Limescale in a boiler

1 What scale does

The Drinking Water Inspectorate says hard water "causes scaling in hot water systems", kettles, irons and appliances, and that scaling shortens heating element life and makes appliances less efficient¹. It also reports the World Health Organization's 2017 statement that water at 200 mg/l or higher will produce scale¹. South East Water says hardness can build up limescale in kettles, boilers and hot water pipes².

No official source read gives a figure for how much efficiency or life a scaled boiler loses, so none appears here.

Scale is a different problem from the black iron-oxide sludge in heating water. Baxi's quick guide treats the two apart: a filter and inhibitor for sludge, and a device on the mains water for scale³. See sludge, inhibitor and filters.

2 What makers say

Those are claims and instructions from each maker for its own boilers. They do not show that any one device suits another boiler.

Worcester adds a point worth raising with an installer: a salt-based softener should follow the local water company's requirements, and a bypass valve must be used so the heating system is filled and topped up with untreated water⁴. A Worcester oil boiler installation manual says outright that salt-based softened water must not be used to fill the central heating system⁷.

3 What to do about it

Treatment is covered under hard water treatment, including what the building rules expect when a boiler goes in. Scale inside a boiler is an engineer's job, because going inside the casing is not a homeowner task on any gas boiler; see who may work on a boiler. Mention scale at the annual boiler service if the area has hard water.