If you've ever looked inside a kettle in a hard water area, you already understand the problem. Now imagine that same crust forming on the tiny moving parts inside a thermostatic mixing valve, parts that have to slide freely and react within seconds. That's why limescale is the single most common reason we find TMVs that aren't doing their job.
Autumn is a good time to think about it. The heating's coming on, hot water use goes up in the colder months, and a sluggish valve that scraped through the summer can start to struggle.
Why hard water is hard on TMVs
Hard water carries dissolved calcium and magnesium. When it's heated, some of that comes out as limescale. The hotter the water, the more readily it happens, and the hot side of a TMV sees some of the hottest water in the building.
Inside the valve, scale builds up in three main places:
- On the thermostatic element, which senses the mixed temperature and moves to adjust it. A coating of scale slows its response.
- On the shuttle or piston and its seats, so parts stick and don't seal properly.
- In the inlet strainers, where flakes of scale from the pipework collect along with any other debris.
Much of the south and east of England has hard water, but scale can turn up anywhere. Even in softer water areas, a valve on a very hot supply or a system that's been disturbed can collect debris.
The warning signs
Scaled-up valves rarely fail overnight. They get gradually worse, and the signs are often there before a test picks them up:
- Temperature creep. The mixed temperature slowly moves away from its set point between visits.
- Hunting. The water surges warmer and cooler for a while before settling.
- Slow response. When someone turns on a nearby tap or flushes a toilet, the TMV outlet goes noticeably hot or cold.
- Weak flow. Partly blocked strainers cut the flow, and sometimes on one side more than the other.
- A struggle at the fail-safe test. The valve is slow to shut down when the cold is isolated, or doesn't fully shut down at all.
That last one is the serious one. If a valve fails its fail-safe test, the outlet should come out of use until it's fixed. We've covered what to do in your TMV failed its fail-safe test.
What strainer cleaning involves
Most TMVs have a strainer and a non-return valve on each inlet. Cleaning them is usually the first job on a service visit. The engineer isolates the supplies, removes the strainers, clears out the debris and scale, and checks the non-return valves are moving freely and sealing. Damaged strainers or worn check valves are replaced.
It's a quick job, but it makes a real difference. A valve with clean, balanced inlets has a much easier time holding its temperature.
What descaling involves
Descaling goes a step further. The valve is stripped down and the cartridge, element and other internal parts are cleaned of scale. Usually that means soaking them in a descaling solution that's suitable for the valve, following the manufacturer's instructions, then rinsing thoroughly before reassembly. Some manufacturers have specific guidance on what can and can't be used on their parts, so it's worth following it.
Once it's back together, the valve is recommissioned: the mixed temperature is set at the outlet with a calibrated thermometer, the fail-safe test is repeated and the results recorded.
Sometimes descaling isn't enough. If the element is badly scaled or worn, a new cartridge or service kit is the better fix. And if the valve body itself is heavily scaled or corroded, or parts aren't available, replacement might be the sensible option.
How often?
There's no single answer, because it depends on how hard your water is, how hot the supply is and how much the valve is used. What we'd suggest is letting the results guide you, which is exactly the approach HTM 04-01 takes. If a valve is drifting or sluggish at every test, it needs servicing more often. If it's clean and stable, the interval can stay where it is. Our post on how often TMVs should be serviced explains how to set it.
Keeping records of what was found at each visit helps. If the register shows the same valves scaling up every six months, that's useful information, both for planning servicing and for deciding whether something needs to change upstream.
Treating the problem at source
On sites with persistent scale problems, it can be worth looking at the water itself. Options like water softening or scale reduction equipment on the incoming supply might reduce how quickly TMVs scale up. That's a decision for the whole system, not just the valves, and it should involve whoever manages your water safety, because any change to treatment needs to fit with your Legionella controls under ACOP L8.
Whatever you decide upstream, regular servicing is still needed. Treatment can slow scale down. It doesn't replace testing.
A typical scenario
Imagine a leisure centre in a hard water area with a bank of TMV-controlled showers in the changing rooms. Users have started complaining the showers run hot, then cold, and the flow on a couple of them is poor. On inspection, the strainers are packed with scale, and two cartridges are so crusted that they barely move. Those two get new cartridges, the rest are descaled and recommissioned, and the service interval for the bank is shortened so scale doesn't get the chance to build up that far again.
Choosing valves for hard water
If you're replacing valves on a site with hard water, it's worth thinking about serviceability. A valve whose cartridge can be removed and cleaned easily, with good access to the strainers, will be quicker and cheaper to look after over its life. Fitting isolation valves either side makes servicing simpler too. Our installation page covers how we approach that.
For everything included in a routine visit, including strainer cleaning and descaling, see our TMV servicing and testing page. Or read why TMV outlet temperatures drift for the other usual suspects.
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Tell us roughly how many TMVs you've got and where you are. We'll come back with a clear, fixed quote.