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How to Backwash a Pool Filter

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Before you bought a pool filter, the word backwash probably meant something kind of gross to you, right? Well, when it comes to pools, it has a literal meaning. And it’s something good.

Your pool is only as clean as your filter is. The good news is, when you backwash a pool, cleaning the filter doesn’t always entail taking it apart and washing every little crevice.

Why Backwash a Pool?

So we’re talking about how to backwash a pool filter here, but a lot of people often use the shorthand of just “backwash a pool.” You know what we mean, right? Of course you do.

You also know you have to keep your pool filter clean. But when it comes to how to clean a pool filter, why is backwashing a good method?

As pool water passes through your diatomaceous earth (D.E.) or sand filter, it leaves behind dirt, oils, and other debris, which are collected in the filter medium—either D.E. powder or sand.

Over time, as water continues to pass in just one direction, the filter medium will become clogged, which will reduce your filter’s effectiveness. When you backwash a pool, you send water backward through the filter, and out the waste or drain port. This forces all the debris caught in the filter to dislodge so you can easily remove it and restore the filter’s normal functioning level.

Think of it like using a strainer in your kitchen sink. Bits of food can get stuck along the bottom of the strainer, and the only way to get them out is to turn the strainer over and run water through it from the opposite direction.

Backwashing a pool works on the same principle. While backwashing generally works the same way regardless of the type of pool filter, the procedures do vary a bit.

How to Backwash a Sand Filter

Can you imagine if you had to remove the sand and wash it? How messy would that be?! Luckily, when you learn how to backwash a sand filter, you quickly realize it’s a much easier process. The sand never leaves the filter.

Before you get started, you’ll need just one item.

What You Need

Heavy Duty Pool Backwash and Drainage Hose with Clamp
$39.96

When backwashing your pool filter, use this easy-to-roll up and store backwash hose.

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How to Backwash Using a Multiport Valve

This is a pretty straightforward process.

Important: Never turn the multiport valve handle while the pool filter is on and running. This could cause the rubber diverter gasket inside the valve to come loose or break, which will in turn cause water to leak out of the wrong ports while it’s running.

How to Backwash a D.E. Filter

Before you get started, you’ll need a few supplies.

What You Need

Diatomaceous Earth Powder - 24 lbs.
$46.80 ($0.11 / oz)
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What is Diatomaceous Earth?

It’s a white powder composed of fossilized skeletons of one-celled organisms called diatoms. The powder is added to the filter and attaches itself to the grids inside creating a D.E. “cake.”

This cake, along with the filter grids inside, helps to filter out very fine particles.

How Much D.E. Powder Will You Need?

This will vary depending on the type and size of filter you have. Always refer to your owner’s manual for specific information.

In any case, D.E. powder is measured in pounds, so use a one-pound D.E. scoop to make measuring easy. In a pinch, you can also use an empty one-pound coffee can.

Standard DE Dosage Chart

How to Backwash with a Multiport Valve

You’ll want to set aside a little time to do this, but don’t worry. It’s not an arduous task that’s going to take all day.

  1. Turn off the filter system.
  2. Connect the backwash hose to the filter’s waste port.
  3. Turn the multiport valve to Backwash. If you have a push/pull valve, open the backwash gate.
  4. Turn the filter system back on, and let it run for about two minutes. This will allow pool water to flow out of the backwash valve or port.
  5. Turn the filter system off.
  6. Turn the multiport valve to Rinse, turn the system back on, and let it run for about another minute.
  7. Turn the filter system off.
  8. Turn the multiport valve back to Filter.
  9. Add D.E. powder to the filter.
    1. Refer to your filter owner’s manual to determine how much D.E. powder you’ll need to add.
    2. Prime the pool pump.
    3. Remove the strainer basket lid.
    4. Fill the basket with water, and allow some water to run through the incoming line.
    5. Turn on the pool pump.
    6. Mix the appropriate amount of D.E powder with enough water to make a slurry, which is a thin, creamy solution.
    7. Make sure the pump is running.
    8. Pour the slurry directly into the pool skimmer.
    9. Run the pool pump for at least 30 minutes to allow the D.E. to distribute evenly over the filter grids.

Important: Never turn the multiport valve handle while the pool filter is on and running. This could cause the rubber diverter gasket inside the valve to come loose or break, which will in turn cause water to leak out of the wrong ports while it’s running.

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How to Backwash with a Push/Pull Valve

This procedure differs only slightly from the multiport valve process.

