needing a detailed walkthrough on how to backwash a sand or DE filter, look no further than our how-to guides on backwashing a sand filter, or DE filter. For the visual learners out there, you can watch our how-to backwash video guides embedded below.

But enough about what backwashing is, let us get to the meat of the issue. When should you backwash your pool filter?


Hayward 102D-204D-3
Dry Case Pressure Gauge

An increase of a quater or a third of your filter’s “clean” operating pressure is a sign of a dirty filter. Pounds per square inch (psi) is a standard unit for measuring pressure and the most common factor used in determining when to backwash a pool filter. A filter’s pressure gauge is the tool used to measure a filter’s psi. The face of the gauge is clearly marked off in increments of ones or tens and normally has a maximum value of 50-60 psi. The “clean” psi rating for individual pools will differ due to variables such as filter size, pump strength and feet of head.  The “normal” pressure is the baseline psi reading after a heavy filter cleaning cycle or after the installation of new filter media (new sand or recharged DE grids). For example, if your normal baseline psi reading is 12 psi then when your gauge reads 15-16 psi (20 – 25 percent psi above your filter’s normal operating pressure) it is time to backwash your pool filter.

There are two types of pressure gauges generally used on filters: dry case and liquid (oil) filled. The dry case gauges are by far the more common and often the type that is packaged with a brand new filter. The liquid filled gauges are my personal favorite as they prove to be more durable than dry case gauges. The liquid (glycerine or silicone) dampens the vibrations which are the archenemy of the fragile innards of the pressure gauge.

Go with the Flow

Blue White Pool Flow Meter
Blue White Pool Flow Meter

A flowmeter is a quick and simple tool for checking filter health by measuring the gallons per minute (GPM) rate through your plumbing line. A GPM drop of a third of the normal flow rate is a sign that your sand or DE filter requires a backwash. For example, if your normal flow is measured at a rate of 30 GPM, a reading of 20 GPM means your filter is screaming, “Clean me!”

There are two common flowmeter styles used on residential pools. The one is the float and tube and the other is a digital format that uses a paddle-wheel mechanism to measure the flow. The float and tube is my personal preference because of the minimal moving parts. Fewer moving parts mean fewer things can go wrong. Digital flowmeters are super accurate, but for the simple practice of determining when to backwash a pool filter they may be overkill.

Flowmeters are tools that you can use to determine the health of your pool quickly and easily. Unfortunately, they are often overlooked or left out of pool designs. The reason for this escapes me, but I make sure to tell anyone who is building a pool to add a flowmeter. If your existing pool does not have a flowmeter, do not fret, because they are easily added to plumbing lines. For those who are curious, feel free to read our how-to guide on installing a pool water flowmeter.


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SafeDip 6-in-1 Electronic Tester
SafeDip 6-in-1 Electronic Tester

Pool Water TDS Levels, AKA Clarity

Measuring a pool’s total dissolved solids (TDS) level is to measure its water clarity or lack thereof due to suspended particles. Essentially it is measuring how dirty your water is down to the parts per million (PPM). A freshwater pool should not exceed 2,000 PPM, because anything higher risks sanitation chemicals, like chlorine, losing their effectiveness. The most common method of measuring TDS is by checking the water for a “cloudy” appearance or testing water samples. Most local pool stores offer free water testing services, but for DIY guys and gals a home kit is just as effective. The SafeDip 6-in-1 Electronic Tester measures TDS levels as well as free chlorine, pH, ORP, saline levels and temperature. It is a one-stop shop for avid water testers.

As usual, thank you for reading and if you have any further questions feel free to leave a comment below or call our toll-free helpline at 877-372-6038.

75 thoughts on “When to Backwash a Pool Filter

    1. If you’re asking what is the minimum PSI required to backwash, it is going to depend on your system, and how dirty your filter is. If you have a dual speed motor, I would backwash on high speed. With a variable speed pump, I would backwash the filter on the same speed you use to vacuum your pool.

  1. I added chemicals to my pool cause I just opened it and my psi is reading 20 it normally stays at 30 what should I do?

  2. I just opened my pool and add a opening kit I don’t want to loose the chemicals backwashing. The pool is dirty the cover ripped over the winter. When would you recommend backwashing

    1. You’re going to lose the chemicals by them killing bacteria or being affected by your green pool, so it just has to be done. It is going to be the same schedule we mentioned in the article.

