TMV - Thermostatic Mixing Valve
What is a TMV?
Thermostatic Mixing Valves - TMVs - come in various designs, each suitable for different applications. Some for installation prior to the outlet (i.e. tap or bath) or some are combined in the actual tap or shower. Either way they so the same job, and all work in the same way.
The TMV takes a hot and cold supply and blends the water to a ‘safe’, controlled outlet temperature which is usually determined by the installer who will commission the valve to the desired temperature. This then ensures the temperature of the water will not exceed the set point.
Most TMV’s also have a failsafe mechanism so if there is a failure with either the hot or cold water supplies the valve will close off, allowing virtually no water to pass through and hence protects the user and at the same time highlights the fact that there is a problem with the plumbing system.
Why are TMVs so important?
TMVs accurately control water temperatures for bathing, showering and hand-washing. They maintain pre-set temperatures - even if water pressure varies when other appliances are used. TMVs can be fitted under baths and basins, be part of shower fittings, or as a feature of exposed hot and cold water mixers.
TMVs help to prevent scalding, which can cause very serious and even fatal injuries. Every year around 20 people die as a result of scalds caused by hot bath water and a further 570 suffer serious scald injuries.
Installation of TMVs is recommended in hospitals, care homes and schools.
How does a TMV work?
Three possible types of mechanism are used to operate a TMV.
The wax capsule mechanism expands and contracts according to the temperature of the incoming water and thus, via the valve’s ‘shuttle’ system, will maintain a stable temperature of the outgoing water.
The bi-metallic coil mechanism also reacts to temperature changes of incoming water and adjusts the proportion of hot and cold to maintain a stable temperature.
The electronic/digital TMV uses a system of motorised pumps and valves to control the outgoing water temperature.
What do they look like?
TMVs come in a variety of designs.
• T-pattern valves are typically situated under the washbasin or bath to control the temperature of the outgoing water.
• TMVs may be found within the body of the mixer tap itself.
• TMVs are most commonly used in shower controls, either sequential or dual control for built-in or exposed models.
What factors affect the choice of product?
Many factors affect the final choice of TMV but all of them should comply with the Water Supply (Water Fittings) Regulations 1999 and the Water Bye-Laws Scotland 2000. Also there are a number of British and Industry Standards which cover the performance and safety aspects of TMVs.
TMVs may be approved to the appropriate ‘BuildCert’ Scheme. In domestic situations the valve must comply with the specifications laid down in TMV2 and in the more strict applications of commercial or public buildings TMV3 becomes relevant.
Other factors affect choice. Water Efficiency and ‘Green Issues’ are becoming increasingly important. TMVs also need to matched to the existing plumbing system. For instance a TMV designed to work in a high pressure system will not work effectively in a low pressure system. The valve mechanism will not respond to the low pressure water supply, and may even become dangerous.
It is also important the boiler system is considered. A TMV will work well with a combi boiler provided that it is of the ‘fully modulating’ type.
How are TMVs installed and maintained?
The correct installation, commissioning and maintenance of a TMV is critical to its correct operation. Water supplies must be connected correctly – if they become swapped the TMV may fail to do its job and scalding could result. Supply pipes should be flushed after installation to remove debris and service valves should be installed to allow regular and effective maintenance.
What temperature are TMVs set to?
Temperature stability is key to the operation of TMVs and wherever they are used they must maintain a set mixed hot water temperature of ±2°C.
In order to prevent scalding, the TMV must default to shut off in the event that the cold water supply is lost.
BS7942 and NHS D08 show tables which indicate the maximum temperatures relating to different bathroom applications:
• A TMV controlling water to a washbasin or shower should be set at a maximum of 41°C
• A TMV controlling water to a bath should have a maximum of 44°C
• A TMV controlling water to a bidet should be set at a maximum of 38°C.