Backwashing a Pool Sand Filter – Full Step by Step Guide




Maintaining a crystal-clear pool requires understanding the intricacies of pool equipment, and one of the most vital components in this equation is the sand filter.

Over time, as the filter collects debris and impurities, it becomes essential to refresh its filtering capabilities. This involves the process of backwashing.

In this post, we will guide you through the step-by-step procedure on how to backwash a sand filter, ensuring your pool remains in pristine condition and your filter operates at peak efficiency.

How to Backwash a Sand Filter
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How to Backwash a Sand Filter? Backwashing a sand filter is a simple, straightforward process that involves switching the filter valve setting of the pool sand filter to “Backwash” and then running the pool pump for a few minutes. This reverses the flow of water in the filter which flushes the dirt out of the sand and discharges it through the water outlet.

Why do you need to backwash a sand filter?

There are three main reasons you may need to backwash the pool sand filter:

  • the pressure on the pressure gauge (normally located on top of the sand filter housing) is showing a back pressure that is higher than normal. This is often represented with a red quadrant on the gauge. This occurs because the sand becomes blocked by the debris that has been collected in the filter which slows down the flow of water and increases the back pressure.
  • when you are vacuuming you notice that you do not have very high suction, making picking up the particles on the bottom of the pool difficult. This also occurs because the sand becomes blocked by the debris that has been collected in the filter.
  • you have just vacuumed the pool, which was particularly dirty and it is good practice to do it then as part of your regular cleaning routine.

What does backwashing a pool do?

When you backwash a sand filter system, you are reversing the flow of pool water through the pool filter sand.

When on the filter setting, water passes through the sand from the top, trapping debris in the sand as it flows through and then returns to the pool as filtered water.

But on the backwash setting, the water passes through the filter from the bottom, rather than from the top, and dislodges all the dirt particles lodged in the filter sand to restore optimum filtration. The dirt is then expelled, either to the sewer or the yard/garden, depending on the setup.

backwash sand filter diagram
pool filter settings diagram

How to backwash a sand filter – step by step guide

Follow these simple steps to successfully backwash your sand filter:

1 – Run out the backwash hose

If your pool drains into your garden/yard etc and you have a backwash hose attached to the drainpipe then roll it out and position the end where you want the water to run. You can buy backwash hoses quite cheaply on Amazon.

If your pool drains directly into the sewer then disregard this step.

2 – Open the waste valve

how to backwash a swimming pool
The waste pipe valve is the red handle

If you do have a valve on the waste pipe then open it so that once you start backwashing the water can flow out.

Again, if you don’t have a valve on the waste exit pipe skip this step.

3 – Turn off the pump

If you haven’t already done this after vacuuming the pool, then turn off the pump. You should do this every time before you change the setting on the multiport valve. If you do not, and leave the pump running, it could damage the valve.

4 – Set multiport valve to Backwash

how to backwash sand filter

Push the handle down to release the bottom from the current position. Twist it to the backwash position and then lower the lever to lock it in place.

It doesn’t matter whether you turn the valve clockwise or anticlockwise.

5 – Switch on the pump

6 – Watch the sight glass observation tube

how to backwash a pool sand filter

Keep an eye on the water that is passing through the sight glass observation tube.

Initially, it will probably be very cloudy with particles of dirt in it. After a few minutes, it will start to clear. When it does run clear go on to step 7.

7 – Switch off the pump

8 – Set multiport valve to Rinse

After you have been backwashing for a few minutes the sand in the filter will be quite loose. So turn the multiport valve to rinse as the rinse cycle will settle the sand back down.

9 – Switch on the pump

Run the pump for between 30 seconds and one minute. This will help to settle the sand to make it filter properly. Then switch off the pump.

10 – Set multiport valve to Filter

You are now almost finished so make sure the pump is off and set the lever to the filter position.

11 – Switch on the pool pump

You can either run the pump for a while or put it on a timer setting so it comes on later. I would normally let it run for a while as this is the stage I would normally test the water and add any chemicals necessary.

Once you add chemicals you should run the pump to circulate them around the pool otherwise you will get a build-up of very strong chemicals in one spot.

12 – Check the pressure gauge

Check that the pressure gauge on the filter is now showing a normal operating filter pressure (the dial pointing to the green sector on the dial).

how to backwash a pool filter - pool filter pressure gauge

If the dial is showing little or no pressure then this will be because there is an airlock somewhere in the system.

Let the pump run for a few more minutes as the airlock will probably dissipate by itself and the pressure will rise.

If you find you still have an airlock in the pump, which doesn’t clear itself quickly, then you will need to bleed the air out. Read – How to bleed the air from a pool pump and filter.

13 – Roll up the backwash hose if you have one

14 – Go and have a well-earned rest!

Man relaxing after backwashing a sand filter

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How often should you backwash pool filter?

There are some differing opinions about how often should you backwash a pool. Some say you should only do it when it is absolutely essential (as in the situations in the section above), others say you should backwash every week as part of your regular weekly pool maintenance while others believe you should do it every time you vacuum your pool.

Certainly, the amount of use the pool gets will make a difference to the frequency of cleaning and therefore the frequency of backwashing. When it is just my wife and I here, perhaps swimming only once or twice a week, then I may go 2 weeks between cleaning and backwashing. If we have young family staying and it gets used all day, every day, then I may need to clean and backwash every few days.

