Swimming Pool Blog

Pool Filter Troubleshooting Guide

Pool Filter Troubleshooting Guide
by Rob Cox January 16, 2015

Pool Filter Problems - Solved!

So - your pool filter is having problems, and you are seeking answers. Sit down grasshopper, have a cup of tea, and free your mind from worry. Enlightenment is here.

Pool filter trouble can take many forms, and not all issues with filtration are filter problems. Clogged or closed pool skimmers, pump or valves can all cause pool filter problems. Check other equipment for anything that could be obstructing water flow - to and from the pool filter.

Here's a list of common pool filter problems, and suggested solutions that you can try.



When the pump is off, or the valve is set to recirculate or drain to waste, the filter pressure should be zero. If the filter pressure gauge reads zero or close to zero with the pump running and the valve in filter or backwash mode, something is wrong. Give the gauge face a flick - with your fingernail to see if it pops up. Inspect the gauge dial or face for warping, which can prevent the dial from moving.

If you are certain that water is moving through the filter at a normal rate, either the gauge has failed, or there is a clog within the air bleeder assembly that the gauge screws into. To check, shut the pump off and remove the pressure gauge with pliers or a small wrench. Run a stick or small screwdriver down into the hole to clear out any clogs from debris that can block the pressure gauge or air relief assembly. $8 pool filter gauges are cheap instruments that will fail in time; you may want to keep a spare gauge on hand.

If the water is not moving through the filter at a normal rate, there is a clog or obstruction, before the pool filter. It could be a clogged pump basket, clogged impeller, clogged pipe or clogged skimmer. It could also be a closed or broken valve before the pump. Or, the pump may have an air leak, and is drawing in more air than water, which will also reduce the filter pressure (and filtration).


If your pressure is higher than normal, this usually indicates a dirty pool filter. They say to backwash your filter (or clean your cartridges) when the pressure rises 8lbs above the clean, start-up pressure.

If your pressure is much higher than that, there could be a foreign substance in the filter (paper products, plastics, mulch, oils) that is really clogging it up. There could also be a closed return valve or bypass valve after the filter, causing the pressure to rise to very high levels. The filter and the filter valve (multiport or slide) could have broken internal parts which have lodged themselves inside of a pipe or fitting.

Many pool filters operate in the range of 8-15 lbs of pressure, although I have seen them range from 3 psi to 30 psi. If your filter pressure has jumped unexpectedly, to a higher than normal reading, there may be internal filter problems, or filter valve problems. Check the return side valves, or heater bypass to be sure they are open. Return-side pipes or valves after the filter don't normally clog with debris, but broken filter parts or valve parts could restrict the water flow after the filter; raising the filter pressure.

Very high filter pressure can be dangerous! Filter lids of clam shell type filters have been known to blow off when the clamp band fails under high pressure. Be sure that all pipes and valves are open after the filter, and if you see the pressure spike to 40 psi - shut off the filter immediately.


Most pool filters will run for several weeks before the pressure rises 5-8 lbs higher on the pressure gauge. Very large filters can take months, which is a nice feature for a pool owner. Some filters can have problems however, that allow pool water to bypass the filter and return to the pool unfiltered. This can cause a pool filter pressure to rise slowly, or take weeks to clean up adverse water conditions. In DE and sand filters, problems with the filter valve (multiport or slide), can cause water to bypass the filter. Internal problems (broken, missing or misaligned filter parts) inside a filter tank can cause water bypass on all pool filter types, and an oversized pool pump can push the dirt right through a filter. 


If your filter pressure rises rapidly after backwashing (within hours or days), the filter media (sand, cartridge or grids) may be clogged with oils or minerals, requiring deep cleaning with a pool filter cleaner product made for your filter type, or replacement of the filter media. DE filters will also see a rapid pressure increase when insufficient amounts of DE powder is used, or if the filter isn't opened up and the DE grids cleaned thoroughly at least once per year. During very poor water conditions, having to backwash daily or even more often can be considered normal - but with clean and clear water, a well sized and well operating pool filter should have a filter run of 3-6 weeks, ideally.


As mentioned above, clamshell type pool filters can be dangerous. These are a common type of filter tank design for cartridge and DE pool filters with two halves and a center clamp to hold them together. If the clamp band is leaking water, it is not making a safe seal.

