By The Zagosa Team. Published .
Reviewed or updated .
The default advice is a fresh fill every 12 weeks for an average household. That number comes from a straightforward ratio: over a quarter, roughly 1 in every 3 litres of your tub ends up carrying bather load. Use the tub more than three times a week, or run a busy home, and 8 weeks is the sensible interval.
The 12-week rule without the jargon
The PWTAG sum is litres divided by ten, then divided by daily bathers. So a 1,200-litre tub used by two people every day gives a refill window of about 60 days. Most UK homes land nearer 90 days simply because the tub gets two or three uses a week rather than seven.
The point of the rule is to keep Total Dissolved Solids (TDS) below the level where the water turns slick, stops balancing, and starts eating sanitiser even while the readings look fine.
Clues that a refill is overdue
The water turns off-smelling within a day of shocking. That is usually TDS or organic load pushed past the point chemistry can recover from.
The pH will not settle. You add the right dose, retest next morning, and it has wandered off again. Fresh water is the fix.
Foam appears the second the jets kick in. A defoamer masks it for one soak, but the real culprit is a build-up of body oils and product residue.
Doing it without upsetting the chemistry
Run a pipe cleaner through overnight before you drain. The biofilm hiding in the pipework is what reseeds bacteria into your fresh water within days, and a pipe purge product loosens it off.
Drain to a soft soakaway or a foul drain, never into a watercourse. The neighbour's lawn does not count as a soakaway.
If you live in a hard-water postcode, refill through a hose-end pre-filter. It strips the metals that cause green tinging and spares you a calcium fight from the first hour.
Common questions
Do I still refill if I only use it twice a month?
Yes, though every six months is fine rather than every twelve weeks. Still water develops biofilm even when nobody is getting in.
Refill before a holiday or after?
After. Refilling before you go wastes the water if the tub then sits idle for two weeks. Refresh on your return so the water is at its best when you actually want it.