Pipes & Heat in Old Cambridge

A reading site on water and heating in older Cambridge and South Cambridgeshire homes. Not a plumbing or heating firm.

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Hard water ยท In brief

Scale inhibitors and magnetic devices: what the evidence covers

A scale inhibitor on a combi boiler aims to limit scale on the hot-water side, but Part L names no device and DWI says magnetic devices do not soften water. Ask for evidence and sale-or-return terms.

Most useful for: Anyone offered a scale reducer with a new boiler or by a salesperson

Compiled from the sources at the end. Updated .

Cut end of a metal pipe with thick layered grey-white scale narrowing the bore.
A cut pipe with thick layers of scale narrowing the bore.

The word "inhibitor" covers two different things, and quotes do not always make the difference clear. Central heating inhibitor goes into the circuit that feeds the radiators. A scale inhibitor or reducer is usually sold for the cold feed on its way to a combi or water heater. Limescale in a boiler sets out which water each one deals with.

What the rules and DWI say

Approved Document L asks for "suitable measures" to reduce limescale in hard water areas when a new heating appliance goes in. It names no specific device, so its wording is no endorsement of any product.

The Drinking Water Inspectorate (DWI) is blunter about one type: magnetic devices do not soften water. It suggests buying on sale-or-return terms and asking for in-home performance data. This site has no independent evidence that any device of this kind works in Cambridge's water, so it makes no such claim for any of them.

Questions worth asking the seller

  • What does it change in the water, and does it reduce hardness at all?
  • Where is the in-home performance data, and was it measured in water as hard as yours?
  • Can it go back if you see no difference?
  • Does the product meet regulation 4 of the fittings regulations?
  • If it is being offered as the Part L measure, why that one?

DWI also warns against door-to-door sellers offering water tests and says to report them to trading standards. If the real aim is softer water, a softener is a separate decision with its own cautions for old pipes.

Sources, read 4 November 2026: Drinking Water Inspectorate: water hardness; Drinking Water Inspectorate: filters and softeners; Approved Document L, Volume 1 (2021 with 2023 amendments).