Northamptonshire sits on chalk and limestone, giving it some of England's harder water — typically 200–300 ppm (parts per million) hardness. This affects central heating systems in two key ways.
Scale on heat exchangers
Hard water calcium deposits (limescale) build up on the inside of heat exchangers, reducing thermal transfer efficiency. A 1mm layer of scale reduces boiler efficiency by approximately 10%. The problem worsens in combi boilers where the domestic hot water circuit is directly heated by the primary heat exchanger.
Scale in the domestic hot water circuit
Scale accumulates in the hot water side of combi boilers and in indirect cylinders. This reduces hot water flow rates and can eventually block the heat exchanger.
Prevention
Scale reducer/inhibitor: Chemical scale inhibitors (Fernox DS40, Sentinel X400) can be dosed into the primary circuit. For the domestic side, a whole-house scale reducer on the mains supply is the most effective solution.
Electrolytic scale inhibitor: Fitted on the cold mains supply, these use an electrolytic process to prevent scale formation throughout the domestic water system.
Water softener: The most complete solution — eliminates scale throughout the entire property. Significant investment (£600–£1,500 installed) but protects every water-using appliance.
What we recommend in Northamptonshire
At minimum, a Fernox Combimag scale reducer on the cold supply to combi boilers, and inhibitor in the primary circuit. We include this in all our new combi installations in the area.