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Thermostatic Shower Mixing Valves

A very important component to consider when installing or upgrading a shower area.....
Shower Valve Frequent Questions


Compatibility – will they work with your water supply system? Consider the nature of the hot water supply e.g. combination boiler and the minimum Water Pressure – what minimum pressure do TMVs require to give a good flow rate?

Spare parts – what is available and for how long in the life of the product?

Frequently Asked Questions
Q
Why should I install a TMV?

A
A TMV can be used to control the maximum hot water temperature available at the hot tap outlet thus significantly reducing the possibility of scalding the end user. A TMV therefore offers the most cost effective solution to reduce the risks of scalding in the bathroom environment.

Q
What type of TMV do I need to fit and why?

A
Typically in domestic situations a valve that complies with BS EN 1111 and or BS EN 1287 or indeed has been certified as TMV2 or 3 approved is suitable. These TMVs are aimed at providing suitable safety for the average able bodied person in the home. A wide variety of products are available to meet the specific needs of the application. If the end user could be categorised as less able by virtue of a physical or mental condition and are therefore deemed to be at greater risk of injury in their use of domestic hot water than would be the case for a normal able bodied person then a TMV that complies with either BS 7942 or NHS D 08 or a TMV that is TMV3 approved should be used.

Q
Can I install a TMV with unequal pressures?

A
In short – yes. However, care should be taken to check the manufacturers’ information regarding the ability of the valve to cope with unequal pressures. Many manufacturers will quote the maximum amount of imbalance as a ration (e.g. 10:1 – this literally means that the valve will work with one pressure being up to 10 times larger than the other pressure). It is important that any valve is installed with check valves (if they are not an integral part of the TMV) where an imbalance of pressure exist as the check valves will prevent cross flow. It is also worth noting that where one supply is high pressure (typically the cold) and the other supply is low pressure a better solution would be to reduce the high pressure to a lower value (by us e of a pressure reducing valve) to nominally equalise the pressures.

Q
Will fitting a TMV affect my flow rate?

A
The fitting of any control device to a water supply will affect the flow rate. However, on high pressure systems any affect on flow rate should be negligible. The situation on low pressure systems can be somewhat different. This is especially true if a TMV designed for high pressure systems is fitted to a low pressure supply. The flow paths through the valve may not be big enough to allow adequate water flow. For TMVs designed to be used on low pressure systems the water flow may still be affected but this will be dependent on the amount ‘head’ between the outlet of the valve and the water supply cisterns. 1m of head equates to approximately 0.1 bar of pressure. Most TMVs have a recommended dynamic supply pressure requirement of 0.2 bar (equating to 2m head). For shower TMVs while there may be 2 m between the actual valve part and the cistern there may actually only be 1m between the shower head and cistern and this will be the limiting factor on the flow rate.

Q
Can I use a TMV with a combination boiler?

A
TMVs are suitable for use with combi boilers as long as the boiler is a fully modulating type (i.e. the gas flame is regulated in sympathy to the flow of water through the heat exchanger).

If the combi is not a fully modulating type the TMV will not be able to maintain a stable outlet temperature. This is due to hot water from the non-modulating boiler increasing in temperature as it enters the TMV, the TMV will reduce the amount of hot water it mixes with cold water to try to maintain the stable set temperature. In turn, the flow through the boiler is reduced to the point where a thermostat within the boiler switches off the gas burner. The water temperature from the boiler will then reduce until the boiler thermostat resets thus allowing the water flow to be heated again. This results in the outlet from the boiler, and also therefore from the TMV to constantly cycle between hot, and cool temperatures.