How to Backwash a Sand Filter

Backwashing your swimming pool filter is one of the easiest ways to keep your sand filter running efficiently and smoothly through the summer months and should be done the right way once a week on your maintenance schedule. How to Backwash a Sand Filter?

  • Turn the filter system’s power off
  • Attach backwash hose
  • Set to Backwash position on the multiport valve
  • Turn the filter on & backwash for 2-3 min or until the sight glass clears.
  • Turn the filter off again
  • Set it to the Rinse
  • Turn pool filter on & rinse for 30 sec.
  • Turn pool pump off-Set to Filter position


Each filter and pump system has its own normal operating pressure psi which you should be able to find out from your manufacturer’s guide or an internet search for your particular model but the best way is to get readings when you first start a new filter and pump.


How to Backwash a Sand Filter


One of the most important maintenance jobs to perform on a swimming pool is backwashing your pool’s sand filter. The sand filter is the heart and blood of your pool’s health and if it’s operating inefficiently then your swimming pool will lose a critical part of treatment. The first treatment is filtering the second is chemical treatment. Both have to work to achieve a clear and clean pool.

As a general rule of thumb, you should backwash and rinse your filter about once a week. The optimal time is right after you vacuum the pool. However, if your pool has had a lot more use than normal, it may be necessary to backwash it twice a week.

You can also tell when it is time for a backwash by checking the sand filter system’s pressure gauge and the sight glass on the sand filter. Come on the hot summer days when the pool is being used more and more, and you may have to schedule backwashing according to the usage. I’ve found there is no real way of scheduling backwashing.

If you backwash your pool pump for too long, it runs the risk of introducing air into the system. Air pockets can cause pumps to lose the constant flow of water that they need to keep working. If this happens to your system after a backwash, you may need to prime your swimming pool pump. If the problem persists, then there may be a more significant problem, in which case you should get in contact with an expert supplier who can help solve your issue.

If you backwash too long, you can also lose sand from the filter. If you know your filtration system and you keep it chemically balanced you can keep your pool filter backwashing to a minimum off maybe once a week, according to filter system pressure and pool water clarity.


Pool Sand Filter Pressure


The one part of your main filter sand system that really tells you what’s going on and when you need to clean the filter is the pressure gauge. A pressure gauge is typically a round gauge that looks like the manual speedometer on many cars. Most gauges range from 0-60 PSI. A small needle will point to a number in that range, and that is your filter pressure in PSI (pounds per square inch).

Every filter has its own ideal working pressure. Typically that will be between 10-20 PSI, but not always. To find the normal functioning pressure of your filter, you will have to test it. Clean and backwash your filter, then run it a bit so it is primed. When the pressure holds steady, write down the PSI. That is your ideal working PSI.

As the filter gets dirty, the pressure may rise. Other issues can also cause a spike in pressure. As long as the PSI is no more than 10 PSI above the ideal, you don’t have to do much. But when the PSI reaches 10 or more PSI over your ideal, it’s time to clean the filter. Pull out the cartridge to clean it, or backwash a DE or sand filter. If the pressure goes back to the ideal, you’re done.

Low pressure in your sand filter is much less dangerous than high pressure, but it means your pool filter isn’t doing its job. Low pressure will almost always be caused by a problem at or before the pool filter pump. That’s because after the pump, the water is under pressure, so any problems would produce high pressure, not low on the system. Low pressure is caused by an obstruction or other problem that keeps water from reaching the pump.

Unlike high pressure, there is no normal range for low pressure on the pool pump. If your PSI drops more than 5 PSI from the normal, it’s time to inspect the pool filter for a problem. Here are a few common problems to look for:

  • The skimmer or pump basket to the filter is clogged
  • The pump impeller is clogged
  • The filter valve directs water around the filter
  • Skimmer or main drain valves are closed or partially closed
  • Skimmer or main drain pipes are clogged or collapsed
  • Air is pulling into the filtration system at the pump intake, pump lid, or valves

Backwash Sand Filter When Pool Filter Pressure is High


When you filter your pool water material will collect inside the pool filter vessel that is trapped by the porous sand. This could be inert material like chemical sludge, grease, grass, or hair, or organic material like algae alive or dead.

Some of the debris is big enough sucked up through the vacuum from the bottom of the pool or small enough coated on the sand particle that you can’t see until you backwash the system. Here’s how. When you backwash you are reversing the direction of flow inside the vessel and wasting the dirt and debris out of the system and onto the outside via the backwash hose.


