What is Backwash or Backwashing?

Swimming Pool Maintenance

backwashing pool
Beatrice Murch/Flickr/CC BY-SA 2.0

Backwash or backwashing is the process of thoroughly cleaning a swimming pool's filter by a method of reversing the flow of water to flush out contaminants. This way, you don't have to clean the filter manually, and it usually takes just a few minutes. Continue the process until the water runs clear through the waste line.

You'll know that it's time to perform the backwashing routine when the pressure gauge indicates a pressure rise of 8-10 pounds above the clean or "startup" pressure.

Dirt Passing Back into Pool

If dirt or DE (diatomaceous earth) gets through the filter and into the pool, check for:

  • Damaged grids, laterals or cartridges
  • Broken manifolds or retainers
  • Backwash valves that have bad gaskets or O-rings

Maintenance and prevention are key: when you feel a backwash valve becoming difficult to turn, do a teardown and lubricate before any leaks occur. While you're at it, carefully examine grids, laterals, cartridges, and manifolds each time you break down a filter for cleaning. Don't be in a rush when reassembling—sloppy and careless reassembly after cleaning is the cause of most filter leaks.

Filters and Backwashing

While each type of filter will keep your pool clean, the key to keeping it that way is by making sure the filter is the correct size and is regularly cleaned. So, what's the best type of filter? The one you're most likely to keep clean. Cartridge filters are often a popular choice because they are easy to maintain.

Choose the filter that fits the size of your pool.

Backwashing a DE filter should be considered a temporary solution when a complete teardown and cleaning aren't practical. A sand filter works well with backwashing. Since there is no DE to add, it removes the potential for errors that may result in a dirty pool.

How to Backwash a Filter

Instructions vary according to your filter type; it's best to consult the website of the filter manufacturer, if possible. Basically, this is how it is done with a DE filter:

  1. Turn off the pump and turn the backwash valve (plunger or multivalve) to the backwash setting.
  2. Turn on the pump again until the filter view-glass appears clear. Try to alternate between backwash and rinse (or filter for plungers) a few times to remove more DE. Don't forget to turn off the pump when changing valve settings.
  3. Discard the DE in the yard or in the trash. Some units include a DE separation tank that captures and contains the DE.
  4. Turn off the pump to make sure that the timer or automation system doesn't turn back on while the filter is being disassembled.
  5. Drain the filter by opening the release valve at the top of the filter to allow the water to drain. For thorough draining, remove the drain plug (sometimes by using a screwdriver) at the bottom of the filter so the water can drain from the tank's bottom.

How Often Should You Backwash?

Backwashing and teardown depend on how often you use your swimming pool and just how dirty it actually gets with normal use. DE filters are usually torn down and cleaned at least six times per year.

 Unless your pool gets really dirty, you should not need to backwash it beyond your scheduled maintenance.

Another theory recommends to backwash when the pressure gauge is about 8 to 10 psi (pound-force per square inch) over the starting level. Also, backwash if a major storm or weather event has occurred in your region or a there is a major algae outbreak (any color).

Sand filters, however, can be backwashed once a month and are typically torn down two times per year. To backwash a sand filter:

  1. Shut off the system 
  2. Clamp the backwash hose to the water outlet
  3. Position the backwash valve to either push or pull—the water will flow through the hose or the equipment
  4. Turn the handle to the backwashing position
  5. Backwash for a few minutes or until the water clears
  6. Turn off the system to stop the backwashing process
  1. Roll up the hose
  2. Move the valve handle to its previous position, which will permit water to flow through the system. Lock it in place
  3. While turning on the system, open the air relief valve, which is located at the top of the filter. Leave it open until water flows through
  4. Close the valve and shut off the system

Saving Energy

You can save energy by running the pump at a lower speed after cleaning the filter. This can be achieved without reaching a point of no-flow or low-flow.

Keep That Pool Clean!