Backwash a Pool Sand Filter

All sand filters use the same basic mechanics: when set to Filter, water flows from the pool, through the filter, and back into the pool. The sand inside the filter system blocks debris, dirt, and oil. A typical sand filter multiport system has 6 different settings – Filter, Backwash, Rinse, Circulate, Closed, and Waste.

However, when you vacuum your pool a concentrated level of dirt and debris get deposited into the sand – the result being that the sand becomes dirty and clogged with contaminants that, if left unchecked, will reduce the sand’s ability to filter and, ultimately, simply end up back in your pool.

Backwashing prevents this. The backwash reverses the flow of water, lifts up and flushes the sand, and then expels the dirty water via a waste line into the ground or drain.

In order to ensure against residual blowback into the pool, once you’ve finished backwashing it is highly advisable to rinse the filter. Just as the backwash lifts and flushes the sand, the rinse reseats the sand in its original position for optimum filtration.

How to Backwash a Pool Sand Filter

  1. Turn off the pool pump.
  2. Set the filter valve handle to the BACKWASH position and ensure the handle locks in place.
  3. Turn on the pump and backwash for 2 minutes or until the water in the sight glass (located on the filter) is running clear.
  4. Turn Off the Pump.
  5. Set the filter valve handle to the RINSE position and ensure the handle locks in place.
  6. Turn on the pump and run the rinse process for 1 minute or until the water in the sight glass is clear.
  7. Turn Off the Pump.
  8. Close the skimmer valves to prevent water from entering the filter from the pool. Empty the filter skimmer basket and clean out the hair catcher.
  9. Re-seat and seal the skimmer basket and hair catcher.
  10. Reopen the skimmer valves.
  11. Reset the Filter Valve to Filter and ensure the handle locks into place.
  12. Turn On the Pump.

How Often to Backwash a Pool Sand Filter

As a 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 twice a week.

Also, if your pool is particularly prone to leaves, more frequent backwashing may be necessary since leaves, other debris from trees, and contaminants from birds etc. will be entering your pool.

But, you can also tell when it is time for a backwash by checking the sand filter system’s pressure gauge. Normal operating pressures are between 50 and 75Kpa (kilopascal). When the sand gets dirty and clogged up, however, the pressure reading rises. If the Kpa pressure on the gauge is showing north of 80Kpa, it’s time to backwash.

When Not to Backwash

If your pool contains an unusually high level of dirt (for instance, as a result of a nearby construction site, a protracted period of being untended and uncovered, or possibly from runoff caused by flooding) it may be advisable to by-pass the filter entirely.

This is also the case if your water had, or has had, an algae problem. Live algae easily pass through the filter sand and re-enter the pool. And the same goes for dead algae following a shock treatment.

In these cases, instead of vacuuming and backwashing, it may be better to vacuum directly to WASTE, and to send the vacuumed water directly down into the drain.

A Few Important Backwashing Tips

  • The one downside of sand filter backwashing is that, because water is expelled from the system and sent into waste, is that it…well…wastes water. While this is unavoidable, the key to not overdoing the backwash process and flushing out more water than strictly necessary is to keep a careful eye on the colour and quality of the water in the sight glass.
  • Don’t backwash more than necessary, or for longer than necessary. Sand filter systems operate most efficiently during the middle of the filtration cycle. This is partly due to the positioning, at any given time, of the sand in the filter, and partly because of the cyclical build-up of collected dirt and debris in the filter. The net effect is that too much or to frequent backwashing actually reduces the sand filter’s efficiency.
  • Because backwashing inevitably washes out some of the sand along with the dirt and debris, from time to time it will be necessary to top up the amount of sand in the filter. When you do add extra sand, run the filer system on RINSE for a minute or so to reduce the amount of sand blowback into the pool.
  • Finally, absolutely make sure you always turn off the pump before you either mover or reset the filter valve setting. If you do, you’re going to damage, and maybe destroy the system.
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