When to Backwash A Sand Filter

A sand pool filter uses sand to trap particles in the pool water as it flows through the sand. The sand in a pool filter is usually #20 grade sand, which is specially sized and shaped to capture particles in the 20-100 micron range. As a sand filter collects more particles it becomes less efficient, collecting more and more dirt in the sand. It’s time to backwash a sand filter when the pressure gauge on the filter reads 8-10 pounds higher than it did when the sand was new and clean.

Sand Filter Maintenance

There are three main types of pool filters: Sand filters, DE filters, and cartridge filters. In general, a sand filter lasts longest between cleanings and is easiest to maintain. A sand filter usually only needs to be cleaned every 5 years or so. A DE filter requires cleaning at least once annually. To clean it, remove the internal grid assembly and clean it well. A cartridge filter doesn’t backwash. Instead, the cartridge is removed and cleaned well with a hose. The frequency of cartridge cleaning depends on the size of the cartridge and the type of filter.

The general rule is that the sand in a sand filter should be replaced about every seven years. This is due to gradual deterioration of the sand. The gradual deterioration may be hard to notice, but there are some signs to look out for. If you have to backwash your filter more often, you could have deteriorating sand. Other signs are visible in the pool water. If the pool water is dirtier than usual, if it requires more chemicals to keep it clean, or if the pump needs to be run more frequently, chances are your sand needs to be replaced.

When to Backwash A Sand Filter [infographic]

Problems with Pool Filter Sand

Two common problems that arise with pool filter sand are “channeling” and “mudballing”. When sad 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 in the water builds up in the sand and starts 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, literally turning the sand into sandstone.

Other issues with water balance can also affect the sand in your sand filter. If your pool pump is too large relative to the filter, that can also shorten the life of your sand.  But a properly sized sand filter that doesn’t have any of the above issues can easily last for ten years before requiring new sand.

If there are no other problems, the need to replace the sand in your filter will arise when the sand has become to worn down. Years of water being forced through the sand will slowly wear down its sharp edges, making each ganual more round. The rounded edges make it harder for the sand to effectively trap dirt particles.

What is Backwashing a Filter

Take note of the filter pressure after the last time you backwashed the filter. When the pressure gauge on your filter reads 8-10 pounds higher than that, it is time backwash again. Backwashing the filter is a relatively simple process. To clean the sand, you just run the water backwards, loosening and knocking out all the built up particles. That’s why it is called “back-washing”.

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

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 pipe.

Types of Sand Filters

There are generally two types of sand filters. The first type has a simple two-way valve that is either set to “filter” or “backwash”. It is usually set by pulling a plunger up or down.

Another type of filter is the multiport filter. This type of sand filter has more options.

It is important that any time you are switching the valves, you turn off the filter. Failing to do so could rupture gaskets and cause serious damage to your filter. It is possible you could even rupture filter tanks, valves, or pipe fittings. Flying debris from such a rupture could seriously injure someone nearby.

A two-way port filter (sometimes called a slide valve) has only two settings. A multiport filter usually has the following options:

  • Filter. This is your default setting. Unless you are performing maintenance, this is how the filter should be set.
  • Rinse. You can use this setting for 10-15 seconds after a backwashing cycle. It will rinse out the sand filter tank.
  • Backwash. This setting runs the water backwards through the filter and out the waste pipe. Be sure to open valves and roll out your backwash hose.
  • Closed. This setting closes of the filter form the pool. It is usually a setting used for maintenance. It can also be used to keep water frwom flowing backwards into the pool. Never run the filter on this setting.
  • Waste/Drain. This bypasses the the regular path that sends water back to the pool. Instead, the water flows out the drain or waste pipe. This is good for lowering the water level in your pool or for removing heavy amounts of silty debris or algae.

How to Backwash Two-Way Valve Filter

To backwash a two-way valve filter, follow these steps:

  1. Turn off the filter motor. If there is a backwash hose, roll it out now.
  2. Unlock the plunger T-handle (usually you just have to twist it) and then pull is up 2 or 3 inches.
  3. Open up the filter’s air bleeder assembly and turn the pump back on
  4. Watch the pressure gauge and keep an eye on the hose to look for kinks. Be prepared to turn off the pump very quickly if the pressure spikes.
  5. Once the hose is filled with water, let the pump run for 2 or 3 minutes or until the wastewater runs clear.
  6. Shut off the pump and reset the T-handle to its original locked position.
  7. Turn the pump back on. Take not of the new, lower pressure. Using a permanent marker, write down the old and new pressure on the filter tank, so you will have a reference to know when to clean it next.

How to Backwash Multiport Valve Filter

To backwash a multiport valve filter, follow these steps:

  1. Turn off the pump motor.
  2. Push down on the valve handle and turn it from FILTER to BACKWASH.
  3. If there is a backwash hose, roll it out now. If there are valves on the waste line, open those now.
  4. Open the air bleeder assembly on the pump and turn the pump back on.
  5. Watch the pressure gauge and keep an eye on the hose for kinks. Be ready to turn off the pump very quickly if there is a sudden spike in pressure.
  6. Let the pump run for 2 or 3 minutes or until the wastewater runs clear.
  7. Turn of the pump again and move the valve to RINSE. Turn the pump back on and let it rinse for about 15 seconds. This will reset the sand in the filter and wash out any leftover dirt in the waste line.
  8. Shut off the pump again and turn the valve to FILTER.
  9. Turn back on the pump and mark the old and new pressure on the filter tank, so you will have a reference so you will know when to clean it next.

Professional Pool Maintenance

If you suspect that there is a problem with your sand filter, or if you just need help washing or replacing your sand, The Pool Butler is here to help. We offer filter cleaning and sand and grit replacement. We also have regular maintenance plans that run weekly, bi-weekly, or even seasonally. You can find out more about all of our services here. When you are ready to learn more about getting your pool serviced, just contact us and we will be happy to figure out what you need and give you a free estimate.

The Pool Butler