  3. Is it best practice to close any of the valves (return or drain) for the pool while backwashing, or can I leave them all open?

    1. Depending on how quick your pool drains, you may want to close the skimmer and pull from the main drain only. While backwashing, your water level may dip below the skimmer causing you to lose prime.

  4. I Have a Doughboy 18′ round pool, with sand filter. The filter has backwash only, no rinse or site glass to see when water is clear. Any recommendations on how to know when I’ve backwashed enough? Will residual sand spit into pool since I have no rinse option?

    1. Do you know if the sand is coming from an outside source, or the sand coming from your pool filter?

      If the sand is coming from an outside source, then yes, that is somewhat normal. if the sand is coming from the sand filter, then no, that is not normal and a sign of a cracked laterals or a standpipe. To figure out the latter, have a read of this blog: Sand in Your Pool?

  5. I am adding chlorine and it is still staying very low. I don’t normally as so much and cannot understand why it is staying so low

  6. If your PSI level is staying less than 8 above it’s baseline for weeks, should you just go ahead and backwash once a week or so regardless or is it ok to wait until it actually goes up the 8 psi?

    1. I think the article says that if the PSI rises by a quarter or a third of the baseline then you should backwash it. Unless you start seeing ill-effects of a dirty filter like low suction pressure then it should be ok to skip a week.

      1. I used a whole bottle of sparkle conquest sequestrant to try and clear some stains in my pool. Left filter on overnight. The conquest bottle says to backwash the filter this morning. I have a DE filter I backwashed it about 10 days ago do I need to add more DE powder after I backwash, do I definitely need to backwash? Also the FC has dropped to 0.5 so I’m assuming I need to add chlorine after I backwash. My pH is about 7.2 TS 80.

        1. If the bottle instructions say to backwash the filter, then that is a necessary part of the process. When you backwash a DE filter you are purging all the dirt but also all the DE powder in the filter. You should always recharge your filter with new DE after a backwash. To answer the question of how much DE do you add after a backwash refer to this guide: How Much DE to Add?

  7. I could use some insight if anyone has any. I bought something at Home Depot to raise the PH Level in my in ground salt water pool and not sure if it has anything to do with it or just bad timing, but I now seem to have sand in the bottom of the pool and very little pressure on the filter. I have backwashed and rinsed but the viewing ball is completely green and doesn’t seem to be getting any clearer..

    1. To get an idea what we are working with, we would your latest water test results. Algae like high pH, so if you add too much PH increaser, you inadvertently started an algae farm. The low suction and the green sight glass sounds like your filter is clogged with algae. You have to kill the algae, and clear the dead remnants before things get better.

      The sand in the bottom of the pool could be that you filter is so clogged that it can’t pull it into the filter. Or, something like a lateral or standpipe is broken in your filter.

      1. I’m having the same problem. Mine is a brand new system with brand new sand. After a few hours, my line is in the yellow which means I need to backwash to release pressure. My pool is crystal clear, I don’t understand why I have to do this everyday.

  8. Can you please help. I have just purchased a house which has a pool. I am a complete novice! I attempted a backwash and water spurted everywhere. The waste water has an inline valve which was frozen closed. I replaced the valve and this time water spurted out at the join between the filter box and the outlet pipe. I would have thought that the waste water would not be under pressure. So my question is , Why is the water under pressure?

    1. Your wastewater is going to pump out the same strength as if your valve was set to normal filtering. The only difference is that the valve is diverting to the waste port instead of the return line.

      By the way you describe this leak, there could be a bad bulkhead fitting or a crack in the filter casing. or you could use some teflon tape and a new oring to seal the joint. You may want to try posting this question in Inyo’s Forum so you can post pictures along with your summary of the issue.

  9. Hey Matthew, i am about to open my pool this weekend. The water is obviously dirty from the Winter season and i have the shock and other chemicals ready to go. My question is regarding when to do the backwash…Is that something i do first, and then shock the pool and add chemicals? Or do i shock the pool and get it cleared up before doing the backwash? Thanks for your help…

    1. Good question. I guess it couldn’t hurt to do a backwash before adding your spring cleaning chemicals. If only, to churn up compact sand and eject the super old crud that has been sitting in your filter.

