Backwashing A Sand Filter Too Much?

(Neenah, WI)

I care for a 37,000 gal pool with a sand filter. Based on what I just read I'm backwashing quite a bit more than every 2 - 3 weeks. In a 2 week period I've backwashed 4 times. I was told that I needed to backwash whenever the GPM went below 100.

Typically the GPM ranges from 150 - 200 & then suddenly drops to 50 or less. The PSI typically reads at 15.

Above you stated:

"A swimming pool sand filter needs back-washing after the pressure has increased about 8-10lbs. higher than the "just back-washed" pressure."

In order to know the just back-washed pressure I'd have to keep track of prior readings. Just to make sure I understand... that would be based off the PSI, not the GPM. Correct?
Basically over the past 2 weeks the PSI only dropped to a 10 once. It's always been 15.

So was I told incorrectly to backwash based on the GPM? What causes the GPM to be low? The water level? Dirty pump filter?

Very good question

First we need to establish the difference between GPM and PSI. GPM is "gallons per minute". You need an actual gauge for this because it cannot be determined by any filter reading.

I checked the GPM at the YMCA pool using a gauge. The gauge fits into a small hole that is drilled into the plumbing. When the water flows by it pushes up on a little metal cap within a chamber inside the gauge. If you've ever seen a ping pong ball float in a clear plastic tube, that's what it's like.

PSI is "pounds per square inch". This is the reading you get from the gauge on top of the sand filter. It's this gauge that you look at when considering doing a back wash.

If you were told to backwash when the GPM decreases, that could be partially true, but again, you need a GPM gauge and the most accurate reading will come from a functional gauge on the sand filter itself.

A low GPM could have multiple issue, just like a decrease in the PSI of a sand filter. You should keep track of when you backwash and your filter readings. You can so something as simple as putting a piece of tape on the filter with the date and last filter pressure reading you have.

A sand filter needs a little bit of pressure to filter properly so back washing too often is actually a bad thing to do. With normal maintenance, use, and dirt level, you shouldn't need to back wash any more than once per month.

A low water level and dirty filter will cause the GPM to decrease. You want to keep the water level up between 1/3 - 1/2 from the bottom of the skimmer. Write down your "just back washed " pressure and back wash again when the pressure get 8 - 10 PSI over that pressure. So, if your just back washed pressure is 8 PSI, back when the pressure gets to 18 - 20 PSI.

Hope this answered your questions and good luck with your pool. You can find your post on the Q&A page in the "Swimming Pool Filter Backwashing" category.

Swimming Pool Questions and Answers


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Backwashing And Started Spitting Out The Sand

by Christy B

We just got our pool. He dumped some algaecide into pool and said to backwash.

We accidentally turned it to WASTE and we turned pump on.

We caught what we did..shut it off then turned to BACKWASH.

It started spitting out the sand.

Do you know why?

Thanks for the question Christy

There could be many reasons why sand get returned to the pool.

Here are some good posts to look over:

Replaced Lateral But Still Sand In Pool

Sand In Pool After Changing The Filter

Why Is Sand Going Back Into The Pool After Backwashing?

A simple reason is that while the filter was on WASTE the sand could have gotten into the pipes.

When you turned it to BACKWASH the sand went back into the pool through the returns.

I would say if this was a one shot deal and hasn't happened again, just vacuum the sand up and don't worry about it.

But, if it keeps happening on FILTER or it happens after you back wash, then the above posts will help.

The short of it is this:

1. Broken lateral and/or diffuser

2. Channeling

3. New sand

4. Old sand

5. Not doing the RINSE after backwashing

6. Not checking the "freeboard". This is the measurement in inches from the top of the sand to the top of the filter.

You did the right thing in turning off the system before making any valve adjustments.

You always want to turn the system off before you do anything with the valves.

A broken lateral means that the dirt is not being captured in the sand. The dirt returns back to the pool through the returns.

For a broken lateral you need to scoop out the sand, replace the lateral, then replace the sand.

For a reply, you can see your post on the Q&A page in the "Sand Filter" category.

Swimming Pool Questions and Answers

Hope this helps and have a fun and safe swimming season.


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Losing Sand When Backwashing Sand Filter

I have followed all the directions for backwashing my sand filter.

Problem is when I backwash the filter I loose over half of the sand. Is that normal?

Also, when I hook up my vacuum I only have suction for about 5 minutes and then loose most of the suction, even if there is no debris going through the filter, only water.

Thanks for the question

First I would direct your attention to these very similar posts:

Sand Filter Pressure Increases And Low Suction

Backwashing And Started Spitting Out The Sand

Replaced Lateral But Still Sand In Pool

Check this post for spider gasket:

Water Coming Out Backwash Hose During Filtration

About low skimmer suction:

How Do I Fix Low Skimmer Suction?

As you can see there are many other pool owners with sand filters that have the same issues as you.

The above posts are the answers to their questions.

If, after reading the above posts and using the troubleshooting tips you still have questions, please feel free to contact me again.

To post a reply, or if you have a similar question, you can see your post on the Q&A page in the "Sand Filter" category.

Swimming Pool Questions and Answers

Due to space and similarities, I will merge this question with Backwashing And Started Spitting Out The Sand on July 12, 2011.

You will find your post with that question after that date.

Check back to this post for answers.

Hope this helps and have a great Summer.


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Sand In Pool After Changing The Filter

by Mrs. Chambes
(Lakewood, NJ)

I have a 16 X 32 inground pool that is getting sand in the bottom when the pump is running. Our pool person told us because the filter is so old (13 years) it would be better to replace the filter rather than changing out the laterals. We replaced the filter on 6/15/11.

Cleaned the sand out the following day & today (6/17/11) I noticed sand again on the floor. Could there be a different problem?

Thank you for your question

If you have a new filter and new sand, it will take some time for the sand to settle down.

What you're seeing is the residual dirt and debris that is in all new pool sand. Sand going back into the pool with new sand is unavoidable but will decrease in time. A sand filter need a little bit of pressure to work properly.

The best thing to do is to try to vacuum to WASTE to get rid of the excess debris. Keep an eye on the water level because you're going to lose some water. 1/3 - 1/2 up from the bottom of the skimmer is best.

Here are some posts that will help you:

Change The Sand In Above Ground Pool Sand Filters

Why Is Sand Going Back Into The Pool After Backwashing?

Sand Returning Back To Pool After Replacing With New Sand

Another thing that might contribute to sand going back into the pool is not doing a RINSE after back washing. Check out the above links.

You shouldn't have an issue with the sand after a few day.

Have a fun and safe Summer.


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