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.

The Conflict: Legionella or Burns?

Not much of a choice, but this conflict is at the heart of every plumbing system. Water heated and stored at sufficiently high temperatures to control and kill bacteria such as legionella, but it can also cause severe scalding injuries in a matter of seconds. If water is heated and stored at safe non-scalding temperatures, this provides the ideal medium and temperatures for bacteria growth.

Legionnaire’s Disease

Legionnaire’s Disease comes from a naturally occurring organism that can be found in low levels in the water supply. It is a bacterial disease that is contracted from small droplets of water that are contaminated with legionella bacteria and which have become suspended in the air and may cause pneumonia. The time from infection to start of the illness (the incubation period), is between 2-10 days and the disease can be particularly deadly to the very young or old, especially if infirm.

Aerosol droplets that allow transmission of legionella are found in: whirlpool spas, showers, cooling towers, taps with sprays etc. Legionella can grow in any water system that is not properly maintained.

Elimination of legionella from a system

Methods to eliminate legionella include chlorination. This method of treatment is used when a system is cleaned prior to commissioning and has real disadvantages if a system is being used. Another option is to use heat treatment (i.e. running water above 70°C for 30 minutes in the whole sanitary system), though this can also have serious drawbacks if no temperature control is used on the outlets.

If temperature control down to a completely safe level is exercised at the water heater (i.e. turning the cylinder down to a non-scalding temperature so that all the stored water is below 50ºc), the following will occur:

  • The system will not comply with building regulations,
  • Water usage will increase as users run taps for longer periods, in the hope of getting hot water,
  • Users will not get a hot bath unless the water heater is close to the bath because of the temperature loss from the pipework between the water heater and the point of use,
  • Washing up becomes a problem as lukewarm water will not shift grease.

Building regulations state that the circulation of hot water must be at temperatures sufficiently high to stop the legionella that naturally occurs in the water supply from multiplying to a level that will cause health problems to susceptible people. In the UK building regulations stipulate that hot water should be stored at no less than 60ºC and circulated at no less than 55ºC to prevent the growth of legionella.

The Burns Issue

Every year 23 people are killed by being immersed in hot water by mistake by a carer or nurse or by falling in to a bath and not being able to get out quickly enough. These are sobering statistics when you consider that the burns suffered by scald victims are every bit as painful and destructive as those suffered by victims of fires or explosions.

Almost 90% of the 570 people who suffer serious scalds each year, which require hospitalisation, are children. Other groups considered to be at high risk are the elderly and disabled. While children are normally scalded because they do not identify or understand the risk, the elderly and disabled are more likely to be injured or killed as a result of not being physically able to remove themselves from the scalding situation when they find themselves in danger.

A typical scenario is a carer or nurse filling a bath of hot water and leaving the person to get in by themselves; quite often the bather will sit on the side of the bath and swing their legs over and into the water. At 60 degrees an adult will suffer third degree burns after less than six seconds of immersion, with an elderly person this time is likely to be even less due to the more sensitive nature and reduced thickness of their skin. Regardless of skin sensitivity, however, it is clear that anyone who is even marginally impeded in their movements is going to suffer a serious scald injury at such temperatures. 90% of all people who are killed each year by scalding are the elderly aged 65 and over.

Legionella or Burns? The Solution

Water below 50°C can be considered ‘safe’ as even for a child to receive a second degree burn it would take 45 seconds, however water stored below 50°C creates a breeding ground for legionella bacteria to breed.

The best solution to both problems is to fit a thermostatic mixing valve at the point of use (i.e. local to the taps). This will allow the hot water to be stored at a sufficiently high temperature in the water heater to prevent bacteria growth but the TMV will mix cold water and hot together and discharge it out of the tap at a controlled and stable temperature, typically 38-44°C in a hospital or nursing home, to prevent scalding the end user.

I addition, Reliance UK also offer an ‘Anti-Legionella Valve’ designed to convert a standard single connection expansion vessel into a ‘flow through’ type so water is continually renewed to prevent water stagnation & bacteria growth.

TIU - Thermal Interface Unit
What is a Thermal Interface Unit (TIU)?

TIUs form part of a Community Heating Network (CHN) that provides central heating and domestic hot water to individual properties or apartments within a single development.

Traditionally in the UK, housing stocks have been built with individual gas combi boilers located in each apartment. Conversely, across mainland Europe Community Heating Networks – (or District Heating Systems as they were previously known), have long been the solution to provide heating and hot water in large community developments, with a centralised energy centre coupled with a low temperature hot water network.

In recent years we have seen a resurgence of CHN developments in the UK as the available technology improves and becomes more popular in helping comply with current legislation for energy efficiency and reduced emissions. Community heating networks operate from a single energy source (the energy centre) that distributes energy in the form of hot water circulating within a primary network throughout a building or housing development. Each individual house or apartment has a secondary network which is connected to the primary network via a thermal interface unit that heats DHW and space heating water via plate heat exchangers to meet the individual demands of the property.

The Radiance TIU range from Reliance UK offer the perfect solution as they can be adapted to exact requirements when an instantaneous or stored hot water system is required in a Community Heating Network installation.

As a leading supplier to the UK heating industry for many years, we have gained a wealth of knowledge and experience in the design and production of innovative products. The RWC brand is recognised for quality and reliability and our TIUs, created specifically to suit UK systems, have been designed, manufactured and tested to ensure they provide the same high levels of satisfaction and longevity.

We understand that no two project requirements are the same, that is why we provide a dedicated technical and design support service to work alongside you. This enables us to develop custom-built solutions to meet the exacting needs of your project. We won’t just focus on the TIUs, we will ensure your whole system operates as effectively and efficiently as possible to meet every demand.

Our Radiance TIUs are manufactured in the UK ensuring lead times are minimal and deliveries can be made to suit a site’s requirements.

High efficiency solution for community heating networks

  • No requirement for gas throughout the building avoids the cost of testing/regulations for installing gas
  • No requirement for flues saves cost and allows for an aesthetically pleasing building exterior
  • No planning constraints
  • Simple design; reduced maintenance
  • Easily integrated with renewable heat sources if desired (CHP, biomass, heat pump, solar)
  • Ultra-safe well-known technology using only low pressure hot water
  • Individual metering options available, including automatic meter reading or a prepayment meter

Are you looking for a Thermal Interface Unit for your project? Use the form provided below and we can help find the perfect solution to meet your needs.