It depends too whether you have a sand filter or a DE (diatomaceous earth) filter. DE filters should be backwashed less often than sand filters, perhaps as little as every 4-5 weeks.

How long do you backwash a pool for?

How long do you backwash a pool for?
Glass inspection tube

Most sand filter setups include a small glass inspection tube so that you can see the dirt that is coming out of the filter system and flowing down the waste pipe. I watch this while the filter is on backwash and turn the pump off when I see that the water is running clear.

This normally takes between 1 minute and 3 minutes, depending on how dirty the filter was.

Do you rinse after backwashing a pool?

Once I have turned the pump off I always turn the multiport valve to Rinse and run the pump again for between 30 seconds and a minute. The reason for doing this two-fold:

  • it removes any remaining dirt and dust in the filter sand
  • it helps to resettle the sand filter material in the filter housing leaving it ready to begin filtering again

Running the filter on rinse also sends the water down the filter’s waste port so if you have a backwash valve on this pipe you must leave it open until rinsing is finished.

Can you backwash while vacuuming?

While you are vacuuming it is possible that you may start to notice a loss of suction as dirt on the bottom becomes more difficult to pick up. This can happen, particularly if the pool is quite dirty, as the filter may become blocked which then builds up back pressure, reduces suction and lowers the sand filter’s efficiency.

If this happens then you can backwash without having to disconnect the vacuum hose etc. but of course, you will have to stop actually vacuuming while you are going through the backwash process above.

Should I backwash after shocking pool?

Generally speaking, this wouldn’t be a very sensible idea as you will be flushing some of the chlorine shock straight to waste. It would be much better to backwash, rinse and only then shock your pool.

Do all pools need to be backwashed?

Both pools with sand filters and DE (diatomaceous earth) filters need to be backwashed, although DE filters less often (see the section on how often you should backwash above).

Can you backwash a cartridge filter? Cartridge filters are not built to allow backward water flow so you cannot backwash. You have to remove the cartridge filter and clean it manually.

Can you vacuum pool on backwash?

No, you cannot vacuum a pool when the multiport valve is in the backwash setting. This is because the water flow is reversed with the water from the pool (or the vacuum hose if it is connected) going from the bottom of the sand filter to the top and then into the waste pipework.

Any dirt you pick up while vacuuming is likely to get trapped in the sand at the bottom of the filter and then when you finished and put the system into the filter cycle much of the dirt would be sent into the pool. Refer to the how to backwash a pool guide above.

See Can you vacuum a pool on backwash for a full explanation.

Do you lose water when you backwash?

Where to discard backwash pool water

Yes, you do lose pool water when you backwash as the backwash water does not recirculate into the pool but instead is sent to the waste line and on into the sewer or the yard/garden.

This isn’t normally much of a problem since you are only likely to backwash for 1-3 minutes so the water loss will not be significant.

Some pool systems exit the waste water directly into the sewerage system so you don’t have to worry about it.

Other systems do not, but instead, the pool water is expelled from a waste line. Often there is a flexible pool backwash hose that can be rolled out. The end can be positioned in a part of the garden where you want the waste water to flow (and move it around too to distribute the water). Alternatively, the pipe can be positioned in a drain so the water flows away.

Some pool owners position the backwash hose so the dirty water runs into the road by their property. Before you do this I would suggest you check with your local authority to ensure that this is allowed and not illegal.

When you are just backwashing, or even vacuuming on waste then the amount of water doesn’t normally cause a problem if it is flowing into the garden.

If you are emptying the pool completely, which would be pumped out into the waste system using the waste setting, then the sheer volume can cause problems. In this case, it may be better to empty the pool in stages to allow the water time to soak in and distribute.

Read my post Where does backwash pool water go?

Can I backwash my pool onto my lawn?

The chlorine in pool water could damage the grass if the level is fairly high. Low levels of chlorine will not damage lawns. After all, when you use a sprinkler on the lawn directly from your water supply this normally contains low levels of chlorine and that doesn’t damage the lawn

Ideally, you should run the backwash water into an area of the garden that will allow the chlorinated water to leach down through the soil.

If you have a salt water pool then this should have less effect on the lawn unless you are planning to completely empty the pool in which case this may affect the lawn.

Backwash Video

Frequently Asked Questions

Why is dirt coming out of my pool jets?

This is likely to be because the sand pool filter requires cleaning or backwashing as it has too much dirt and debris in it. You would backwash if it is a sand filter or remove and clean if it is a cartridge filter.

Why is my pool filter pressure gauge reading zero?

This could be because the gauge is faulty. But most likely it is because you have an airlock in the system so no water is passing through the filter. Try loosening the valve on top of the filter to let any trapped air out.

How often do you backwash a saltwater pool?

As far as backwashing goes, then a saltwater pool with a sand filter is no different from a freshwater pool with a sand filter. It should therefore be treated the same and backwashed as shown above.

What happens if you don’t backwash your pool?

If you do not backwash your sand pool filters regularly then the filter will become more and more clogged with debris leading to increased back pressure, lack of suction in the skimmers and much reduced filtering of your pool’s water.

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