How to flush a hot tub before a water change: dose 200 ml per 1,500 L, run the jets, soak overnight, then drain. Removes the biofilm behind cloudy water.
To flush a hot tub, add a pipework flush product such as ClearSpa Hot Tub Flush to the old water just before a drain and refill: remove the cartridge filter, dose about 200 ml per 1,500 litres, run the jets on high for 30 minutes with the cover off, leave to soak for 8 to 12 hours (ideally overnight), run the jets again briefly, then drain completely, wipe and hose down the shell, refit a clean filter and refill. Flushing removes biofilm, the slime layer inside the pipework where most hot tub bacteria actually live, and it is the step that stops cloudy water and high sanitiser use coming straight back after a water change.
Biofilm is a slime made of polysaccharides and trapped organics that bacteria build to anchor themselves inside hot tub pipework and jets. Most of the bacteria in a hot tub live in this pipework slime, not in the water itself, which is why it hides from your sanitiser and keeps chlorine or bromine demand high. Draining and refilling changes the water but leaves the biofilm on the pipe walls, so the fresh fill inherits the same problem: cloudy water that returns within days, a musty or sour smell from the jets on start-up, and sanitiser that vanishes faster than it should.
Brown or grey particles, foam or a strong smell during the flush are the treatment working - that is old biofilm and trapped oils lifting off the pipe walls. Keep the cover off and ventilate; the smell clears soon after the jets are switched off.
Use the flush only when you are draining the tub - never bathe in flushed water. The concentrate is alkaline: wear nitrile gloves and splash-proof goggles, avoid breathing vapours while the jets run, never mix it with chlorine products during the flush, and drain to a sewer rather than a soakaway. Always follow the instructions and safety guidance on the product label.
With clean pipework, the new water starts with far lower sanitiser demand. Refill, balance total alkalinity and pH first, then establish your free chlorine or bromine level and test regularly. If your water problems persist after a flush and refill, work through the hot tub water troubleshooting guide, and see how often to drain and refill a UK hot tub to plan your water-change routine.
Flush at every routine water change - for most UK hot tubs that is roughly quarterly. Flush sooner if sanitiser demand creeps up, the jets smell musty on start-up, or cloudy water keeps returning days after a refill.
No. A pipework flush is used only when you are about to drain the tub, and you should never bathe in flushed water. Drain completely, rinse the shell and refill with fresh water before using the tub again.
Usually because biofilm in the pipework survived the water change. The slime layer keeps feeding bacteria and organics into the fresh water, so sanitiser is used up quickly and cloudiness returns. A pre-drain flush removes the biofilm so the new fill gets a clean start.
Yes. Remove the cartridge filter before dosing the flush and do not refit it until after the refill - fit a cleaned or new cartridge so the fresh water is not re-seeded from a dirty filter.