How to Clean a Pool Sand Filter

Sand filters are one of three options for filtering your swimming pool water. The other two are diatomaceous earth filters and cartridge filters.  Sand filters are compact and tend to cost less money to maintain and operate.  Sand filters utilize pool filter sand which is #20 Silica sand.  This grade of sand is 44-55 millimeters.  At first, as the filter gets dirtier, it can actually work a little better.  But as the debris starts to accumulate, the pressure on your filter goes up.  You can check your filter’s pressure by looking at the reading on the gauge on the very top of the filter.  When we first install a filter, or immediately after a clean, this is known as your “clean starting pressure”.  You’ll want to mark this.  A lot of gauge dials have a spinning outer rim that allow you to set one arrow to “start”.  When the gauge gets to the “clean” arrow, which is unusually 8-10 psi (pounds per square inch) above your clean starting pressure, it is time to either backwash, or do a full filter clean.

Your Sand Filter Valve

Typically, your sand filter will have a valve with several ( approximately six) different outlets, or ports.  These are called a “multiport valve.”  There will be different outcomes from selecting each port.  One setting is “Filter”.  This is what you will have it set to when it is filtering the water like normal or if you were going to be vacuuming the swimming pool. “Rinse” is utilized to clean the water inside of your filter tank.  You’ll want to use this if you are adding or replacing sand.

“Closed” is used when you want to completely shut off the water flow to the swimming pool and filter.  If you want to bypass the filter tank and have the water expelled without it returning to the pool, you would use the setting “Waste”.  For example, if you wanted to lower the swimming pool water.

“Circulate” would also bypass the filter, but it would be returned to the swimming pool. “Winter” is only used if you are closing the pool, typically in areas where water can freeze.  This is part of winterizing a swimming pool.  Finally, “Backwash”.  This is the setting we will be using when our filter pressure is high, but it is not quite time to replace all the sand.  Backwashing works by pushing the water reversely through the filter.  It passes through the sand and then will drain out your waste pipe.

When to Backwash a Sand Filter

Ok, so you have checked your gauge and the pressure on your sand filter is “high”.  This means it is time to backwash your filter.  Backwashing removes any algae, dirt & debris and human by products from your filter.  It basically “makes room” for the filter to catch new junk.

In season, it is recommended to do this once a week.  You would also want to do it after an algae bloom.  If your swimming pool water is cloudy, it also may be time for a backwash.

How to Backwash a Sand Filter

It is very important to make sure your pool pump is off before attempting to move the valve to backwash.  You’ll want to move your handle on your multiport valve into the “backwash” position.  Make sure it is securely in place.   Now you turn your pump back on and allow your sand filter to backwash.  Usually you’ll allow this for 3-5 minutes.  You’ll know when to stop because the water in your sight glass will become visibly clear.  Your sight glass is located on the top of your filter in most cases.  You lose water and chemicals when backwashing so keep an eye on it.  You can lose up to 130 gallons of water during a backwash.

Once the water is clear, turn off your pump again.  We do this because we will be moving our multiport valve to the “rinse” outlet. Once the valve is locked into place, turn the pump back on and rinse until the water is clear.  This usually only takes a minute or two.  Since we now need to move the valve setting again, you guessed it, turn off your pool pump.  Then move the valve into the “filter” setting.  This sets up the filter to resume normal operation.

Changing the Sand in Your Sand Filter

Because our media is sand it uses the rough edges to catch all the dirt and debris from the swimming pool.  Over time, these rough edges can tend to smooth out, rendering your filter less effective.  An averagely used sand filter will need a complete clean usually every 2-3 years. More or less depending on usage.  Not only are the edges getting smoother as time goes on, but the sand will start to “ball up”, leaving empty channels. 

Let’s start with the valve. You will have to remove the multiport valve from atop of your filter.  Hopefully it was installed with unions.  Otherwise, you will need to cut it out and replumb.  Next, make sure you have any needed supplies such as #20 silica sand, the tank o-ring, teflon tape and pool lubricant.  If you had to cut out your valve, you will also need unions.  Check the manufacturer guidelines for your specific filter to see how much sand you will need.  #20 Silica sand usually comes in 50 pound bags.

Make sure the pool pump is off.  You may even want to flip the breaker to make sure no one else turns on the pump.  We need to release the filter tank’s pressure by turning the drain cap at the bottom of the filter.  Remove the cap completely.

You’ll need to disconnect the “waste hose” from the waste line on your valve. If you haven’t already, remove your valve by unscrewing the unions.  These unions are attached to the pump and return ports.  If you had to cut it out, make sure you leave enough room and stagger the cuts.  We don’t want to end up with a union on top of the filter.

Now you will want to remove the tank flange.  This holds the filter onto the tank.  Simply screw off the bolts.  This will separate the clamps.  Don’t lose the vinyl protector caps on the screws!

Gently lift the valve.  If you pull too hard you may damage the laterals at the bottom.

Put some duct tape over the top of the exposed pipe.  This will prevent any sand from getting down there.  Now we are ready to start scooping out the sand.  Not a fun task.  Use a cup or can.  If this is too arduous, you can use a wet/dry shop vacuum.

Carefully pull the pipe and the assembly of laterals out of the tank.  Before you remove it completely, rotate the laterals up the side of the pipe and out.  Lift the pipe/lateral assembly out of the tank.  You’ll want to take this opportunity to check your laterals to see if any of them are broken.  Replace the broken ones.  Sometimes the laterals get clogged, so a good soak in a bucket of water with a mild clean solution should do the trick.

You can use your everyday garden hose to rinse out the tank.  Replace the drain cap at this point.  You will want to fill your sand filter tank with water, forming a cushion for the laterals as you add the new sand in.  About halfway full is a good level.

Now, we do everything in reverse.  Put the pipe/lateral assembly back into the filter.  Make sure the laterals are rotated in the “up” position. Then rotate them 90 degrees into position. You’ll also want to make sure the pipe is centered. Keep the pipe centered when you pour in the sand.  Pour in the sand slowly until you cover the laterals.  You’ll want about 6” of space between where the sand ends and the top of the filter begins.  Level out the sand.  Don’t lift the lateral assembly or you will get sand underneath and then not be able to push it back down.

Replace the o-ring on the bottom of the multiport valve.  We don’t change our sand very often so this is a good idea to do every time.  Make sure you lubricate the o-ring!  Now you can take the tape off the pipe and slip the control valve onto the pipe.  Reinstall the clamp and tighten using a screwdriver.  Use a rubber mallet to “beat” out any slack around the clamp as you are tightening it.  Don’t forget to put those vinyl protector caps back into place!  Replace the unions to the pump and return ports.

Turn your valve to “backwash”.  You’ll want to ensure your waste line is rolled out to an ok place to dump the water. Turn on the pool pump.  You may need to prime it.  Run the pump for a couple of minutes to flush out fine sand particles.  Now turn the pump back off and turn the handle to “rinse”. Run the pump again until the sight glass is clear.  Just like you would during a backwash.  Turn the pump off again so you can turn the valve to “filter”.  Once securely in place, turn on your pump to resume its normal filtration mode.  Reset your gauge and viola!  You now have a clean sand filter.

Seems like a lot of work?  It is.  If you have never done it before or want to make sure it's done right, you may want to hire a pool professional, at least the first time, to see how it is done.  See you poolside.