A common concern we’re asked is: “Will softened water damage my boiler?”
The short answer is no — in fact, softened water can protect your boiler and improve efficiency.
Extensive research now confirms that softened water does not harm boiler heat exchangers. Some manufacturers, including Viessmann, actively recommend filling boilers with softened water during installation. Earlier concerns about increased corrosion in aluminium heat exchangers have been disproven. A British Standards Institute corrosion report (December 2012) found no long-term adverse effects from softened water on aluminium heat exchangers.
This misconception originated from an outdated standard, BS 7593:1992, which was revised in 2006. The updated BS 7593:2006 permits the use of softened water in heating systems, including those with aluminium heat exchangers, provided a suitable chemical corrosion inhibitor is added and maintained.
Leading inhibitor manufacturers such as Fernox and Sentinel have long confirmed their products are fully compatible with base-exchange softened water. Fernox Technical Bulletin 21 and official Sentinel statements confirm their inhibitors provide effective protection when correctly dosed and serviced.
Several boiler manufacturers have also softened their position, moving from “cannot be used” to “not advised.” Worcester Bosch, for example, confirms softened water is acceptable on the domestic hot water side of their boilers, with no negative effects. Other manufacturers, including Grant UK, Ferroli, and Vaillant, raise no objections to softened water use.
Before installing a new boiler, we always recommend checking directly with the manufacturer. Some plumbers and heating engineers may not yet be fully up to date, despite this information being widely published in trade journals such as Professional Plumbing and Heating Installer and H&V News.
The UK Water Treatment Association (UKWTA) confirms that softened water is suitable for central heating systems when used with an appropriate inhibitor. While some manufacturers previously cited warranty concerns based on the 1992 standard, the UKWTA has since worked with boiler manufacturers to clarify guidance and recommends confirming compatibility, particularly for stainless steel heat exchangers.
The Energy Saving Trust also recognises the benefits of softened water. Limescale caused by hard water significantly reduces boiler efficiency, increases energy use, and raises carbon emissions. Ion-exchange water softeners remove hardness salts, preventing limescale buildup and improving appliance performance. This evidence led to changes in UK Building Regulations (2006) requiring water treatment in hard water areas.
Research by the University of Plymouth demonstrated the impact clearly:
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No limescale: 90 minutes/day to supply hot water
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5 mm limescale: over 4 hours/day
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10 mm limescale: over 6 hours/day
This inefficiency can add hundreds of pounds per year to energy bills. As a result, water softeners became mandatory in new homes in hard water areas in 2006, with many local authorities now installing them during property refurbishments.