Backwashing the sand filter: step by step, how often and what the valve positions mean

Short answer: a backwash reverses the flow of water through the filter, so accumulated dirt is flushed from the sand to the drain. Do this every 2 to 4 weeks or as soon as the pressure gauge rises: pump off, valve to Backwash, pump on and flush for about 60 seconds until the water is clear, pump off, valve to Rinse, rinse for 30 seconds, pump off, valve back to Filter. The one rule you never break: only change the valve position with the pump off.

How a sand filter works

The pump pushes the bath water down through a layer of filter sand; dirt, hair and skin cells stay caught in the sand and the clean water returns to the bath via the bottom laterals. The more dirt the sand holds, the higher the resistance: you see that on the pressure gauge as rising pressure and in the bath as decreasing clarity. Think of it like a vacuum cleaner bag: full is full, and then only emptying it helps.

The multiport valve positions

For a winter setting or intermediate position, you relax the valve before winter; see winterising a hot tub.

What you need

• The backwash hose supplied with the filter, with the supplied hose clamp.
• A place where tens of litres of dirty water can go: a drain, flowerbed or street gully.
• Possibly a bucket to catch the first, dirtiest water.

Step by step

1. Pump off. Switch off the filter system before changing anything on the valve.
2. Connect the hose. Unscrew the cap from the drain connection, attach the backwash hose with the clamp and place the end at the drainage point.
3. Valve to Backwash and pump on. The water now flows in reverse through the sand to the hose.
4. Flush for about 60 seconds, or until the water in the sight glass or at the hose is clear.
5. Pump off, valve to Rinse.
6. Pump on, rinse for 30 seconds. This settles the sand and flushes dirty water out of the valve, so it does not run back into the bath.
7. Pump off, disconnect the hose, replace the cap.
8. Valve to Filter, pump on. Check the pressure on the gauge and note it: that is your reference for the next round.

Afterwards, top up the bath to the usual level; a backwash uses tens of litres.

How often

• By default every 2 to 4 weeks.
• Immediately if the pressure is clearly above your reference value.
• Immediately with cloudy water, after a busy evening and after changing the water.
• Not more often than necessary: every backwash also flushes away a bit of sand and water.

The sand itself

Filter sand lasts for years, but not forever. Replace it on average every 3 to 5 years, or sooner if:

• the sand clumps together or feels hard;
• the pressure rises quickly again right after a backwash;
• sand ends up in the bath (usually the bottom laterals are also damaged in that case).

When replacing it, check the filter housing, the laterals and the lid seal, and use only filter sand of the prescribed grain size.

Common mistakes

• Switching the valve with the pump running: damages the valve and the seal.
• Skipping Rinse: the first dirty water runs back into the bath.
• Backwashing too briefly: the sand is not yet clean and the pressure rises again immediately.
• Using Waste instead of Backwash: the bath drains without the sand being flushed.
• Forgetting to top up: a water level that is too low lets the pump draw in air.
• Filtering with the valve on Recirculate: the water bypasses the sand and stays cloudy.