All About The TMV3 Scheme & NHS Model Engineering Specification D08

History of the NHS’s D08 Specification

In 1996 the UK market for thermostatic mixing valves (TMVs), was very similar to the rest of Europe and North America: regulations and standards were either non-existent or hopelessly full of holes. The Department of Health soon realised that the issues of scalding within NHS properties was a serious threat to both patients and staff, and that the tempering valves being fitted were woefully inadequate to protect end users from scalding. Part of the problem was that the tempering valves used at the time were mainly of continental or North American design and they could not cope with the vagaries of UK supply conditions such as low pressure from gravity fed systems or having a mixed pressure system (ie high pressure mains cold and low-pressure gravity hot water). Another far bigger problem was that the UK standard at the time, BS 1415 part 2, was a self-certification standard so anyone could claim compliance; the standard was also very weak in terms of how much of the functionality of the valve was tested and virtually everything was done at low, equal pressures.

To address this problem the NHS Estates enlisted the help of WRc and industry, including Reliance Water Controls, to discuss what could be done to make sure that the thermostatic mixing valves fitted in hospitals and nursing homes were:

(a) of suitable quality and built in an ISO 9001 accredited facility,
(b) were capable of functioning under extreme supply conditions that were and are still common in the UK,
(c) were identifiable and traceable after installation, and most importantly
(d) were independently third party tested by an approved test house and not self-certified.

From these initial discussions the 1997 NHS model engineering specification D08 was born. The key to why the D08 specification has worked so well is because they were written together with the UK regulators, test houses, industry and NHS Estates who understand that the unique qualities found in UK plumbing systems create challenges that are different from anywhere else in the world. The specification was written to cover performance at both high and low pressures, and with mixed high and low pressures and to cover a multitude of other requirements never before considered, such as thermal shut down on hot or cold supply failure (ie the failsafe test), testing for hot spikes, testing for temperature control at equal and unequal pressures, testing for temperature stability under changing inlet temperatures as well as many others.

In the years since the D08 was first written it has had some updates, but it has proven its validity and quality as the core of the specification has remained largely unchanged from the 1997 version and it has become a benchmark for many countries upon which to base their own national standards for thermostatic mixing valves.

The TMV3 Approval Process

In order for a manufacturer to get a valve approved to the D08 specification and market it as a TMV3 scheme approved product, there is a very difficult approval process to go through. It starts with the application by the manufacturer in writing that they would like to submit the valve for testing to TMV3. All the relevant information is submitted, including material specifications, drawings, marketing information, instructions etc. Once the application has been processed they will then ask the company to submit samples of the valve for testing. Once the valve is on the test rig it is subjected to a battery of tests including but not limited to:

  • Endurance testing of the thermal element
  • Temperature overshoot on start up (the hot spike test)
  • Temperature response on temperature adjustment
  • Thermal shut off
  • Temperature stability on changing pressures
  • Temperature stability on changing inlet temperatures
  • Temperature stability on reduced flow-rates.

As the above list shows, TMV3 is very comprehensive in its performance testing, and necessarily so for a valve that is really a safety valve.

Reliance UK TMVs

Reliance UK are one for the market leaders for Thermostatic Mixing Valves (TMVs), providing high quality solutions for a variety of applications & sectors. The Ausimix, Heatguard Dual and Heatguard Style as just some of the key products we supply to a variety of customers and you can find out more about them in the Products area of our website or by contacting our friendly customer services team using the short online form provided below.