Low calcium hardness is a quiet killer of heaters and pumps. Here is why soft-water postcodes need to dose calcium up at every refill.
Soft mains water sounds great. No scale on the kettle, no white marks on the shower screen. In a hot tub, however, low calcium is a problem. The water becomes corrosive, leaching metals from the heater and damaging the inner surfaces of pumps. Soft-water UK postcodes need to add calcium hardness at every refill.
Water that is below about 80 ppm calcium hardness is undersaturated with respect to calcium carbonate. Given the chance, it will pick up calcium from anywhere it can: tile grout, the heater element, the pump impeller, even the walls of acrylic shells.
The effect is gradual but cumulative. Over a year of operation in soft water, an unbalanced tub can lose noticeable thickness from heater elements and develop pinhole corrosion in copper components.
The water also feels different. Soft water is excellent for skin in a shower but can feel slick or slimy in a hot tub at low calcium hardness.
Generally soft-water regions: most of Scotland, the North West, North Wales, South West Wales, and pockets of Cornwall and Devon.
Generally hard-water regions: most of the South East, East Anglia, the Midlands, and the South coast.
Check your water company's hardness map. Many publish it as a postcode lookup. If your hardness is below 60 mg/L (CaCO3 equivalent), you are in a soft-water area for hot tub purposes.
Buy calcium hardness increaser (calcium chloride flakes). Dissolve in a bucket of warm tub water before adding, with the jets running.
For a 1,200-litre tub starting at 30 ppm, you need around 250g to lift to 200 ppm. The product label will give a per-litre dose for your specific brand.
Add at every refill. Calcium hardness does not change much with use, so once dosed it holds for the life of the refill.
No. Bath salts are sodium-based and do nothing for calcium hardness. They contribute to TDS without solving the underlying issue.
At 100 to 250 ppm, no. That is the band that bathers find comfortable. Hard tap water is often 400 ppm or more, which is well above the hot tub band.