The Pentair FNS Plus pool filter is a diatomaceous earth (D.E.) filter which removes impurities from pool water down to as small as 5 microns. Regular filter cleaning is important to keep the filter running efficiently and the pool water sparkling clear. With the right technique, the normal cleaning operation, called backwashing, is a quick and easy process.
Clean the filter when the pool water begins to turn cloudy or when the filter pressure gauge read about double its normal clean running pressure. For example, if the pressure is 12 pounds when the filter is clean, it's time to backwash when the pressure reaches 24 pounds.
Understand the principle of the D.E. filter. The filter has eight curved grids covered with a tightly woven Dacron material. The D.E. is introduced through the skimmer and forms a thin coat over the entire surface of the grids during normal operation. The pump forces the water into the filter through the D.E. and the filter grids. The filtered water flows out through the center of the grids into a collection tube and out of the filter.
Backwash the filter to reverse this process to force the water in through the center of the grids and out through the material, washing the D.E. off the grids to clean them. The D.E., debris and dirty water flows out the backwash valve and into the P-trap drain.
Turn the pump off. Set the filter valve to backwash, and open the air release valve. You will hear the air being sucked into the filter as the water drains out of the filter tank. Wait until the tank is empty and the sucking sound stops.
Turn the pump back on. You will hear the air rushing back out of the air release valve as the water fills the tank again. Close the air release valve when water begins to flow out of it. If the backwash line has a clear sight glass, continue to run the pump until the water is clear. If there is no sight glass, watch the outflow line from the backwash to the P-trap to check for clear water. This process takes only a couple of minutes.
Turn the pump off when the water begins to run clear. Open the air release valve again and drain the tank once more. Repeat the process of turning the pump on, bleeding the air out and running the backwash cycle until the water runs clear again. After the second backwash cycle, turn the pump off, turn the filter valve back to filter and restart the pump. Bleed the air out of the tank one more time.
Keep the pump running on the filter cycle to re-coat the grids with fresh D.E. The formula is one pound of D.E. by weight for every 10 square feet of filter area. If you have the Pentair FNS 60, for example, you have 60 square feet of filter area. You need 6 pounds of D.E. to coat the grids. This translates to 12 1-pound coffee cans of D.E. since a 1-pound coffee can holds 1/2 pound of D.E. by weight.
Fill the five gallon bucket about 2/3 full of water. Add four coffee cans of D.E. to form a slurry. Make sure the skimmer valve is all the way on. Remove the deck skimmer lid and pour the slurry into the skimmer. When the water in the skimmer clears, repeat the process until you've added all six cans of D.E.. Return to the filter and bleed out any air remaining in the filter to be sure the D.E. coats the grids all the way to the top.