Hot water stored at 60°C is great for keeping Legionella down. It's not so great if it comes out of a bath tap onto a frail resident or a small child. Thermostatic mixing valves, or TMVs, are how buildings deal with that clash. They solve one problem but create another, so they need a bit of thought.
September means schools are back, care homes are planning for winter and a lot of building managers are looking at their maintenance schedules for the year. It's a good time to check your TMVs are pulling their weight.
What a TMV does
A TMV blends hot and cold water to deliver a set, safe temperature at the outlet. Inside is a thermostatic element that reacts to changes in the incoming supplies. If the cold supply suddenly fails, a working TMV should shut off or reduce the flow to a trickle rather than send scalding water out. That's called the fail-safe.
You'll see two main designations:
- TMV2 valves are for domestic and similar settings, and are tested against BS EN 1111 or BS EN 1287 depending on the supply pressure.
- TMV3 valves meet the NHS specification for healthcare premises, where users are more vulnerable and the performance requirements are stricter.
Why they're needed
Scalding is a real hazard, particularly for older people, young children and anyone with reduced mobility or sensation. HSE guidance for health and social care says that where vulnerable people could be exposed, water at outlets used for whole-body bathing, such as baths and showers, shouldn't exceed 44°C.
So you can't simply turn the hot water down. Drop the stored temperature below 60°C and you give Legionella a much easier time. The TMV lets you keep the system hot and the outlet safe.
The Legionella catch
Downstream of the TMV, the water is warm. Usually somewhere in the high 30s to low 40s °C, which is right in the range where Legionella can grow. If there's a long run of pipe between the valve and the tap, that's a length of warm water sitting there between uses.
The HSE's technical guidance, HSG274 Part 2, deals with this in a few ways:
- Fit the TMV as close to the outlet as possible, so the length of mixed water pipe is kept short.
- Only fit TMVs where the risk assessment shows they're needed. A TMV on a staff kitchen sink in an office may be adding Legionella risk without much scald benefit.
- Avoid one TMV serving several outlets through long runs, where it can be helped.
- Keep them maintained, because scale and debris build up inside and give bacteria somewhere to live.
Healthcare buildings follow HTM 04-01, which goes into more detail on TMV selection, siting and testing.
What TMV servicing involves
A TMV is a precision valve that sits in hard, warm water all day. Without servicing, it drifts, scales up and eventually stops doing its job, sometimes without anyone noticing. A typical service covers:
- Temperature check. Measuring the mixed water temperature at the outlet and comparing it with the set point.
- Fail-safe test. Isolating the cold supply to make sure the valve shuts down or reduces to a trickle, as it should.
- Cleaning. Strainers and filters are cleaned, and the internal parts are descaled or replaced as needed.
- Check valves. Making sure the non-return valves are working so hot and cold don't cross over.
- Re-setting and re-testing. Adjusting the valve to the right temperature for the outlet and repeating the fail-safe test.
- Recording. Each valve gets logged with its location, readings and any work done.
How often?
There isn't one figure for every building. Servicing frequency should follow the manufacturer's instructions and your risk assessment, and it can be adjusted based on how each valve performs. In healthcare, HTM 04-01 describes a way of setting the interval by checking how much each valve drifts between tests. In hard water areas like London, scale tends to build up faster, so you may find valves need attention more often than you'd expect.
Flushing and TMVs
TMV outlets that aren't used much need weekly flushing just like any other. Where the controls allow, it's worth running them so that both the hot and cold supplies get turned over, not just the mixed water. Our flushing guide covers the basics.
One more thing to know: if your system ever needs a thermal disinfection, the TMV will stop hot water reaching the outlet at full temperature. The valve may need bypassing or adjusting following the manufacturer's instructions, so make sure whoever's doing the work knows the TMVs are there.
A typical scenario
Imagine a primary school that had TMVs fitted to all its pupil washbasins years ago. Nobody's serviced them since. Some outlets are running barely warm, which tells you the valves have drifted, and a couple fail their fail-safe test. The pipes from the valves under a long row of basins also run a fair way before they reach each tap.
A sensible plan would be to service or replace the failing valves, look at whether the long mixed-water runs can be shortened, and set up a regular service schedule. If any basins turned out not to need TMVs at all, the risk assessment could recommend removing them.
Getting the balance right
TMVs aren't good or bad. They're a tool. Used in the right places, sited properly and serviced regularly, they protect people from scalds without making Legionella much worse. Used everywhere and forgotten about, they can make it worse.
Your risk assessment should say which outlets need TMVs and why, and your written scheme of control should say how and when they're checked.
If your TMVs haven't been looked at for a while, our TMV servicing team will test, service and log every valve. We can also fit it around your routine Legionella maintenance visits.