How to Backwash a Pool Filter

When to backwash — and when not to

The rule is pressure, not the calendar: backwash when the gauge reads 8 to 10 psi above the pressure the filter shows when it is genuinely clean. That means you have to know the clean number — write it on the tank with a marker the day you change the media, because nobody remembers it in August.

Backwashing on a schedule is worse than useless. A slightly dirty sand bed filters better than a spotless one, because the trapped debris does some of the straining. Backwash a sand filter every week and you keep resetting it to its least effective state, and you pour away water and chemistry doing it.

Sand: the sequence

Turn the pump OFF. Turning a multiport valve under pressure destroys the spider gasket.
Roll out the backwash hose so it drains where it is legal and where it will not flood a neighbour.
Set the valve to BACKWASH and lock it down.
Run the pump. Watch the sight glass; two minutes or until the water runs clear, whichever comes first.
Pump OFF. Valve to RINSE. Pump on for 20–30 seconds to settle the bed.
Pump OFF. Valve to FILTER. Restart and note the new pressure — that is your fresh clean number.

Painted illustration: a backwash hose running across a sand deck to a drain — The hose run out across the deck before anything was switched.

DE: backwash then recharge

A DE filter backwashes like a sand filter and then needs its media replaced, because you have just sent the diatomaceous earth down the hose. Recharge through the skimmer with the pump running, at the weight on the tank label — under-charging leaves grids bare and lets fine debris through, over-charging raises pressure straight away and has you backwashing again next week.

If the pressure climbs back within days of a recharge, the problem is not the media. Pull the grids and inspect for tears.

What it costs in water

Filter	Backwash + rinse	Chemistry impact
Sand, 24 in tank	200–300 gal	Dilutes stabiliser and salt slightly
DE, 48 sq ft	150–250 gal	Same, plus media cost each time
Cartridge	0	None — it is hosed off, not backwashed

Volumes are the tool’s own working estimates for typical residential tanks; measure yours once with a meter if the number matters to a client’s bill.

Painted illustration: a pressure gauge on a filter tank, needle in the high band — Pressure gauge sitting high — the reading that starts the whole job.

What goes wrong

Turning the valve with the pump running is the expensive one. After that: forgetting the rinse cycle, which sends a cloud of fines straight back into the pool; leaving the valve between positions, which slowly bypasses the filter; and backwashing a pool whose real problem was an over-driven filter all along.