Look-up › Cold weather and hard water
Hard water treatment
Checked against its sources: Updated
In a hard water area, a new boiler's feed water should be treated to reduce limescale under England's Approved Document L, which defines hard water as over 200ppm and asks for "suitable measures" rather than one product.1 Wales and Scotland have similar wording.23
- Covers
- England, Wales and Scotland, each with its own document. No Northern Ireland rule was confirmed. Hardness classes are for England and Wales.
- You
- Find your hardness from your water company, and ask what treatment a quote includes.
- A professional
- Choosing and fitting treatment on the boiler's feed, by the installer. A qualified plumber for a softener.
1 What the rule asks for
Approved Document L for England, 2021 edition, says that in hard water areas suitable measures should be taken to treat the feed water to water heaters and the hot water circuit of combination boilers to reduce limescale1. That sits in the commissioning paragraph, so the installer deals with it at fitting. From 24 March 2027 the 2026 edition keeps the point at para 7.84. Wales has the same treatment wording in its 2022 edition2. Scotland's 2022 compliance guide has a 200ppm rule, read from a consultation copy3.
The wording is "treatment". Trade material often says a softener is mandatory, but the Approved Document does not. An older Compliance Guide of 2013 for England and Wales listed chemical inhibitors, combined inhibitors, polyphosphate dosing, electrolytic scale reducers and water softeners as options, and the 2021 Approved Document no longer cites that guide7. Quotes may offer any of these; ask how the choice meets the paragraph for your country. Building rules for a new boiler has the wider picture.
2 Magnetic devices
The 2013 list does not name magnetic devices. The DWI says they do not soften water and were designed originally for large industrial systems, not homes, which have long periods with no flow5. No source read shows that one meets the rule, so none is claimed here.
3 Knowing your number
| Class | Range |
|---|---|
| Soft | up to 1005 |
| Slightly hard | 100 to 1505 |
| Moderately hard | 150 to 2005 |
| Hard | 200 to 3005 |
| Very hard | more than 3005 |
The DWI uses mg/l and Part L uses ppm; treating the two as equal is an inference from how they are used, not something either document says. UK regulations set no standard for hardness in drinking water5. Your water company is the place to ask for a figure.
4 If you choose a softener
The DWI advises keeping an unsoftened supply at the kitchen tap for drinking and cooking, because softening adds sodium, and says softened water should not be used for baby feeds6. It also advises a qualified plumber, checking with the water company first, and regular salt top-ups and backwashing6. Softened water at 100 mg/l or less can be more corrosive to pipework5. Worcester requires a bypass so that its heating systems are filled with untreated water8. See limescale in a boiler for what makers say.
Sources
- Volume 1 of Approved Document L, para 8.8 and glossary, in-force 2021 text read 3 November 2026.
- Building Regulations Part L guidance, Welsh Government, Volume 1, 2022 edition read 3 November 2026.
- Scottish Government consultation copy of the 2022 Scottish compliance guide read 3 November 2026.
- Volume 1 Dwellings, 2026 edition, England, para 7.8 read 3 November 2026.
- Water hardness page, Drinking Water Inspectorate, England and Wales read 3 November 2026.
- Domestic water filters and softeners, Drinking Water Inspectorate read 3 November 2026.
- Domestic Building Services Compliance Guide, 2013 edition, hosted by the Welsh Government read 3 November 2026.
- Worcester Greenstar 4000 manual, hard water tip, p6, 2020/09 read 3 November 2026.