When Should I Replace My Pool Filter Grids, Cartridges or Sand?

One thing that is often overlooked when your pool has water quality issues is the filtration system. Whether it is cloudy pool water, algae forming every week or a combination of both, we often treat the symptom of it and not the cause. The pool’s filtration system is an essential part of the equation and the age of the filter elements – grids, cartridges, and sand is sometimes the deciding factor.

When to change your filter grids. Cartridges or sand can seem like a lot of guesswork. There is no set-in-stone time frame for each so you will have to do a little detective work to find the correct time. Obviously, if your D.E. Filter is spitting out D.E. through the return lines it is time to inspect and probably change the girds. Even a tiny hole in the grids can shoot back a large amount of dirt, D.E., and debris. Typically D.E. filter grids are good for 4-5 years, although if your filter gets very dirty and is under constant high pressure (high PSI on the gauge), then changing them after 2-3 years would not be a bad idea.

Cartridge filters are a bit tougher to diagnose and you can’t always rely on the filter pressure (PSI). Since the larger 4 cartridge type filters have such a large filtration area the PSI tends to stay at the clean range for months at a time. If you clean your 4 cartridge type filter and the PSI is sitting at 20 PSI, don’t be surprised 5 months later even with very dirty cartridges that the gauge is still reading 20 PSI. For the single cartridge filters or single bullet filters, as we call them, the PSI will rise about 10 PSI indicating when the cartridge is dirty.

Typically a 4 cartridge filter will go about 3 years before you need to replace the cartridges. The only exception to this rule is the Sta-Rite System 3 Cartridge filter where the large and small cartridge can go 5 years or more between replacement. The single-cartridge filter should be replaced every year and sometimes sooner depending if it is on a very large heavy use pool. The way I gauge a cartridge filter and when it needs new cartridges is by the flow and water quality of the pool in question. If the flow is poor after cleaning the cartridge(s) it is probably time to change them. Again, usage and the size of the pool are a factor and use the pool’s return flow and water quality as your indicator – cloudy water or algae, not holding chlorine are some common signs.

For a Sand filter knowing when to change the sand can be even more difficult. Typically you are using the water quality as your gauge and also the age of the filter and sand. Of course, if you purchased a house with an existing sand filter you may not know either so that is not always a helpful indicator. If you backwash your sand filter and notice the PSI on the pressure gauge jumps up by 10 PSI after a few days, this may indicate the sand is no longer working. It could also be the water quality itself causing the PSI to rise rapidly so you can also do a test using Floc.

If the pool water is very cloudy adding a Floc agent is a good way to clear I up and test to see if you need new sand. You would add the Floc agent and run the filter on recirculate for 1 hour. Then turn the pool pump off for 24 to 48 hours. The Floc agent will drop everything down to the bottom of the pool and you can then vacuum to waste. If the pool water stays clear for a few days and the chlorine is holding steady, then your sand is probably still good. When the water quality turns bad in a sand filter it can affect the filtration dramatically. That is why the Floc test is a good way to check to see if the sand is good or bad. If you Floc the pool and then the water clouds up quickly again the filter is probably not filtering effectively and will need a sand change.

One major way to assess if the filter elements need to be changed is by the water quality and the return flow of the pool. If you notice the pool is not holding chlorine and the water is cloudy I would suggest inspecting and replacing the filter elements to see if that corrects the problem.

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