Backwashing Your Sand Filter

How and When to Backwash a Sand Filter

When the pressure gauge on your sand filter is reading 8 – 10 lbs above the clean, starting pressure, it is time to backwash your sand filter. This process involves turning a valve so that the water will flow through the filter backwards, flushing out the dirt – Hence the name “back-washing.” It is also advisable to backwash your filter after vacuuming in order to remove the debris and impurities you just vacuumed up.

Pool water normally flows top to bottom in a sand filter, water enters at the top of the tank and pushes down through the sand bed into the laterals at the bottom of the tank. During backwashing, the water flows in reverse, from bottom to top, out of the laterals and up thru the sand, to flush out the trapped dirt from the sand bed, and send it out the waste line.

Sand filters can have either a push-pull valve (also known as a slide valve) or a multiport valve. The multiport valve has multiple ports on the valve, usually 6 positions, hence the name “multi-port”. Slide valves have only two positions, filter or backwash, with a plunger being pulled up or pushed down to change positions.

Always shut off the filter pump before turning the filter valve, or it could damage the gasket or internal filter parts. It can create a brief dead-head or water hammer effect, which could rupture filter tanks, pipe fittings or valves, and possibly injure those nearby.

  1. FILTER: Keep it here all the time, except when backwashing, rinsing, or wasting
  2. RINSE: Use this setting for 15 seconds after backwashing to rinse the sand filter tank
  3. RECIRCULATE: A filter bypass setting. Use this if your filter’s broken or leaking; at least you’re circulating the water.
  4. BACKWASH: Use this setting to reverse the flow in the filter and send water out of the waste line. Make sure valves are open and your backwash hose is rolled out.
  5. CLOSED: Put here to close off flow from the pool, usually to work on the equipment, or prevent backflow of water out of the filter. Do not operate the pump with the valve in the closed position, and don’t use it as a winterizing setting, pop-up the handle midway between two positions for winter.
  6. WASTE/DRAIN: Another filter bypass setting, but this setting sends the water out of the waste pipe (hose), instead of returning it to the pool. This setting is used to lower pool water level or to vacuum to waste, which is best when vacuuming algae or heavy amounts of dusty, silty debris.

To backwash a sand filter with a multiport valve;

  • Shut off pump motor, roll out backwash hose (if you have it)
  • Twist to unlock plunger T-handle, pull/twist plunger upwards 2 – 3″ (Pentair valves are reverse)
  • Open-air bleeder assembly on the filter, and turn pump on
  • Watch pressure gauge for back pressure (over 30 PSI) and hose for kinks. Be prepared to shut off the pump quickly if the pressure gauge spikes
  • After the hose fills with water, backwash your sand filter for 2 – 3 minutes or until water runs clear
  • Shut off pump motor and push T-handle back down into locked position
  • Turn pump back on and note lower pressure. Mark it on the filter tank!*

All pool filter systems have different operating pressures, and your filter system will also have its own specific pressure range, from clean to dirty.

A properly sized sand filter should, in most cases, be able to operate continuously for a period of 4 weeks between backwashing. A “Filter Run” of less than 4 weeks may indicate sand problems (or sizing problems), or unusual water conditions being cleaned up.

Most manufacturers suggest that a filter pressure of 8-10 psi greater than the clean, start-up pressure is when you should backwash. However, if your flow seems to diminish considerably at only 5 psi higher, or if pressure rises very rapidly, you may have a sand pool filter that’s a bit undersized, overworked or in need of a sand change. Definitely keep an eye on the flow as well as an indication of when to backwash.

As mentioned before, sand pool filters operate a bit more effectively when they are half dirty than when they are fully clean. Backwashing a sand filter too often could lead to cloudy water. Assuming that your pressure gauge is functioning properly, resist the urge to backwash a sand filter until it’s climbed at least 5 psi higher than the clean pressure.

Filter Sand in the Pool?

Bad news. If sand has not blown in, or been carried in on the feet of swimmers, it’s likely coming from the filter. A broken lateral or standpipe may be the cause. You’ll need to empty the tank, locate and make the repair, refill with pool filter sand, and test.

If you have overfilled your sand filter during a recent sand change, it is not uncommon to experience a small amount of “blowback” into the pool after backwashing. You may also see some sand discharged from the backwash hose after backwashing. Both normal, but continued sand leakage during filtration usually indicates a lateral breakage. Don’t confuse dust with pool filter sand. Dust and silt will ‘dust-up’ off the pool floor when you touch it with a pool brush, while filter sand grains will roll around on the floor and not dust up very much.

It is common to experience a small amount of dust returning into the pool after backwashing a sand filter, even when using the Rinse setting afterward. If you have a problem with fine dust constantly passing through your sand filter, you can vacuum this out to waste after filling the pool first. If this problem continues, you may have sand bed problems or a sand filter that is too small for the pump.

In areas of high dust or pollution, clarifiers can help by increasing the particle size by coagulating smaller dust into more easily filtered clumps. Sand filter cleaners or enzymes are helpful products to remove trapped oils and gunk, which gets trapped, mostly in the top 2 inches of the sand bed. In fact, replacing just the top 25% of your filter sand with fresh pool filter sand is a quick way to rejuvenate an overworked sand pool filter, and extend the time between complete sand replacements.