Frequently Asked Questions on Thermostatic Mixing Valves
TMVs accurately control water temperatures for bathing, showering, hand-washing and bidets. They are designed to maintain the desired water temperature, even when the incoming water pressures/flow rates change. By contrast, a mechanical mixing valve cannot adjust to changes in supply temperature, pressure or flow rate.
Digital mixing valves are also available. For different types of TMV see questions 2 and 4.
TMVs can be fitted under baths and basins, be part of shower fittings, or as a feature of exposed hot and cold water mixers.
Hot and cold water entering the valve is mixed to a temperature pre-selected by the user or installer. A thermally sensitive mechanism within the valve automatically proportions the amount of hot and cold water entering to produce the required blend. The mechanism then automatically compensates for any reasonable variations in supply pressures or temperatures within the design range to maintain the pre-selected temperature. In the event of cold water supply failure, the thermostatic mixing mechanism will automatically shut down the flow to prevent discharge of dangerously hot water. The flow will also be shut down in the event of a hot water supply failure to prevent thermal shock. In the event of a sudden supply temperature increase (‘spike’), the TMV will quickly (but not absolutely instantly) adapt and deliver water at a safe temperature. For more on temperature spikes see question 15.
A temperature sensitive element which expands or contracts depending on the temperature of the water surrounding it. When the thermostatic element senses a temperature change, it moves a piston which changes the proportion of hot and cold water being mixed in the valve. This movement enables the valve to remain stable and to shut down in case of cold or hot water failure.
Usually connected to the thermostatic element, the piston moves back and forth over the cold and hot ports of the valve, changing the proportion of hot and cold water entering the valve depending on the temperature of the water.
When the thermostatic element expands it moves the piston under its own energy and compresses the return spring; when the thermostatic element is cooled, the thermostatic element contracts and the return spring pushes the piston back.
Many thermostatic mixing valves have a separate temperature adjustment (usually beneath a lock shield cover). Typically, this can be adjusted to change the position of the piston and therefore the proportion of hot and cold water entering the valve.
Digital Mixing Valves
Digital thermostatic mixing valves control water temperature and flow by means of accurate temperature measurement and electrical/electronic control. As with all TMVs they can comply with TMV2 and TMV3 approval schemes and maintain the same level of thermostatic control. Digital products can utilise manual or touch free control by the user, the latter providing a degree of infection control for surface transmitted infections. The digital nature of these products can also provide additional duty flushing or thermal disinfection regimes, programmed by facilities management personnel.
Over three quarters of severe scalds are suffered by children under five years of age, and almost three quarters of the fatalities are people aged 65 and over. By limiting water temperature and stopping sudden increases in water temperature, TMVs help to prevent scalding, which can cause very serious and even fatal injuries. Hot bath water is responsible for the highest number of fatal and severe scald injuries in the home. Every year around 20 people die as a result of scalds caused by hot bath water and a further 570 suffer serious scald injuries.
Young children and older people are most at risk from bath water scalds because their skin is thinner and therefore less tolerant to higher water temperatures than that of other age groups. As a result, they sustain scalds more quickly, at lower water temperatures and often with a greater depth of burn.
People with a reduced ability to perceive risk or react to hazardous situations – for example those with mental or physical disabilities – are also at greater risk of injury. TMVs help to protect the population from scalding injuries.
There are third party approval schemes that are based on regulatory requirements.
The latest version of Building Regulations (Part G, 3.66 - Prevention of Scalding) states: “acceptability of in-line blending valves can be demonstrated by compliance with the relevant European standards EN1111 and EN1287.....to demonstrate that the maximum temperature of 48°C cannot be exceeded” (note – this applies to baths only). BuildCert approval schemes (called TMV2 and TMV3) require TMVs to comply with these standards. BuildCert is the only third party approval company in the UK, and their approval schemes are widely followed. The schemes are designed to meet different needs, and cannot be directly compared.
The BuildCert TMV2 scheme is targeted at TMVs used in domestic properties. It ensures that valves meet the requirements of either or both BS EN 1111 and BS EN 1287, and requires adequate documentation, manufacturing processes and testing. Additional information on the TMV2 scheme can be found in the TMV2 section of the Buildcert website
The BuildCert TMV3 scheme is targeted at TMVs used in healthcare and commercial settings. It ensures that a thermostatic mixing valve complies with the requirements of D 08, an NHS Model Engineering specification, so valves approved under this scheme can be used in NHS institutions. Additional information on the TMV3 scheme can be found in the TMV3 section of the Buildcert website.
You can search for approved valves in the approval section of the Buildcert website
A standard, closely related to D 08, is BS 7942:2011 Thermostatic Valves for use in care establishments (the previous version is BS 7942:2000). NHS document D 08 is based on this, and BS 7942 contains similar content but is a British Standard. The TMV3 scheme references D 08, not BS 7942.
Two NHS Health Technical Memoranda recommend the use of TMVs in the specification and design of new and existing sites. They are: HTM 04-01: The control of Legionella, hygiene, “safe” hot water, cold water and drinking water systems; and HTM 64: Sanitary Assemblies. They recommend the use of TMVs compliant with D 08 for most hot water outlets.
Building Regulations Part G for domestic properties state that in new or materially changed homes, the maximum bath water temperature must not exceed 48°C.
NHS institutions have a duty of care that requires hot water temperature to be controlled. Maximum temperatures are 41°C for handwashing and showers, 44°C for baths and up to 46°C only for supervised baths. Injury from scalding is on the Department of Health’s ‘never events’ list, which means it must never happen and can be prevented in public hospitals. The requirements of D08, an NHS Model Engineering Specification for TMVs, are covered in the TMV3 section of the Buildcert website. These signify strong approval for the effectiveness of TMVs.