Shut off the pump and drain the filter tank by opening the air bleeder. Remove the filter clamp band and the filter o-ring, aka belly band o-ring. Clean the o-ring with water and a rag to remove any dirt and debris, and also clean the rim of the bottom tank half, where the o-ring sits. Inspect the o-ring for damage. If the o-ring is no longer round, but triangular in cross section, or if it is cracked or has dry-rot, it should be replaced.

Lubricate the filter o-ring with Teflon pool lube, and replace the top half of the filter tank. Secure the clamp band around both tank halves and tighten down the nut all the way, until the springs are touching (new style clamps) or the two ends of the clamp are within 1/4" of each other on older types. Tapping the clamp band lightly with a heavy wrench or mallet around the tank while tightening improves the seal. 

In rare cases, a tank may become deformed or warped, preventing a good seal with the clamp band. In this case, contact your dealer or the filter manufacturer for possible warranty coverage.


After backwashing, it is normal for a small amount of dust to blow into the pool. Using the multiport valve Rinse setting for 20 seconds will fix that problem. For sand filters, a small amount of silty dirt may pass through the filter all the time, and be considered normal. Using a filter cleaner or replacing the sand can help, in addition to using a clarifier. Just be sure that it's not sand that is coming into the pool. Hit it with a brush, and if it poofs up into the water it is dust; if it just rolls around, it may be filter sand escaping through a broken filter lateral.

DE filters can lose DE powder into the pool (which looks like sandy dust) through small tears in the fabric, a loose filter grid assembly, cracks in the manifold or from a missing air bleeder screen or standpipe o-ring. Cartridge filters also need to be installed properly into the tank to keep water from bypassing the cartridge, or flowing around the cart. Or there could be holes in the fabric, from an oversized pool pump or from chemicals, or from aggressive cleaning (with a pressure washer).

Finally, the problem of dirt blowing back into the pool could be caused by the pool filter valve on sand or DE filters, bypassing some portion of the water back to the pool unfiltered.


This is a valve problem, and not a filter problem. In a multiport valve, the spider gasket and rotor make a seal together to prevent water from going to the backwash port when the setting is other than Backwash. If there is a trickle or stream of water constantly draining out of the backwash line, open up the multiport valve and inspect the gasket, which may be twisted, deformed or broken. If the valve handle is 'floppy', and has little tension, you may also need to replace the spring under the handle.

As a temporary measure, you could install a threaded plug or a valve on the backwash line to stop the water flow, but this should not be permanent solution for two reasons. One, someone will forget to open the valve during backwashing, which could be dangerous to the operator and damaging to the equipment. Secondly, if the valve gasket is not sealing up the backwash port, it may not be sealing up the return port, which allows for water to bypass the pool filter - and return to the pool unfiltered.


Why is my pool still cloudy? It could be many things, but let's start with your water balance and sanitation levels. As we have preached on this blog for many years, for clean and clear pool water, you need the trio of filtration, sanitation and circulation. And you also need good water balance (pH, Alkalinity and Calcium).

If your sanitation, circulation and water balance are spot on - it's time to look closely at the pool filter. Is it running long enough each day? Are the pump and filter sized properly for the pool and for each other? Is there a chance that water is bypassing the filter sand, grids or cartridges, and returning to the pool unfiltered? 

If your pool filter is a bit undersized, or the media is tired, try using a clarifier to help coagulate suspended particles. You can also use a flocculant, to settle suspended particles to the floor for vacuuming (to waste). In many cases where it takes weeks to clean-up poor water conditions, replacing the filter media (sand, cartridge or DE powder) can help tremendously.


Trick question? Normally the pump is what makes the noise, and while in operation a pool filter is actually quiet. Loud pool pumps are normally caused by cavitation (starved for water) or by bad internal motor bearings. A pool pump with an air leak can put lots of air into the filter, which can be a little noisy as the airy water is pushed through the pool filter. If you have a constant stream of air bubbles in the pool, or in the see through pump lid, look for and patch any suction leaks (before the pump), with sealant.


Another trick question! The pool filter is not electric, only the pump turns on. If the pump won't turn on, it could be whole range of things which are beyond the scope of this article. Look for this answer in our upcoming blog post Pool Pump Troubleshooting Guide.


- Rob