‘No corrosion in boilers’  At last it’s official, after an extensive tests conducted with 2 identical boiler systems, one on hard water and one on softened water, the corrosion coupons proved that soft water does not corrode boilers, even ones with aluminium heat exchangers:-

Here is the statement from UKWTA:-

Above: Typical Coupon Bundle after Removal from Test


It’s official!

Over the lifetime of a modern central heating system, filling with base-exchange softened hard mains water would appear to present no significantly greater risk of corrosion of system metals than filling with hard mains water.

That’s the main conclusion of the Report by BSi following six months corrosion testing in hard and softened mains waters in identical central heating systems.

In accordance with the agreement with BSi, the Report is now being circulated in its entirety to professional bodies and associations in the plumbing industry and the UKWTA is writing to boiler manufacturers to ask them to consider their position regarding the suitability of their boilers with base-exchange softened water in light of the evidence presented.

The UKWTA is also approaching WRc to address the issue of incorporating appropriate tests into the BuildCert test protocol to evaluate the performance of chemical corrosion inhibitors in base-exchanged softened water. The results of the report show that the current ‘open pots tests’ do not reflect performance under normal heating system conditions.

The UKWTA is now embarking upon the testing of proprietary corrosion inhibitor chemicals in the heating systems filled with base-exchange softened water to benchmark background corrosion performance levels as the basis for a future test method and British Standard.

The challenges for the BuildCert experts are now to explain how the rationale behind testing products in the existing 2-week pot test under aerated conditions relates to long term service in the oxygen deficient environment of a real heating system and how to develop a series of appropriate accelerated tests which demonstrate the benefit of adding an inhibitor in reducing corrosion below the background levels.

On behalf of the UKWTA, I would like to take this opportunity to again thank member companies and sponsors who’s financial support in funding the research made this study possible.


Dr. Carl Jasper