By The Zagosa Team. Published .
Reviewed or updated .
Across the South East, tap water ranges from very hard to extremely hard. Calcium hardness in the supply often tops 300 ppm and sometimes 400. Fill a hot tub from the mains in those postcodes and you start above the recommended ceiling before a single chemical goes in. Here is the strategy that keeps it manageable.
Where you start
Test the fill water before it goes in. Calcium hardness above 300 ppm means hard-water problems are already on the way.
The symptoms are scale on the heater element, milky cloudiness as pH drifts up, rough deposits along the waterline and pump impeller wear over time.
Ignoring it will not help. Hot water makes the problem worse than cold mains, because evaporation leaves the calcium behind in the tub.
What to do at refill
Use a hose-end pre-filter. The carbon-and-resin canister type traps a good chunk of the calcium and drops your starting point to something workable. Around 20 pounds, and it lasts a few refills.
Dose the fresh water with a sequestering agent, a phosphonate-based stain and scale inhibitor. It binds calcium loosely so it stays in solution rather than settling on the heater. Top up monthly.
Keep pH at the lower end of band, 7.2 to 7.4. Slightly more acidic water holds back scale. The trade-off is marginally faster sanitiser use, which is fine.
Ongoing care in hard water
Wipe the waterline with a soft cloth weekly. Scale settles there first because the air-water line evaporates fastest.
Descale the heater element once a year. Drain the tub, fill with a 5% citric acid solution, run for an hour, drain and refill. The element comes out looking new.
Replace the cartridge filter a touch more often, every 12 to 15 months rather than 18. Hard water clogs the pleats with mineral deposits on top of the usual organic build-up.
FAQ
Can I use a domestic water softener?
Yes, but only the sodium-exchange type, and the water comes out at almost zero hardness, which is then too low. You have to add calcium back in measured doses. Most owners find the hose-end pre-filter does enough.
Does hardness affect bromine and chlorine differently?
No. Both sanitisers behave the same in hard water. The deposits on heaters and waterlines are the same whichever one you use.