Care homes may also have a duty of care requirement that can encourage them to use TMVs, both to limit the risk of scalding and to ensure that water is at a comfortable temperature that does not discourage hand washing. Where there are vulnerable residents bathing, BEAMA recommends the use of TMV3 valves.
In schools, there is no legally prescribed setting for TMVs but there is a legal ‘duty of care’ obligation on the school to ensure that the facilities can be used safely. The temperature of the hot water at point of use should be based on a risk assessment that the school is required to undertake to identify potential risks and actions to ensure users will be safe.
Local authorities may also refer to ‘Building Bulletin 87, 2nd Edition Guideline for Environmental Design in Schools’, which recommends a maximum hot water temperature for school wash basins of 43°C (this would mean TMVs being set to 41°C to account for a possible ± 2°C range of accuracy – BEAMA and BRE also recommend 41°C as the maximum). However, 41°C or 43°C may be too hot for children to comfortably wash their hands. Therefore, the risks of children not washing their hands should also be taken into account in a risk assessment, meaning a school may choose to set TMVs at a lower temperature than the maximum recommended by Building Bulletin 87 and other guidance.
While there are some differences between Building Regulations in Scotland and Northern Ireland compared to England and Wales, the maximum hot water regulation of 48°C applies in all areas of the UK for TMV2s, and was established in the Scottish regulations first.
The NHS in England, Northern Ireland, Scotland and Wales all endorse the health guidance note that recommends TMVs as a way to help health employers meet their duty of care.
Homeowners and landlords are not currently required to ‘retrofit’ TMVs in existing buildings.
In a normal office environment the decision whether or not to retrofit TMVs should be decided as part of an overall risk assessment of the building. It is not a legal requirement to fit TMVs but it may be appropriate as part of a building owner’s duty of care.
The BuildCert TMV2 scheme states:
“It is a requirement that all TMV2 approved valves shall be verified against the original set temperature results once a year.
When commissioning/testing is due the following performance checks shall be carried out.
- Measure the mixed water temperature at the outlet.
- Carry out the cold water supply isolation test by isolating the cold water supply to the TMV, wait for five seconds if water is still flowing check that the temperature is below 46°C.
If there is no significant change to the set outlet temperature (±2°C or less change from the original settings) and the fail-safe shut off is functioning, then the valve is working correctly and no further service work is required.”
In essence, what is required by TMV2 is an annual functionality check, but many BEAMA members recommend annual preventative servicing.
Several factors may determine the required service frequency of a TMV, most notably water hardness and frequency of use.
TMV3 valves must also receive a regular in-service test to check they are performing adequately. If they are not, the valve must be serviced. In the absence of other instructions or guidance, the regime described in D 08 (an NHS Model Engineering Specification) should be used. D 08 states that the first test must take place 6-8 weeks after commissioning, and the second after 12-15 weeks. Following this, there is no fixed required test frequency, but the valve should be tested at intervals between which there is shown to be little or no change in mixed water temperature. Depending on the change in mixed water temperature between commissioning and the second test, D 08 recommends the third test to take place between 18 and 28 weeks after commissioning.
TMVs are fitted within or near a tap or shower unit.
For TMVs not within a unit, a useful guide to recommended distances from a unit is the Health Technical Memorandum 04-01, published by the Department of Health, paragraph 9.49 which reads as follows:
“Particular attention should be given to ensuring that pipework containing blended water is kept to the minimum. Generally, the downstream dead-leg should not exceed 2m, and the complete length of the spur should not exceed 3m.”
Furthermore, the TMVA Code of Practice (available here), Section 4.2 ‘Group Mixing’, states:
- The operation of one or more outlets should not affect the operation of any other outlet.
- When one valve is used to supply mixed water to a number of outlets the length of the pipe run and the volume of mixed water after the valve should be kept to a minimum.
- The maximum pipe run after the mixing valve should be such that the required mixed temperature, at the furthest outlet, should be reached within 30 seconds.
TMVs will respond quickly to changes in selected temperature. This is verified by the respective TMV schemes that ensure certified products provide water within known anti-scald temperature ranges by conducting third party testing.
It is not recommended that users make very sudden changes to their shower temperature control. However, if a user tries to change their water temperature straight from cold to the maximum temperature, occasionally this can cause a temporary temperature increase of 7°C above the chosen temperature, potentially supplying water at a higher temperature than the recommended maximum. The NHS D08 TMV Model Engineering Specification makes reference to this, and permits a 7°C temperature ‘spike’ to last for 1.2 seconds, with 50°C permitted for a maximum of 0.5 seconds. Therefore as long as a TMV ensures temporary temperature spikes last no longer than these times, it will still be safe.
Non-return valves can be held open by debris or grit. In this case, if the mains pressure and hot water pressure differ, hot water can be forced back down the pipe the wrong way through a non-return valve, meaning the water temperature may no longer be regulated. It is possible that a failed integral non-return valve in a TMV can cause the temperature control system to fail, but there are a range of other potential causes such as an open vented system, localised expansion from water heater, a failed seal in a non-return valve, a particulate contamination and incorrect plumbing.
Correct installation and servicing are also essential to ensure that filtration measures and checks on the product are performed.
It is important that the installation method in the manufacturer’s instructions is respected as failure to do so may prevent the valve from operating correctly. Common installation errors are:
i. Failure to use flow regulators when installing the TMV product to a combi boiler system. This may cause performance issues during winter months.
ii. Failure to remove flow regulators when installing to a vented system, ie a traditional immersion heater. This may reduce the water flow.
All products should be installed according to the manufacturers’ instructions, including supply conditions. If you have any doubt regarding product installation, then please contact the manufacturer.