    1. It would help to know what type of filter it is. But if the sand has been in there for a few years, then you probably need to change the sand. If it is a DE filter, you may be putting in too much DE, not backwashing thoroughly enough, or need to give old DE grids a chemical soak.

  10. Opening my first pool. 14000 gallon. I have a de filter and my water is very hard and rusty. How often should I expect to backwash my filter? It’s been running for about 3 hours now and my pressure is starting to climb. Is a lot of backwashing normal for a circumstance like this?

  11. My question to you is , how long is normal for the water to keep coming out of the drain, I had back flushed it yesterday evening and I still have back flush water coming out of the drain

    1. There should be no flow coming out of the backwash after you switch the setting to normal filter. The diverter and spider gasket should seal off any ports that wouldn’t be specifically used by the setting selected on the valve. i.e. backwash should be dry when filter mode is selected.

      It sounds like you have a leak, read this: Why Is My Multiport Valve Leaking?

  12. I have a question regarding the use of flocculent. The label says to run the vacuum with the waste or drain setting on, wouldn’t that deplete the water of the pool (and flood my yard in the process)? I’m fairly new to the pool owning process, I bought a house with a pool where it was open two years prior when we moved in. Last year the cover ripped, and I couldn’t get the pool clean the whole season. This past winter, I didn’t even bother closing it, and now I’m trying to open it, and I’m at a loss for how to get the pool clean. But I’m nervous as I’m reading that I putting everything I’m sucking up with my vacuum back into my pool. Any advice would be helpful!

    1. The Floc & Vac technique does wastewater and it is a drawback of using it but it works. When the floc is applied it should drop all the dirt to the bottom and that’s when you use the manual vacuum to scoop it up quick to send it to waste. The process of vacuuming up the debris should be quick enough that you are not draining your water below your skimmer.

      Or you can use a broom/sweep to push all the gunk towards the main drain. I’d suggest closing your skimmer if you do this technique.

  13. My inground pool is very full so I don’t think the skimmer is working well. Already added liquid chlorine. Okay to backwash or just let the pump run as is and backwash after 24 hours? Return valves working okay.

    1. If your water levels are high, that would not necessarily mean that an issue with your skimmer. If you have had excessive rain recently that will cause pools to have high water levels.

      If you need to drain a portion of your water, use the drain setting on the filter valve.

  14. Thank you for the very clear explanation.

    Except for this part:

    …if your normal baseline psi reading is 12 psi then when your gauge reads 15-16 psi (eight to ten psi above your filter’s normal operating pressure)

    15-16 is only 3 to 4 psi above 12, not 8 to 10. Am I misunderstanding you, or is this a typo?

  15. Hello

    I just opened my pool 16×32 and the water looks like iced tea with a red tint. The PSI when opened it was around 14 and now it fluctuates between 18-20 I backwash but it only makes the returns/ skimmers good for a couple of hours. I have a DE 2420 Heyward, also I add the required 3 lbs of DE but after a minute I think it is coming back thru the retuns but I can’t tell, usually a backwasing will emit DE and dirty water but last couple I haven’t seen any DE remnints. Do I need to clean inside? Do I have a tear in my fingers that DE sticks too? Any help would be great! Thanks

    1. The only way to know if there are tears in the grids is to open the filter. If your water is so dirty, that you may not be able to tell if DE is in the backwash, then I couldn’t give you a solid assessment as to what is happening to the DE. You can also read this guide: Getting DE Powder in Your Pool

      But if you pool water is that dirty, you have a lot of filtering and shocking to do. You are going to be backwashing often because your filter is going to get caked with that gunk pretty quickly.

  16. Is that why returns almost come to a stop? The filter needs to be backwashed and new DE added? I feel like I am backwashing constantly and adding shock to try to get the water clear?

    1. If your water is greenish, and you are battling it with chlorine, then your filter is probably dealing with loads of alive and dead algae. You are going to fighting that backwash battle until the green is gone. When the green is gone, and the pool is just cloudy, then you are out of the danger zone. The cloudiness is just dead algae you need to get rid of.

      1. If you are backwashing constantly and adding shock/chems a lot….could it be you need to do what some call a “split and clean”? Open the pump and clean out the filters/septums manually…even though not leaking DE into pool? OR could it be the chemicals aren’t right…use an ALL-IN-ONE algaecide in addition to the shock/chlorine? How do you know if it is one or the other approach?

