It is important to know if your swimming pool filter is working. Without knowing you could spend several days trying to clear your pool water and make it look good only to realize the dirt is simply re-circulating to the pool. You could also waste a lot of time trying to clean your pool and even spend needless money on chemicals or clarifier in an effort to get your water back on track.
I have two suggested methods for determining if your filter is working without doing a full disassemble and inspection on the filter and backwash valve.
Note: These methods will show if dirty water is returning. These methods assume if you have a pressure based sweep that uses unfiltered water (such as a Polaris 360) that this line is valved off or closed.
Also Note: If you have a backwash valve it is possible that the valve seal or diverter could be failing which is allowing dirty water to bypass the filter and re-circulate. However it is far more common for the filter to have an internal problem than the backwash valve itself fail… and either way these methods will show you if you have a filter / filter backwash valve problem.
Method #1: If you are unsure if your filter is working you can try adding some diatomaceous earth (DE) to a DE or sand filter to test it. I would suggest just a scoop or two (1lb coffee cup size) into the skimmer. Immediately after adding it go look at the pool jets or returns and see if you can visually detect cloudy water coming back in. If you can then you most likely have a filter issue and will need to address that.
Note: Do not add DE to a cartridge filter. You can add some cellulose fiber based filtration powder though if you want to try and see if anything comes through. DE can pack into the cartridge folds and cause excessive strain on the fabric elements.
Method #2: Another way to test your filter is to connect up a vacuum and hose to a skimmer suction port. You can then see if the jets or returns show any cloudy water coming back through. Of course this method assumes you have dirt on the floor of your pool that you can vacuum through the system to perform the test.
What if my pool filter shows dirt coming back through?
If either of the two methods above shows dirt or cloudy water coming back in then I would recommend a filter cleaning and backwash valve inspection. The filter cleaning should be a full disassemble and inspection of each internal part to determine where the damage is.
If you have a sand filter blowing dirt back in then I would check and see if the sand is more than 3-4 years old. If so you might be ready for a sand change. Over time the water going through the sand particles causes them to “round out” and lose their sharp edges. The rounding out makes the sand more like small marbles or round stones in a stream or natural water bed.
Replacing the sand in a filter typically costs several hundred dollars and can put you at 50% of the cost of a new filter. So if you are a residential pool owner or don’t need your filter running 24hrs a day (which sand filters are the only type that can both do this and backwash) then I would suggest considering a new filter at this time. A cartridge filter can do the job in ½ the time and a DE filter might do the job in ¼ the time it takes a sand filter to clear up a pool. This means you can run your pump less and stop giving so much money to the electrical company and have much better filtration.