As part of a well maintained water system TMVs should be regularly tested and cleaned. HSG 274 Part 2 states:
What are TMVs?
TMVs are valves using temperature sensitive elements to blend hot and cold water to produce water at a temperature to safeguard against scalding. Blended water downstream of a TMV may provide an environment in which legionella can multiply thereby increasing the risk of exposure.
Scalding risk versus Legionella Risk
The use and fitting of a TMV should be by assessing the risk of scalding against the risk of infection from legionella. If the risk of scalding is insignificant TMVs are not required. The most serious risk of scalding comes from the whole body being immersed such as baths and showers, particularly for the very young and elderly. TMVs should be fitted at these outlets. TMVs will also be required at wash hand basins and sinks used by the very young, very elderly, infirm and severely disabled people or those with sensory loss.
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.
- Risk assess if TMV is required;
- Where needed, inspect, clean, descale and disinfect any strainers or filters associated with TMVs;
- To maintain protection against scald risk, TMVs require routine maintenance carried out by competent persons in accordance with the manufacturer’s instructions.
Devices which blend water
The common approach for blending water is a thermostatic mixer valve which is fitted to the hot and cold pipework supplying the asset.
Alternatively and preferably a thermostatic mixer tap can be used which blends the water at the point of use
Another alternative is a tap which mechanically blends hot and cold water within a chamber inside the tap. If the bar on the side of the tap is set to the cold setting only cold water will flow to the outlet. Similarly, if the bar on the side of the fitting is set to hot only hot water will flow to the outlet. This tap can still provide a scalding risk as it works on mechanical adjustment and does not have a failsafe setting.