Of all the results on a TMV test sheet, this is the one that matters most. A valve that's a degree or two out can usually wait for a service. A valve that fails its fail-safe test is a different story. It's telling you that if the cold supply drops out, it might not protect whoever's standing under the shower.
So here's what the test actually checks, what to do the moment a valve fails, and how it usually gets sorted.
What the fail-safe test checks
A thermostatic mixing valve blends hot and cold water to a set temperature. The big risk is losing the cold. If the cold supply fails, perhaps because of a burst, a closed valve somewhere or a drop in pressure, an ordinary mixer would happily send straight hot water to the outlet.
A TMV is designed to stop that. When the cold goes, the thermostatic element should react and shut off the hot, cutting the flow to nothing or to a trickle.
The fail-safe test (sometimes called a thermal shut-off test) checks it does. With the outlet running at its normal mixed temperature, the engineer isolates the cold supply to the valve and watches what happens. The valve should close quickly, within the limits set out in its certification and the manufacturer's instructions. Then the cold is restored and the engineer checks the temperature settles back where it should be.
HTM 04-01 includes this test as part of commissioning and in-service checks, and we do it on every valve at every visit. It's the only way to know the safety feature still works.
What to do straight away
If a valve fails, the outlet it serves shouldn't be used until it's been put right. In practice that means:
- Take the outlet out of use. Isolate the valve if it can be done safely, and put a clear "do not use" sign on the shower, bath or basin.
- Tell the people who need to know. Care staff, housekeeping, the site manager, whoever might otherwise use that outlet or help someone else use it.
- Record it. The failed result, the date, and what you did about it. If anyone asks later, that record shows the problem was caught and acted on.
- Book the repair. Not for next month's round. As soon as you reasonably can.
If it's the only assisted bath on a unit, talk to us. We'll try to fix it quickly, and in the meantime you'll need a safe alternative agreed through your risk assessment.
One thing not to do: turn the set temperature down and carry on. A lower mixed temperature doesn't fix a valve that won't shut off.
Why valves fail the test
In our experience, the cause is usually one of a handful of things.
Limescale on the thermostatic element
This is the big one, especially in hard water areas. Scale builds up on the element and the moving parts around it, and the valve gets slow and sticky. It still mixes, more or less, but it can't move fast enough when the cold disappears.
Blocked strainers or sticking check valves
Most TMVs have small strainers and non-return valves on the inlets. Debris and scale clog them up, and a check valve that's stuck open can let water cross between the hot and cold sides. Either can stop the valve behaving as it should.
A worn-out cartridge
Thermostatic elements don't last forever. After enough years and enough temperature cycles, they lose their response. A new cartridge or service kit often brings the valve back to life.
Not enough difference between hot and mixed
Sometimes the valve is fine and the system is the problem. If the hot supply is barely warmer than the set mixed temperature, the valve doesn't have much to work with. Most manufacturers specify a minimum gap between the hot supply and the mixed temperature for the valve to work as designed, so check the instructions for yours. A low hot supply is also worth looking at from a Legionella point of view, which we'll come back to.
The wrong valve for the job
A valve fitted on a system or outlet it isn't certified for may never perform properly. More on that in TMV2 vs TMV3.
Putting it right
A typical repair visit goes something like this. The engineer isolates the valve, strips it down, and cleans or replaces the strainers and check valves. The element and body get descaled, worn parts are swapped out (often with the manufacturer's service kit), and the valve is reassembled and recommissioned. That means setting the mixed temperature at the outlet, then repeating the fail-safe test.
If it passes, the outlet can go back into use and the new readings go on record. If it still fails after a full service, or parts aren't available any more, the valve needs replacing. Our post on repair or replace? goes through how to decide. Our installation and replacement page covers how we pick and fit a new one.
Afterwards: watch it a bit closer
A valve that failed once has told you something. It's worth bringing its next test forward rather than leaving it for the full interval, just to check the repair has held. If several valves on the same site are failing for the same reason, usually scale, that points to the interval being too long or the water needing attention. Our post on how often TMVs should be serviced explains how results should shape the schedule.
And if the cause turned out to be a low hot water supply, don't just leave it with the TMV contractor. Hot water temperatures are one of the main Legionella controls under ACOP L8 and HSG274, so whoever looks after your water safety needs to know.
A typical scenario
Imagine a spring service round at a supported living scheme. Most valves pass, but one en-suite shower valve doesn't close when the cold is isolated. The shower is taken out of use and the resident is offered another bathroom for the day. On strip-down, the element is crusted with scale and one check valve is stuck. A service kit and descale later, it passes, and the next test for that valve is booked in a few months rather than a year.
Nothing dramatic. But it's exactly the kind of fault the test exists to catch, before it catches someone out.
If you want the full picture of what we check at each visit, have a look at our TMV servicing and testing page.
Got a valve that's failed its test?
Tell us where it is and what the test showed. We'll get an engineer out to repair or replace it, then retest and give you the paperwork.