A thermostatic mixing valve blends stored hot water down to a safe delivery temperature at the outlet. It exists because the temperature that controls legionella is hot enough to scald, and it is the compromise between those two risks. The compromise is not free: the mixed water on the outlet side of the valve sits in the range legionella grows in, which is why TMVs are maintained rather than fitted and forgotten.
What the valve is doing
Taking hot water at storage temperature and cold water, and mixing them to a set delivery temperature at the tap or shower. If the cold supply fails it shuts down rather than delivering scalding water, which is the safety function that makes it a TMV rather than a mixer. TMV2 and TMV3 are the scheme designations, with TMV3 used in healthcare and care settings where the people at risk are most vulnerable.
Why it creates a legionella risk
Everything downstream of the valve is at blended temperature, and the valve body itself holds water in that range. The shorter the pipe run from valve to outlet the less water sits there, which is why guidance pushes for the valve to be as close to the outlet as possible and why a valve serving a whole run of basins is a worse arrangement than one per outlet.
What servicing checks
The delivery temperature against the set point. The fail safe shutdown, by isolating the cold supply and confirming the valve closes. The strainers and check valves, cleaned or replaced. Any scale in the valve body. And for TMV3 in healthcare, the full annual verification against the scheme's requirements. Each check is recorded, because the record is what shows the scalding and legionella controls are both still working.
Where it belongs in the scheme
On the written scheme of control alongside the shower heads and the little used outlets. In practice TMVs are checked on the same visit as the temperature monitoring, which is efficient and also means a lapsed monitoring contract quietly takes the TMV servicing with it.
Questions people ask about tmv
What is a thermostatic mixing valve?
A valve that blends stored hot water with cold to deliver water at a safe temperature at the outlet, and shuts down if the cold supply fails so it cannot deliver scalding water.
Why do TMVs need servicing?
Because the mixed water on their outlet side sits in the range legionella grows in, and because the fail safe shutdown that prevents scalding has to be proved to still work.
How often should a TMV be serviced?
The frequency comes from the written scheme of control and the setting. Healthcare and care settings using TMV3 valves are the most demanding; commercial premises typically check annually with in service temperature checks more often.
Do TMVs remove the need for temperature control?
No. Hot water is still stored and distributed at the temperature that controls legionella; the valve only blends it down at the point of use.