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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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.

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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