Ask ten pool owners how often to backwash a pool and you will get ten schedules: every week, every second week, whenever the water looks tired, whenever Dad did it. Almost all of them are wrong in the same way — they treat backwashing as a calendar event when it is actually a pressure event. Your filter already tells you exactly when it needs backwashing; the answer is on the gauge, and learning to read it saves water, chemicals, sand life and filtration quality all at once.
The pressure rule, once and for all
Every sand filter has a clean baseline pressure — the reading right after a proper backwash and rinse with a clean pump basket. The rule the whole industry actually works by:
Backwash when the gauge reads roughly 20–25% above your clean baseline. Not before, not long after.
Concretely: baseline of 100 kPa → backwash around 120–125. Baseline of 80 → backwash around 100. Write your baseline on masking tape next to the gauge and the guesswork ends today. No working gauge? Replace it — it is the cheapest diagnostic instrument on the entire system.
Why more backwashing is not better filtering
Here is the counterintuitive part: a slightly dirty sand bed filters better than a freshly backwashed one. Trapped particles narrow the gaps between sand grains and help catch finer material. Backwashing too often keeps the bed permanently coarse, sends thousands of litres of treated water down the drain each time, and scours the sand toward early retirement. In a city with Cape Town’s water history, backwashing on a weekly ritual rather than on pressure is genuinely wasteful — of water, of the chemicals dissolved in it, and of filtration quality.
What Cape Town does to the schedule
Pressure-led backwashing self-adjusts to our conditions, which is exactly why it beats any calendar:
- South-easter season: wind-blown dust and debris load filters fast — coastal and West Coast pools may hit the threshold every week or two in summer;
- Leafy suburbs: autumn leaf-load in tree-heavy gardens compacts sand beds quickly — southern-suburbs pools often backwash most in April and May;
- Winter: a covered, lightly used pool may legitimately go a month or more — and that is fine, because the gauge says so;
- After a green-pool recovery or heavy storm: expect to hit the threshold within days — again, the gauge leads.
When backwashing stops working: the filter’s cry for help
The gauge diagnoses more than timing. Three patterns mean the filter needs repair, not another backwash: pressure that rebounds within days of every backwash (sand bed exhausted, channelled or greasy — rebedding due); sand appearing on the pool floor near returns (a cracked lateral inside the filter — a repairable fault we see constantly); and low pressure with weak returns (a starved pump or suction-side air leak, not a dirty filter at all). All three sit squarely in pump and filtration repair territory, and all three waste a season if you answer them with more backwashing. Chronic filtration problems also show up as rising chlorine demand and recurring algae — and if that is paired with a rough, stained surface, the conversation may extend to the surface itself.
The two-minute backwash done properly
- Pump OFF (always, before moving the multiport valve);
- Valve to BACKWASH, pump on, run until the sight glass clears — usually 60–90 seconds;
- Pump off, valve to RINSE, pump on for 20–30 seconds to resettle the bed;
- Pump off, valve back to FILTER, pump on — and note your new pressure reading.
If that final reading keeps creeping higher month after month, the bed is ageing; if it will not come down near baseline at all, book the filter a professional look. A free assessment includes exactly this kind of filtration health-check — along with everything else on our Cape Town repair menu.
Frequently asked questions
So how often will I actually backwash?
Most Cape Town pools land between weekly (peak summer, windy suburb) and monthly-plus (covered winter pool) — but let your gauge, not this paragraph, give your answer.
How long should sand last?
Five to eight years typically — less under heavy leaf-load or after swamp recoveries. Rapid pressure rebound is the retirement notice.
Does backwashing lower the water level?
Yes — you are pumping pool water to waste. Top up afterwards and keep the level mid-weir so the pump never gasps.
Can I skip the rinse step?
Do not — rinse resettles the bed and flushes loosened debris out of the valve. Skipping it sends that debris straight back to the pool on the next filter cycle.
