That is a great question that I hear frequently. The thing is, it’s not so much about frequency as functionality. How often do you wash your dishes? You wash them when they are dirty of course, it’s not on a daily or weekly or monthly basis. You wash them when they need it. A swimming pool filter is the same way.
Every pool filter is sized differently based on the design of the pool.
A filter becomes dirty because of a high bather load, weather, foliage around the pool and other variables. Your sand filter has a base layer of pea gravel and is filled approximately two thirds of the way up with a filtering media which could be silica sand, zeolite, or recycled glass. As water passes through the filter a layer of sediment builds up on the top of the sand bed. As sediment builds up your filter, becomes more effective because the sediment is finer than the filter media itself and after a while it begins to filter even finer particles out.
But that layer of sediment will begin to thicken and after a while will actually prohibit water flow through the filter. That is when it is time to backwash. If the filter slows the flow of the water because it is dirty it also slows the turnover rate of your pool. Turnover rate is how many times the entire quantity of water cycles through your filter per day. Low turnover means less of your water is being filtered, less of your water is passing through your chlorinator, thus less of your pool water is being treated. This can lead to cloudy water and unbalanced chemicals.
Now we know how the filter works so let’s put this knowledge into action.
The first step to knowing when to backwash your filter is to figure out what your starting pressure, or clean filter pressure is. Backwash your filter and run it through a rinse cycle. Once you have done that put your filter back to the filter setting which is your normal operational mode. Check that pressure gauge at the top of the filter.
“The first step to knowing when to backwash your filter is to figure out what your starting pressure, or clean filter pressure is.”
That reading currently is your clean filter PSI. I would slap a piece of duct tape on the side of your filter and write that number down so you don’t forget. Now as your pool water passes through the filter, sediment will begin to build. As that sediment builds you will notice the filter pressure start to rise because that water is having to work harder to pass through the filter. Once that pressure reaches 6-8 psi higher than your clean filter pressure it is time for a backwash, but not until then.
Backwashing too frequently can inhibit the effectiveness of your filter thus causing cloudy, murky, and less than clear water, because you are removing that bed of fine sediment that was increasing the effectiveness of your filter. But if you don’t backwash enough, it can cause a loss of flow also leading to cloudy, murky, and less than clear water because you are not moving enough water through your system.
As the sand in your filter ages, the pressure in your filter may increase regardless how many times or how long you backwash. When it gets to the point that you are unable to keep your water as clear as a solid cube of sunshine it may be time to change out that filter media all together.