I'm writing to address a common question we receive: "Will softened water cause a problem in my boiler?" The short answer is no, in fact, it will help.
Recent research confirms that softened water is not detrimental to boiler heat exchangers. For instance, Veissmann boilers even recommend filling with softened water during installation. The previous concern about softened water increasing corrosion in aluminum boiler heat exchangers is now being debunked. The British Standards Institute's corrosion report from December 12, 2012, indicates no long-term adverse effects on aluminum heat exchangers from softened water.
This historical misconception stemmed from an outdated British Standard (BS7593:1992), which was revised in 2006 to allow softened water when a suitable chemical inhibitor is used. Leading corrosion inhibitor manufacturers like Fernox and Sentinel have consistently formulated their products to be effective with softened water. Fernox Technical Bulletin 21 and Sentinel's statement both confirm that their inhibitors adequately protect heating systems filled with base-exchange softened water when correctly dosed and maintained.
Some boiler manufacturers are also relaxing their stance on softened water, moving from "cannot" to "not advised." While Worcester Bosch's FAQ on water softeners has some ambiguity regarding filling the central heating system, they do state that softened water is acceptable for the domestic heat exchanger side of the boiler with no negative effects. Other manufacturers, such as Grant UK, Ferroli, and Vaillant, do not have issues with softened water.
Before purchasing a new boiler, we recommend consulting the manufacturer for advice on compatibility with softened water. It's important to be aware that some plumbers and heating engineers may not be fully updated on the latest information regarding softened water in boilers, which is being communicated through trade publications like "Professional Plumbing and Heating Installer" (July 2013) and "H & V News" (October 2, 2013).
The UK Water Treatment Association (UKWTA) states that softened water can be used in central heating systems. British Standard BS 7593:2006 now permits filling systems, including those with aluminum heat exchangers, with softened water, provided a specifically formulated corrosion inhibitor is added and properly maintained. While some boiler manufacturers have historically claimed warranty invalidation due to softened water based on the outdated 1992 standard, the UKWTA has contacted all manufacturers to seek clarity. They recommend specific boilers for consumers using water softeners and advise direct confirmation from manufacturers for stainless steel heat exchangers.
The Energy Saving Trust acknowledges the benefits of softened water in modern heating systems. Studies consistently show that using hard water wastes energy and increases carbon emissions due to limescale buildup. The UKWTA highlights that ion exchange water softeners effectively remove hardness salts, preventing limescale formation which significantly reduces the efficiency of domestic water heating appliances. This led to changes in UK Building Regulations (Domestic Heating Compliance Guide – May 2006) to require feed water treatment in hard water areas to inhibit limescale.
For example, research by the University of Plymouth found that a boiler with no limescale took 90 minutes a day to supply hot water, while a boiler with 5 mm of scale took over four hours, and with 10 mm of scale, over six hours, adding hundreds of pounds to annual gas bills. As a result, the government required water softeners in new homes in hard water regions in 2006, and several local authorities are now installing them during property refits.