By The Zagosa Team. Published .
Reviewed or updated .
Soft water sounds like the easy option, and in many ways it is. No scale, no furred kettle, no battle to keep calcium in check. The catch in a hot tub is that very soft water is corrosive and foamy. In the North West the strategy is essentially the reverse of the South East.
What soft water does in the tub
In many North West postcodes, mains calcium hardness is below 50 ppm. Below 80 ppm in the tub the water turns corrosive, stripping calcium from grout, heater elements, pump parts and even nearby tile.
Soft water foams more. Lower calcium reduces the surface tension, so any surfactant, whether body oils or detergent left on swimwear, produces more foam than it would in harder water.
The water can feel slick too. Pleasant in the shower, a little odd in a hot tub.
What to do at refill
Add calcium hardness increaser (calcium chloride flakes) at every refill. For a typical 1,200-litre tub starting at 30 ppm, around 250 to 300g brings you into the 100 to 200 ppm band.
Dose alkalinity to the top of the band, 110 to 120 ppm. The buffer guards against soft water's natural drift toward low pH and greater aggressiveness.
Keep pH at the upper end, 7.5 to 7.7. Slightly more alkaline water tames the corrosivity that low calcium brings.
Keeping on top of it
Test calcium hardness monthly. It can drop very slowly as the water reaches equilibrium with surfaces, so top up with more calcium chloride if it does.
Foam is the steady background nuisance. Rinse swimwear before bathing, soak the filter regularly, use clarifier sparingly, and keep a defoamer only as an emergency for guests.
Check the heater element yearly for pitting or thinning, especially on older tubs. Soft-water corrosion is slow but cumulative, and a heater that has run short on calcium for years can fail without warning.
FAQ
Is rainwater like soft mains water?
Both are low in calcium, but rainwater has zero hardness and no chlorine. Never fill with rainwater. Having no starting buffering at all is much worse than soft mains water.
Will added calcium make my water feel hard?
At 100 to 200 ppm, no. That is well below the level that makes shower water feel scratchy. Bathers consistently prefer water in this band over very soft water.