        1. Doing a thorough clean of the equipment wouldn’t hurt. Also, using old or spoiled chemicals is going to make the algae killing process impossible. Buy some fresh chlorine, algaecide, and make sure the pH and alkalinity are balanced.

  17. My aunt has a sand filter and I just vacuumed her pool yesterday but forgot to backwash afterwards. The pool wasn’t terribly dirty just some leaves and dirt. I plan on doing the backwash when I go back next week. My question is will one week without backwashing the dirt out affect the filter’s performance or impact it’s longevity much? Thanks.

  18. My lever on my sandpro filter broke, (can’t buy the part anymore have to replace whole filter) it is in the filter position. For the time being, is there a way to backwash without putting in backwash position??

  19. I’m confused between the two terms: Backwash vs. Filter Clean. When should you backwash vs. when should you clean the filter, i.e. remove the grids and hose everything down (if DE add new DE).

    1. To backwash is to clean the filter. But the process of completing a deep clean of the filter by breaking down the individual parts, probably only needs doing once or twice a season; the start and/or the end of the pool season. Or if backwashing doesn’t lower the PSI, or if the PSI rises quicker than usual after a backwashing, a thorough deep cleaning of the filter is necessary

  20. Hello when closing the pool the pool safe antifreeze is in the lines. When opening the pool do I run it normally first, back wash or just turn it to waste to flush it out. I have a remote cover so the water will be clear just a couple leaves and such. What order should I run the water. Thank you

  21. When opening an above ground pool (first time here), should I have the sand filter on backwash when I turn the pump on for the first time, or make sure everything works before backwashing?

      1. HELP!!! 🙂 We have an 18x9x4′ oval above ground bestway pool. It came with a TINY cartridge filter, so I ordered a 1500GPH sand filter, and an intex saltwater system, as well as a solar panel (the hard kind, not the mat), but I was so overwhelmed after having set the pool up originally that I paid a pool guy to set the upgrades up…but I think he messed up….
        I know for a fact that he didn’t backwash the sand filter and now I’m worried about what I need to do because of that. He told me he had never worked on this kind of system. When I looked for help with this in my area (so Cal) people only did in ground but this is actually a big above ground pool. I couldn’t find any one and finally this person came out to help but didn’t really seem to understand anymore then I did.

  22. I have just opened my pool. Its an 18,000 gallon in ground pool with a hayward perfect model ec-65 DE filter. Ive noticed my pain is normally around 13-16. But has been creeping up to as much as 20-23psi. When this happens I have stopped the pump and “bumped” my system and the pressure will go back to normal. Then about an hour or two later it will start to creep up again. After bumping a few times and the psi still creeping I will then backwash. The pressure will go back to it normal operating range, but start to creep back up to the 20 psi range again in about an hour or two. Should I just bite the bullet and disassemble the filter and clean the finger tubes by spraying off any DE with a hose or should I go full bore and give the fingers and acid bath ?

    1. If you are opening the pool, are you dealing with any algae or cloudy water? if so, these pressure issues are likely caused by the filter getting clogged with that debris. If there is an algae issure then you will just have to keep bumping and backwashing until it is clear.

      If the water is clear, then your pump may be in need of a deep clean. If I was planned on going through with a full teardown of the filter, I would give the fingers an acid bath just to make sure the job is done right.

  23. I have a DE Hayward filter for in ground pool. Like last year, the pool has phosphates 🤬 so I poured 1/2 bottle in skimmer, then 24 hours later poured the rest. The problem is the returns slow down to practically nothing within a few hours so I have to keep backwashing a few times a day – I have been told it is ok to NOT add earth every time, maybe every other time add about 16 oz – is it normal with phosphates to backwash every few hours?

    1. A lot of phosphate removing agents are not meant to go through the filter media, and should only be circulated with your valve set to bypass, and then backwashed to waste. These types of chemicals clog the grids, which may explain why you are having water pressure issues after the fact.

      And yes, you should add DE after backwashing.

  24. My pool was just opened by a pool company and is still a bit cloudy. (I do normal seasonal maintenance but have them open and clear the pool at the beginning of the season). They are coming every few days to check, but I just noticed that my filter is at the clean filter mark (a bit above). Should I back wash now? I’m not sure what stage we’re at in clean up.

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