A pool filter works by pushing water through a medium that traps dirt; over time that dirt builds up and makes it harder for water to pass through. Backwashing reverses the flow for a short time to flush that trapped dirt back out. Getting the timing and the sequence right makes a real difference to how long the filter lasts and how clear the water stays.
1. Why backwashing is needed
In a sand filter, water passes through a bed of sand (or glass media) that catches dirt as it goes. As that dirt builds up, resistance through the filter increases, the pump has to work harder to keep the same flow, and water turnover in the pool slows down. Backwashing flushes the trapped dirt out the other way so the filter can work efficiently again.
Cartridge filters are not backwashed: the cartridge is taken out and hosed down, and replaced once it wears out. The steps in this guide are for sand filters with a multiport valve.
2. When to backwash
The most reliable signal is the pressure gauge on the filter. Note the reading when the filter is clean; once the pressure rises by a certain amount above that baseline — the exact threshold is set by the filter manufacturer and varies by model — it's time to backwash. Backwashing purely on a fixed schedule (say, "every week") can mean doing it too often in some weeks and not often enough in others; following the gauge is the more accurate approach. A heavily used pool, or one that picks up dust and leaves in the wind, will tend to need it more often.
Having trouble with your pool?
Send a photo or a short video of the water and the equipment. We'll look at it together and send you a suitable maintenance plan in writing.
3. The backwashing steps
On most systems with a multiport valve, the order is:
- Switch off the pump. Turning the valve while the pump is running can damage both the valve and the pump.
- Turn the valve to the "backwash" position.
- Switch the pump back on and watch the sight glass, if there is one, until the water running out turns clear. Once it's clear, this stage is done.
- Switch the pump off again, turn the valve to "rinse," and run it briefly — this flushes out any dirt that loosened during backwashing before it can settle back into the filter.
- Switch the pump off, turn the valve back to the normal "filter" position, and start the system up again.
The water that comes out during backwashing is normally sent to a waste line; check in advance where that line runs and that it discharges somewhere that won't damage the garden or a nearby area.
4. Things to be careful about
The pump is an electrical appliance; before any work on the valve or the pump, disconnect the power and never touch the electrical connections with wet hands. If the valve is turned while the pump is still running, the mechanism can jam or get damaged — the order should always be "pump off, turn the valve, pump on." If a multiport valve is leaking or won't switch cleanly between positions, a part may need replacing; at that point it's worth getting advice from someone experienced in pool maintenance, since the wrong fix can turn a small issue into a full valve replacement.
Frequently asked questions
Where does the backwash water go? Into the system's waste line, which is usually connected to the sewer or a designated drain point. Check that this line is working properly and isn't discharging somewhere that could damage the garden or a neighbouring area.
What if the pool water looks cloudy after backwashing? The rinse step may have been skipped or too short; running the filter for a while after switching it back on usually helps clear it up. If the cloudiness doesn't go away, check the condition of the filter sand — it wears over time and may need replacing.
Do I need to add any chemicals during backwashing? No — backwashing is purely a physical cleaning step. Water balance and disinfection are a separate matter, tracked as part of regular pool maintenance.
For the other checks to run at the start of the season, see the guide on opening your pool for the season; for the general scope of pool maintenance, see the pool maintenance page.
Filter or pump giving you trouble?
If the pressure stays high, the valve leaks, or the water won't clear even after backwashing, send a few photos and a short video of the filter and pump on WhatsApp. We'll work out what's going on together, come for a free site visit if needed, and give you a written quote. Regular maintenance can also be handled while you're abroad — see the pool maintenance page for what's covered.
Having trouble with your pool?
Send a photo or a short video of the water and the equipment. We'll look at it together and send you a suitable maintenance plan in writing.
This guide is for general information. Every home is different; for your own home, ask a company that has seen the work on site for a written quotation.