By The Zagosa Team. Published .
Reviewed or updated .
A pipe flush belongs immediately before a drain and refill. Remove the cartridge, add 200 ml of Hot Tub Flush per 1,500 L, run every jet on high for 30 minutes with the cover open, then leave the solution for 8 to 12 hours. Run the jets for another 10 minutes, empty the tub, rinse it and refill with a clean cartridge. This process lifts the film inside the plumbing that a simple water change leaves behind.
What biofilm is and why a refill leaves it behind
Biofilm is a sticky matrix of polysaccharides and trapped organic material formed on wet surfaces. It clings to the inside of pipes and jets, where normal levels of chlorine or bromine cannot readily reach everything beneath its surface. Emptying the tub removes the old water but does not scrub those internal walls.
If the pipe film stays in place, a new fill may soon turn cloudy, smell stale when the jets start or consume sanitiser unusually quickly. Flushing loosens that residue so it leaves with the waste water.
Signs a flush is due
- You are carrying out the usual quarterly water change.
- Chlorine or bromine disappears faster although use of the tub has not increased.
- The first burst from the jets has a musty or sour smell.
- Fresh water becomes cloudy again within a few days.
- You have taken ownership of a used tub and do not know its maintenance history.
Hot tub flushing method
- Lift out the cartridge filter and keep it out until the replacement water is in.
- Pour the flush directly into the old tub water. Use 150 ml for 1,000 L, 200 ml for 1,500 L, 300 ml for 2,000 L or 750 ml for a 5,000 L swim spa.
- Open all water jets and air controls. With the cover removed, operate the jets at high power for 30 minutes.
- Stop the jets and let the solution stand for 8 to 12 hours, preferably overnight. For a neglected system, extend this to 24 hours instead of adding extra product.
- Restart all jets for 10 minutes, then drain every part of the tub.
- Wipe the shell and hose it down. Fit a clean cartridge, refill, then correct total alkalinity to 80 to 120 ppm and pH to 7.2 to 7.6 before establishing sanitiser.
Foam, dark flecks or an unpleasant smell can appear as residue comes away from the plumbing. Keep the area ventilated and the cover open. The odour should disperse after circulation stops and the treated water is drained.
Safety while flushing
Never bathe while flush solution is in the tub. The concentrate is alkaline and can cause serious eye damage and skin burns. Put on nitrile gloves, long sleeves and splash-proof goggles, and avoid breathing vapour while the jets operate. Do not combine the flush with chlorine donors. Follow the label throughout, then send the waste water to a sewer, not a soakaway.
Setting up the new water
Once you have clean plumbing and a clean cartridge, balance the fresh fill before adding sanitiser. Keep free chlorine at 3 to 5 ppm or total bromine at 4 to 6 ppm, and check it regularly. Our guides to hot tub water troubleshooting and draining and refilling a hot tub can help you plan the next checks.
Hot tub flush questions
How often should you flush a hot tub?
Include a flush with each planned water change, which is roughly every three months for many domestic tubs. Do it sooner when sanitiser use rises, jet odours develop or cloudiness quickly follows a refill.
Can you bathe during a flush?
No. Flush products are only for water that will be discarded. Empty and rinse the tub completely, refill it, balance the water and return sanitiser to its correct band before anyone gets in.
Why can cloudiness return after a refill?
Residue left on the pipe walls can enter the replacement water and increase sanitiser demand. A flush before draining clears the plumbing rather than changing only the visible water.
Does the filter need cleaning too?
Yes. Remove the cartridge before adding flush and only install a thoroughly cleaned or new one after refilling. Otherwise, dirt held in the old cartridge can pass straight into the clean water.