  • Turn the pool filter system’s power off on the on/off switch or the manual switch inside the timer box.
  • Attach your pool backwash hose to the backwash port on your sand filter.
  • Select the Backwash position on your multiport valve handle
  • Turn the pool filter system on and allow it to rush out of the backwash port and through the backwash hose for two minutes or so until the water in the sight glass runs clear
  • Turn the filter off again and set your multiport filter valve handle to the Rinse
  • Turn the pool filter system back on and allow it to run for a minute or at least 30 seconds
  • Turn the pool pump off again and set the multiport valve to the Filter position


Make sure you pull the backwash hose as far out away from the area as possible so it doesn’t kink and obstruct the flow in the middle of backwashing. This could damage the inside parts of the sand filter and you’ll take a bath trying to fix it, trust me. Know how to backwash a pool sand filter and why you are doing it.  Excessive Pool filter backwashing could damage the inside parts of the sand filter, and you’ll take a bath trying to fix it, trust me.


Set Pool Multiport Valve to Backwash Filter Sand


When a sand filter is functioning normally, the water runs from the top to bottom direction inside the filter. Water enters at the top of the filter and is forced downward through the sand trapping debris. From there the water flows into the laterals inside the vessel and back out into the pool through the returns.

Backwashing a filter simply turns that process upside down. Water is forced out of the laterals and up through the sand. The water, carrying all the dirt from the filter, then flows out of a designated waste position on the multiport filter valve and out the backwash hose.

You should backwash your sand filter for around 2-3 minutes or until the water in the sight glass on top of the filter becomes clear. While backwashing, you’re losing pool water down out waste, so don’t do it for too long. You can also push filter sand out of the filter if you backwash too long.

Set Pool Multiport Valve to Rinse After Backwashing


The Rinse setting should be done after backwashing and again run just for a minute or two or at least 30 seconds. In this setting the water is flowing through the filter in the normal direction but once again is being sent to the waste hose and out of the swimming pool rather than being returned to the pool. This is because there is still dirt inside the backwash water being used to clean the sand inside the filter.

Once you turn off the Rinse cycle for the backwash and return the system to the Filter cycle check the pressure gauge on top of the filter and see what psi the system is running at. If the pressure gauge reads normal baseline pressure then the filter is clean.

If the psi on the pressure gauge is still higher than the normal operating pressure when the filter sand is clean you have determined then you need to backwash again to bring it down. 


Backwash Sand Filter Until Filter Pressure Comes Down


The sand in a pool filter is usually #20-grade sand, which is sized and shaped to capture debris in the 20-100 micron size. As a sand filter runs it collects more particles and the material it builds up becomes less efficient, collecting more and more dirt that’s trapped in the sand.

This is the time to backwash the filter. When the pressure gauge on the filter reads 8-10 psi higher than it did when the sand was new and clean. This is determined by you as the normal operating pressure or baseline pressure.

When this jump in psi on the pressure gauge happens then there are two common problems that arise with pool filter sand “channeling” and “mudballing”

When pool filter sand becomes channeled, the water has bored little channels through the sand, so that some of the water is flowing right through without going through the sand. This allows particles in the water to flow through without getting caught.

When sand is “mudballed” oil and grease in the pool water build up in the sand and start to form small clumps, or “mudballs”, in the sand. If you live in an area with hard water, you can also run into calcification. That is when the calcium in the hard water begins to get stuck in the filter sand changing the structure of the sand.

Another sign that it’s time to backwash your filter system is the clarity of the pool water because of channeling and mudballing and an overabundance of debris inside the filter the smallest particle will show up in the pool as a cloudy material as the filter becomes more inefficient. 

You should also backwash your filter if the pressure gauge shows 5-10 psi higher than the normal operating pressure between 10-25 psi. 


Pool Water Backwash Diagram


When Vacuuming Pool The Dirt Comes Back In? | The Rex Garden


In Filter mode, pool water passes through the inlet at the top of the pool filter and runs through the layers of filter media (sand), and back out at the top returning the clean water to the pool.

In Backwash mode, the pool water passes through the water inlet, and the debris and material inside the filter sand are flushed from the bottom to the top and leave the filtering system through the waste hose.



How Long do you Backwash a Sand Filter

You should backwash a sand filter for approximately 2-3 minutes or until the sight glass located on the top of the pool filter underneath the pressure gauge turns clear after which turn the system and set the multi-port position to rinse …………………………… more



JimGalloway Author/Editor


Related Questions:


How often should you backwash a sand filter?
How Often Should You Backwash a Pool Sand Filter? As a general rule of thumb, you should backwash and rinse your filter about once a fortnight. The optimal time is right after you vacuum the pool. However, if your pool has had much more use than normal, it may be necessary to backwash it once a week.
How long do you backwash a sand filter?
After the hose fills with water, backwash your sand filter for 2 – 3 minutes or until the water runs clear. Shut off the pump motor and push T-handle back down into the locked position. Turn the pump back on and note lower pressure.



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