After a period of time, the contaminates in a sand filter start to clog the sand to the point where water flow is significantly diminished and the pressure gauge rises 8 to 10 psi above normal operation readings. To clean out the contaminates, you have to backwash the sand. This guide shows you the steps in backwashing a filter.
Make sure your WASTE hose is laid out to where you want to dump the waste water.
Turn the power back OFF.
Turn the pump back ON and run filter at the RINSE setting for 1 to 2 minutes.
Turn the pool pump ON for normal filter operation. Check your pressure gauge to ensure that the pressure has dropped 8 - 10 psi from where it was before backwashing the pool filter.