  1. Turn off the filter system.
  2. Connect the backwash hose to the filter’s backwash port.
  3. Open the backwash gate.
  4. Turn the filter system back on, and let it run for about three minutes. This will allow pool water to flow out of the backwash port.
  5. Turn the filter system off.
  6. Close the push/pull valve.
  7. Add D.E. powder to the filter.
    1. Refer to your filter owner’s manual to determine how much D.E. powder you’ll need to add.
    2. Prime the pool pump.
    3. Remove the strainer basket lid.
    4. Fill the basket with water, and allow some water to run through the incoming line.
    5. Turn on the pool pump.
    6. Mix the appropriate amount of D.E powder with enough water to make a slurry, which is a thin, creamy solution.
    7. Make sure the pump is running.
    8. Pour the slurry directly into the pool skimmer.
  8. Run the pool pump for at least 30 minutes to allow the D.E. to distribute evenly over the filter grids.

When to Backwash a Pool

When you first set up your filter, whether it’s sand or D.E., take note of the pressure gauge reading. That’s your normal operating filter pressure. Over time, as the filter starts to accumulate dirt and debris, pressure will begin to build up because the water can’t flow as freely through it.

When the pressure gauge reads 10 pounds per square inch (psi) above the normal operating level, it’s time to backwash. If it reads more than 10 psi over the normal level, it’s really time to backwash.

While you could backwash before the pressure reaches the 10 psi over normal mark, we recommend waiting. Yes, your filter functions better when it’s not clogged. But before it gets to the fully clogged point, the presence of small dirt and debris actually helps your sand or D.E. filter work a little better.

Those tiny bits of debris help to catch and stop other tiny bits of debris from getting through the filter medium, and making it back into the pool water.

So, a clean filter is good. A slightly dirty filter is better. A clogged filter is bad, and needs a backwashing.

How to Dispose of Filter Backwash Water

Every time you backwash your pool filter, you could be pulling hundreds of gallons of water from your pool. Where is all that water going to go? Into your yard? The street? The creek behind your house?

None of those are good options, and in fact, may even be illegal, depending on where you live. The water that comes out of your pool is chemically treated, and can damage plant life, groundwater, and aquatic life.

Disposing of backwash water properly is essential, not just to avoid potential fines, but to keep your community and the environment safe from illness and damage.

Storm Drains

Some cities do allow pool water to be released into storm drains. But they may also require that you apply for a permit first.

You may also need to reduce or eliminate chemicals or salt levels in the water before you’re allowed to dispose of it in your community’s sewer system. Check with your local water authority.

Plumbing Cleanouts

You may be able to dispose of backwash water by directing it into one or more of your home’s plumbing cleanouts. Again, it’s best to check with your local water/sewer authority to ensure this is allowed, find out whether you need to treat the water in any way before disposing, and to obtain any necessary permits.

It’s unlikely any municipality will allow you to release pool water into a septic system. You wouldn’t want to do that anyway. Septic systems rely on the existence of active enzymes to dispose of waste. Pouring chemically treated water into the tank—especially water with chlorine in it—would likely kill those enzymes, leaving you with quite a mess.

Dry Wells

These can be a life (or property) saver if you live in an area that sees a high level of rainfall, or experiences tropical storms. They can help prevent flooding and erosion. While a dry well could work for disposing of pool water, that’s not usually its purpose.

Regardless of how many levels of gravel, sand, or other media the water may pass through, it will still end up in the ground, which means it can be a problem if you live in an area that doesn’t allow the disposal of pool water into the ground.

Aside from the issue of permission, a dry well must be constructed by a professional, and it’s a project that can easily cost thousands of dollars.

You can definitely find an easier, safer, and less expensive way to dispose of your filter backwash water. Your local water authority can help you.

Release the Kraken!

Or maybe just some dirty pool water. But keeping a kraken in your pool is all the more reason to keep the water clean. Anyway, when the time comes to backwash your pool, you’ll be ready with the right equipment and procedures.

Take comfort in knowing you’ll be able to help your filter keep your pool clean, which will also help your filter live a longer life, which can save you money. Then you’ll be able to afford that cute little kraken you’ve been wanting to adopt.

Happy Swimming!

Matt Giovanisci is the founder of Swim University® and has been in the pool and spa industry since 1995. Since then, his mission is to make pool and hot tub care easy for everyone. And each year, he continues to help more people with water chemistry, cleaning, and troubleshooting.

Frustrated by adding chemicals and trying to keep your pool clear all the time?

We cut out all the confusion of pool maintenance in this easy-to-read illustrated ebook and video course. It'll help you save $100 right away on pool care!

Click Here to Learn More
The Pool Care Handbook

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