Water hardness, boilers and water heaters
Scale forms when hard water is heated: calcium carbonate settles on heating elements and heat exchangers. Rules and warranties name a limit around 200 mg/L CaCO₃ above which the water feeding a water heater or combi boiler should be treated.
What to do at your hardness
- soft Below 60 mg/L: no scale protection needed; very soft water can be slightly corrosive, so keep the heating circuit inhibitor topped up.
- moderately hard 60–120 mg/L: scale builds slowly; descale water heaters on the maker's schedule.
- hard 120–180 mg/L: fit a scale reducer or inhibitor on combi boilers and water heaters; check the warranty conditions.
- very hard Above 180 mg/L: treat the feed water (softener or scale reducer) for combis and water heaters, as building rules require above 200 mg/L in Scotland.
Rules and warranty conditions
- Where the mains total water hardness exceeds 200 parts per million, provision should be made to treat the feed water to water heaters and the hot water circuit of combination boilers to reduce the rate of accumulation of limescale; also add inhibitor to the primary circuit and follow the boiler maker's instructions. (Source)
- Approved Document L (2022 circular) says: in hard water areas suitable measures should be taken to protect the primary heating circuit against scale and corrosion (e.g. chemical inhibitors) and to treat the feed water to water heaters and the hot water circuit of combination boilers. The excerpt read gives no ppm number. The earlier DBSCG 2013 edition (incorporating 2018 amendments; gov.uk page confirms the edition) is reported by secondary sources to use the same 200 ppm trigger as Scotland, but that England text was not opened. (Source)
- Rheem's limited warranty for its RGT/RGTH tankless water heaters excludes damage from installation where water quality is outside a listed table, which includes total hardness up to 200 mg/L, pH 6.5-8.5, TDS up to 500 mg/L (citing US National Secondary Drinking Water Regulations); it also excludes failures caused by lime, mineral build-up or scale. (Source)
- DWI classes: soft up to 100 mg/L, slightly hard 100-150, moderately hard 150-200, hard 200-300, very hard > 300 (as CaCO3). Hard water causes scaling in hot water systems, kettles, electric irons and domestic appliances. Citing WHO 2017: 200 mg/L or higher produces scale; 100 mg/L or less is more corrosive to pipework. Where softening is used keep a minimum hardness of 150 mg/L. (Source)
- Hard water can cause scale deposition in heated water applications where insoluble carbonates coat surfaces and reduce the efficiency of heat exchangers; excessively hard water can also be corrosive in some conditions. No numeric efficiency loss is given. (Source)
What the evidence says
- Hard water causes scaling in hot water systems, kettles, electric irons and domestic appliances; scale is produced from about 200 mg/L as CaCO3. · strong evidence (Source)
- Scale on heat-exchange surfaces reduces heat-exchanger efficiency (qualitative; WHO gives no percentage). · strong evidence (Source)
- In a DOE-commissioned test of 12 water heaters, the electric unit with heavy element scaling lost 8% energy factor versus about 4% for a comparably aged unit; authors say causation was not confirmed and recommend further study (single-unit observation). · weak or mixed evidence (Source)
- Soft water (about 100 mg/L CaCO3 or less) is more corrosive to pipework; unstabilised soft water has a greater corrosion tendency, and excessively hard water can also corrode. · moderate evidence (Source)