Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

HWA statement on safety devices for unvented systems

The Hot Water Association has released a statement on the use of temperature release valves following press reports

It said: “An article was recently published in the trade press implying that temperature relief valves would no longer be required on unvented systems.

The Hot Water Association (HWA), which represents the interests of manufacturers of hot water storage cylinders, has had a number of enquiries and requests for clarification.

Unvented systems were introduced into the UK over twenty years ago. They have now achieved a 50 per cent share of the hot water storage market and have an excellent safety record.

We believe that this safety record has been largely due to the fact that under UK regulations the unvented systems have to incorporate two independent levels of safety cut out, in addition to the thermostat used to control the temperature of the stored water.

The safety cut outs have to be configured in such a way that the temperature of stored water is unable to exceed 100 deg C.

The preferred method of achieving the two levels of over temperature control has always been the use of a non auto resetting energy cut out and a temperature relief valve.

The big advantage of the temperature relief valve is, of course, that it is totally independent of the means of energy input and, being a self-contained device, does not have to rely on correct wiring or installation of additional devices.

In the last revision of the UK’s G3 Building Regulations it was acknowledged that the temperature relief valve could, in some circumstances, be substituted by an alternative device with the all-important proviso that it has to be proved to be no less effective.

The Guidance to the G3 Regulations is, however, written in such a way as to promote the use of the temperature relieve valve as the preferred choice of the final high level safety device.

Until recently the UK water supply regulations did make the temperature relief valve an absolute requirement and there has been a recent relaxation of the wording simply to make it more consistent with the text of the G3 Building Regulations.

Unfortunately this has been wrongly interpreted in some quarters as a step change in UK regulatory policy and a green light to the general abolition of temperature relief valves.

Its timing is of particular concern since CLG, who are responsible for the G3 Building Regulations, have recently commissioned an independent review of the safety devices used on unvented systems and are still awaiting the report.

The HWA do acknowledge that if an unvented cylinder is supplied as part of a fully approved ‘secure package’, where the only means of heating the cylinder is indirectly from the boiler, then if the cylinder has an energy cut out and the boiler also has an energy cut out to prevent the primary water from exceeding 100 degrees then the G3 provisions are likely to be met.

If however, as is the general UK preference, the cylinder is also fitted with a back up immersion heater, then an additional device, preferably in the form of a Temperature relief valve, is also required.

Even if no immersion heater is fitted the HWA are concerned about what happens in a few years’ time as inevitably the cylinder will have a longer lifespan than the boiler.                                          

If an alternative replacement boiler were to be fitted this might alter the originally approved package and there will be no temperature relief valve to guard against an improperly specified or wrongly installed boiler.

To summarize, at the moment the temperature relief valve is the only universally accepted high level safety device for unvented systems in the UK and this will remain the situation unless the forthcoming investigations by CLG suggest otherwise.

We believe that even if other ‘no less effective” devices do start to be used, most responsible UK manufacturers will still prefer to fit the temperature relief valve due to its unique qualities in being totally independent of the current and future changes to heat input or subsequent bad workmanship.

Finally, it is important that installers never remove a T&P valve from an unvented cylinder on any installation.

Removal of the T&P valve will void approval of the cylinders compliance with building regulations as it would be a change to the safety features with which the cylinder was tested.”