How TMVs maintain a safe water supply?

Anyone that has been scalded knows how painful it can be. Bath and shower water above 44°C increase the risk of scalding particularly among young children, the elderly and other vulnerable groups. As a matter of fact, an average of 20 people die in the UK every year as a result of scalding and around 570 people suffer serious injuries.

The risk of scalding increases rapidly the higher the temperature, with particular care required to manage these risks where water temperatures are circulated above 50°C.


It is critical that domestic, commercial, and public buildings have water safety mechanisms in place to ensure the safety of the health and safety of users. Hot water incidents can easily be minimised by the use of thermostatic mixing valves (TMVs).

What are Thermostatic Mixing Valves (TMVs)?

Thermostatic Mixing Valves are special valves used in plumbing systems to blend hot and cold water to control outlet temperatures to safe levels. They are designed to maintain the desired water temperature regardless of the fluctuation of input pressure and flow rates.

TMVs are used in schools, health and social care environments to protect people by reducing the risk of scalding from very hot water from showers, baths and wash hand basins.

TMVs are fast becoming a crucial part of health and safety management for both domestic and commercial buildings – not only do they safeguard against scaling, they also reduce the risk of water-borne diseases such as Legionnaires’ disease.

Why are TMVs so important?

What makes TMVs so crucial is the fact that they allow hot water to be circulated at high temperatures to prevent the growth of legionella bacteria in water systems and at the same time providing water at a safe temperature from outlets to prevent scalding.

To minimize the risk of legionella outbreak, the water must be stored at a high temperature. Water temperatures above 60°C can kill legionella bacteria and prevent them from colonizing a water system.

However, these temperatures are too hot for most applications and can result in scalding. Children and the elderly are particularly at risk accounting for almost two-thirds of fatalities resulting due to scalding. As children have gentle skin and not able to withstand higher temperatures, they can suffer a scald more quickly and often the burn is much deeper.

Therefore, water must be stored at a high temperature and must be delivered to the point to use at a lower temperature.

TMVs help solve these concerns. They can be installed before water outlets allowing hot water systems to run at higher disinfection temperatures which can be mixed with cold water at the point of use to achieve a pre-set safe temperature from the outlet.

TMVs offer an added advantage in terms of providing safe outlet temperature even when the water pressure and flow rates fluctuate due to the simultaneous use of other appliances.

How does a Thermostatic Mixing Valve work?

The desired temperature of the shower, tap or other water outlet is pre-set on the TMV. This is typically between 39°C 43°C, which is the recommended temperature range for hot water outlets where a TMV is installed.

As hot and cold water enters the valve, the thermostat inside the TMV starts to expand and contract as the water temperature changes. As the valve expands and contracts, this constantly adjusts the temperature of the water coming out of the tap, blending the hot and cold water to exactly the pre-set temperature.

TMVs also incorporate an essential failsafe mechanism that will automatically shut down the valve if either the cold water or hot water supplies fail.

This ensures that people are protected from dangerously high hot water temperatures (interrupted cold water supply), and thermal shock from cold water (interrupted hot water supply).

Where are TMVs fitted?

The highest risk of scalding is with full-body immersion. TMVs are fitted in all new building which have baths and showers.

In many schools, hospitals, and health care settings, TMVs can also be fitted to wash hand basins and sinks to protect vulnerable groups from scaling.

TMVs are usually installed close to the point where the shower or bath water comes out so that it is mixed just before discharging from the tap or showerhead. If it’s located too far away, the water has a higher chance of stagnating within the pipe at a temperature warm enough (but not too hot or too cold) to encourage legionella growth.

The following should be considered where TMVs are fitted;

  • If practicable TMVs should be incorporated directly in the tap fitting and mixing at the point of use is preferable;
  • TMVs fitted with low flow rate spray taps on hand wash basins increases the risk;
  • TMVs should be fitted as close as possible to the point of use to minimise the amount of stored blended water;
  • A single TMV serving multiple outlets can increase the risk;
  • Where TMVs are designed to supply both cold and blended water an additional separate cold tap is seldom needed and can become an infrequently used outlet.

Understand if your water systems are at risk. Get a risk assessment done today!

Choosing the Right TMV

There are two main types of valves, TMV2 and TMV3.

TMV2 valves are designed and tested to be used in domestic and commercial environments to reduce the risk of scalding. They work with water pressures of between 0.1 bar and 5 bar. TMV2 certified valves need to be annually tested in commercial applications.

TMV3 valves are used in high-risk commercial healthcare environments. They offer a high level of protection, reacting much more quickly in shutting off the flow of water if the cold water fails, or a safe temperature is exceeded.  TMV3 fittings are designed to work with much higher water pressures, from 0.2 bar to 10 bar.

TMV3 valves have more rigorous testing processes with periodic checks six to eight weeks after installation. These checks include a thermal element endurance test, a hot spike test, a response on temperature adjustment check, a stability test and a fail stop valve check.

If the valve isn’t checked within the required timeframe it will be marked as non-compliant and no longer suitable for use.

TMV3’s adhere to the NHS D08 regulatory standard, whereas the TMV2 example adheres to two British Standards, but will not meet the NHS requirements.

Maintaining & Servicing TMVs

TMVs play a very valuable role in delivering hot water at safe temperatures, however, they shouldn’t be fitted and then forgotten about. Ensuring the valves are serviced and maintained regularly can help ensure that they are still delivering water at the required temperature to prevent scalding.

It is imperative that all TMVs are well serviced and maintained (at least once a year) to ensure they are working as needed and maintaining a correct temperature within the wider system – as well as ensuring no build-up of dirt that could lead to additional legionella growth.

Maintenance should take into account local conditions (such as a potential build-up of hard water/limescale) and the risk of valve failure.

The outlet temperature should be tested to check that there is no significant change and that the failsafe shut-off activates as intended. If the TMV doesn’t meet these requirements then a full TMV service plus recommissioning, or a new valve will be needed.

As such, the usual approach to monitoring the presence of legionella bacteria and maintaining the water system should be reviewed to account for any new valves that are fitted.

Regular testing and TMV servicing will help to ensure scalding risks are managed effectively and should be seen as an essential component in and regulatory compliance and the Legionella control programme.

If you have questions regarding the installation or servicing of TMVs, feel free to speak with one of our consultants.