A TMV is set up to deliver water at a particular temperature, say 41°C at a shower. Come back a few months later and it might be giving 42.5°C. Or 39°C. Or it might be fine on one test and wandering about on the next. That's drift, and it's the most common thing we find on a routine visit.
A little movement isn't unusual. A lot of movement, or movement in the wrong direction, is a sign something needs attention. Here's what causes it, and what we do about it.
First, a bit about how a TMV works
Inside every TMV is a thermostatic element, a small capsule filled with a material (usually a wax) that expands and contracts with temperature. It sits in the mixed water and moves a piston or shuttle that controls how much hot and cold gets in. If the mix gets too warm, the element expands and lets in more cold. Too cool, it contracts and lets in more hot.
It's clever, and it's entirely mechanical. Anything that makes those parts move less freely, or changes what the valve is being asked to cope with, can push the temperature off its set point.
The usual causes
1. Limescale
If you're in a hard water area, scale is suspect number one. It builds up on the element, the shuttle and the seats, so the valve responds more slowly and doesn't seal properly. The result can be a temperature that creeps up, or one that overshoots and hunts around before settling. We've written a whole post on hard water and TMVs.
2. Debris in the strainers
Most TMVs have fine mesh strainers on the hot and cold inlets. Over time they collect grit, scale flakes and bits left over from pipework jobs. If one side gets more blocked than the other, the balance between hot and cold changes and the mixed temperature moves with it. This is especially common after work on the system, when disturbed debris gets carried along.
3. Check valves that stick
The non-return valves on the inlets stop hot and cold water crossing over. When one sticks or wears, water can pass from one side to the other, which upsets both the TMV and sometimes nearby outlets too.
4. A tired thermostatic element
Elements wear out. After years of cycling, they lose some of their response, and no amount of cleaning brings them back. That's when a new cartridge or service kit is the answer.
5. Changes in the supplies
A TMV can only do so much with what it's given. If someone turns the calorifier or boiler set point down, the hot supply drops and the valve may not be able to reach its set temperature. If a pump is changed or pressures shift, the hot and cold may no longer be as balanced as when the valve was set up. Big swings in demand, such as lots of showers running at once in a hotel or a school changing room, can show up the same weakness.
6. The seasons
This one surprises people. The temperature of the incoming cold mains changes through the year, colder in late winter and warmer in late summer. A healthy valve compensates for that without fuss. A sluggish one doesn't, so the same valve can test fine in October and be out in April. Spring is often when we see it.
7. Somebody adjusted it
It happens. A resident complains the shower's too cool, someone finds the adjuster and gives it a turn. Most TMVs used in care and public settings need a tool to change the setting for exactly this reason, but it's worth bearing in mind if a valve has suddenly jumped.
How much drift is too much?
There isn't one magic number that applies to every valve on every site. What HTM 04-01 asks you to do is compare each test result with the readings taken at commissioning. If the mixed temperature has hardly moved, the valve is behaving and the interval can stay as it is. If it's changed noticeably, the valve needs servicing, and probably checking more often until it settles down.
The direction matters too. A valve drifting up is the more urgent problem, because that's heading towards a scalding risk. One drifting down is less dangerous, but very cool water causes its own trouble: people avoid washing properly, or someone goes looking for a way to turn it up.
What servicing does about it
A proper service tackles drift at the root rather than just turning the adjuster until the number looks right. On a typical visit, we:
- Measure the hot and cold supplies, so we know whether the problem is the valve or the system.
- Clean or replace the strainers and check the non-return valves.
- Descale the internals where needed.
- Replace the cartridge or fit a service kit if the element has had it.
- Reset the mixed temperature at the outlet with a calibrated thermometer, then do the fail-safe test.
- Record before and after readings against that valve.
That last step is what makes the next visit useful. Without a record, you can't tell drift from a valve that was set wrong in the first place.
A typical scenario
Picture an office block with TMVs on the washroom basins. At the spring test, several on the top floor are reading a couple of degrees warm, while the lower floors are fine. Rather than recalibrate each one and move on, it's worth asking why. A likely answer: the top floor valves are at the end of the longest pipe runs, where the cold supply is warmest by the time it arrives, and their strainers are partly blocked. Clean the strainers, reset the valves, and keep an eye on them at the next visit.
Stopping it coming back
You can't stop drift entirely, but you can keep it small and catch it early. Regular testing at the right interval, prompt servicing of valves that move, sensible water treatment in hard water areas and a heads-up to your TMV contractor whenever the hot water system is changed will all help.
If your valves keep drifting despite servicing, it may be time to replace them. Our installation and replacement page explains how we choose new ones. Otherwise, see what's included in our TMV servicing, and read how often TMVs should be serviced for help setting the interval.
Want us to take a look at your valves?
Tell us roughly how many TMVs you've got and where you are. We'll come back with a clear, fixed quote.