Pool Filter Not Backwashing? (The Best Advice You Need!)

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Why Is My Pool Filter Not Backwashing?

It can happen to the best of us. The pool sand filter works perfectly fine when the multiport valve is set on the Filter mode. But it refuses to backwash. There is no pressure build up. So why is your pool filter not backwashing?

Clogged sand media or very old sand are the most common reasons for a pool filter not backwashing. But even experienced pool owners can sometimes make a connection error that can result in the pool filter not backwashing.

The mistake is so simple and so silly that you are sure to laugh about it later. But you will be at your wit’s end till you figure it out. You may even end up wasting some money!

Are you sure you connected each port of the multiport valve of your pool filter correctly? Double check to be absolutely sure. 

There are innumerable instances where pool owners, with years of experience, have connected the pump to the RETURN port of the multiport valve and the return line to the INLET (PUMP) port.

So before you start looking at other solutions to why your pool filter is not backwashing, please check and double check the port connections.

Can a Small Pool Pump Result in Pool Filter Not Backwashing?

A pool pump that is too small for a pool sand filter can result in inefficient backwashing. Overtime you may not be able to backwash at all!

The horsepower of the pool pump required is determined by the following factors:

• Pool Volume (If you do not have it already you can calculate using the formula in my earlier post How To Know The Right Amount Of Algaecide For Your Pool? or use an online pool volume calculator at https://goodcalculators.com/pool-volume-calculator/)
• Desired Cycles per Day (2 times per day is the industry standard)
• Plumbing Head (Usually between 40-50 feet)

The pool filter size is chosen to match the pool filter. Generally speaking, you do not want the pool filter to be too small as that puts unnecessary pressure on the pool pump.

The recommendation is to use a pool filter which is somewhat oversized. For example for a pool pump with a flow rate of 42 gallons per minute (GPM) a 60 GPM pool filter is a good fit.

But if the pool filter is much larger, then the pump may not have enough pressure to push through the dirty sand media, in the backwash mode. 

A pool pump that is too small for the pool filter may lead to your pool filter not backwashing.

How Do I Know If My Sand Filter Is Clogged?

Your pool sand filter traps dirt & debris while it is in the Filter mode. Most of this is collected in the top six inches of the sand media. The bigger debris sits on top of the sand. Minerals go through the sand media and deposit in the bottom layer.

Backwashing of the sand filter is done to clean up the sand media and expel the trapped dirt & debris to waste. In theory the sand media should be as good as new after a backwash. 

In practice, backwashing will not clear the sand media 100%. The mineral deposits and some microscopic solid particles continue to cling. Sand can also form lumps, partly thanks to algae strands and partly due to oils that enter the pool through the swimmers.

These lumps are hard to break in backwashing. Over time the sand will begin to form clumps and start clogging. The pressure from the pool pump is unable to break through the clogged sand during the backwash cycle. 

Ultimately, your pool filter will not be backwashing anymore!

How Do I Unclog My Sand Filter?

Prevention is better than cure!

You are ultimately going to end up with a clogged pool filter. But you can delay that from happening.

Backwash based on the Pressure Reading

Backwashing a pool sand filter too often is counter productive. Actually, a sand filter is more effective after it has trapped some amount of dirt & debris. The pores in the sand become smaller and are able to trap solid particles more effectively.

Backwash your pool sand filter when the pressure reading is around 10 psi higher than the normal psi.

Backwashing will keep the filter clean and prevent clogging for a longer duration.

Use a Sand Filter Cleaner

Use a Sand Filter Cleaner, once in a while, to remove oils, minerals and metals that promote clumping and clogging of the sand. 

Basically the Cleaner, such as the In The Swim Liquid Sand Pool Filter Cleaner, is a chemical that will break down the bonds that oils, minerals and metals form with the sand particles.

Add the Cleaner through the skimmer, with the filter valve on the backwash position. After the cleaner is sucked into the filter, shut off the pump and allow the chemical to work overnight. 

Start it up again the next morning on rinse, to flush out dirt, oils and minerals. Use annually to rejuvenate your filter sand, and keep your filter operating at full effect!

Replace the Top Sand Layer

Most of the debris collection and therefore the clumping takes place in the top six inch layer of the sand media. They call it the “glunk” or “mud balls”.

Once in a pool season you should open the top cover and physically remove all the “mud balls”. Then refill the sand filter with the same quantity of fresh sand.

Follow the above and you may not need to replace all the sand for 7-10 years.

Recommended Pool Sand Filters

Best Sand Media

Aqua Quartz pool filter sand does not stain and ensures easy maintenance at a lower cost by filtering out dust, algae, suntan lotion, oil, leaves and insects. This ecologically safe sand does not solidify and will help prevent clogging and channeling. It is inert, odorless, chemical free, 100% natural sand.

FairmountSantrol AquaQuartz-50 Pool Filter 20-Grade Silica Sand 50 Pounds, White

Best Glass Media

HARSCO METALS & MINERALS Filter Glass is made from 100% recycled materials. The glass filter media cuts down on backwash time as well as loss of pool water and chemicals. Improves water clarity by 25% over sand. 

Filters down to 5 microns, unlike sand only 30 microns. Use 20% less media (by weight) than with sand. Environmentally sound and last 3 times longer than sand.

HARSCO METALS & MINERALS Filter Glass

Best Sand Filter

The Hayward Pro Series Sand Filter for in-ground swimming pools will provide you with hassle-free, effective filtration. The weather-proof tank is constructed of a tough, color-fast polymeric material.

Even water distribution over every square inch of sand is ensured by an integral top diffuser, self-cleaning under drain and 360 degree slotted laterals. Advanced full-flow technology reduces energy costs by achieving excellent filtration, so you can run your filter less.

Thorough backwashing allows for crystal-clear water. Filter is complete with tank, 7-way multi-port valve, sight glass and pressure gauge. 7-way Valve Positions: Filter, Waste, Winter, Closed, Backwash, Recirculation, Rinse.

Hayward Pro Series Pool Sand Filter

Thank you very much for reading the post. I do hope you found it informative and helpful.