TMV Servicing

Thermostatic Mixing Valves – known as ‘TMVs’ are valves fitted close to an outlet (sometimes they are part of the outlet) which blend cold with the hot water supply to provide a constant, predictable and safe temperature at showers, baths and other outlets deemed to be at high risk of scalding, such as accessible toilets or children’s wash facilities. The valves can be controlled to exact temperature, but should not allow water to emit above 44°C. Furthermore, if there is ever a break in the cold water supply, the valve shuts off completely, preventing scalding to the user.

As legionella is most at risk of growth in water between 20-50°C, water downstream of a TMV is at risk of legionella growth and so the distance between valve and outlet should be no greater than 2 metres, avoiding excessive stagnation in the system at lower temperatures between outlet uses.

As TMV-controlled taps and outlets do not show a true hot water temperature, hot pipes feeding each TMV should be checked with a contact temperature probe (see video below), in accordance with the building’s legionella risk assessment, ACoP L8 and HSG 274. A contact temperature probe allows the user to check the temperature of the hot water inside the hot pipework, which is a suitable substitute for checking the TMV-blended hot water to control legionella.

The TMV-blended hot water should also be tested, although this is to ensure the scalding risk remains controlled, rather than to control legionella.

TMVs should be serviced annually, and the service should include dismantling them individually and cleaning the internal parts, before replacing and resetting the temperature. Several parts within a TMV can hold debris and harbor bacteria, which is of particular concern as hot water level at which a TMV emits water increase the risk of legionella growth. It is also important that they are serviced in order to ensure they work correctly and continue to reliably protect building users against scalding.