If you live in London, you already know about the water. It furs up kettles, leaves a crust on shower screens and turns glass tumblers cloudy. What most people don't see is what it's doing inside the thermostatic mixing valves behind their bath panels. Spoiler: nothing good.
Why London's water is so hard
Hard water is water carrying a lot of dissolved calcium and magnesium. Much of London's supply comes from the Thames and the River Lee, plus groundwater from chalk, and that water picks up minerals on its way through the rock. The result is some of the hardest water in the country.
It's perfectly safe to drink. The problem comes when you heat it. Warm it up and some of those minerals drop out as limescale, and a TMV is a device that spends its entire life handling hot water and very warm mixed water. You can see where this is going.
What scale does inside a TMV
A TMV has a few parts that really don't like scale.
Strainers
Fine mesh filters on the hot and cold inlets catch debris before it gets into the valve. In hard water areas they also catch flakes of scale coming off pipes, heaters and cylinders. Over time they clog, flow drops, and the balance between hot and cold goes off. In a hard-water city like London, a partly blocked hot strainer is a classic cause of "lukewarm" complaints.
Check valves
These small non-return valves stop hot water pushing into the cold supply and vice versa. Scale on the seat stops them closing properly. That can let water cross over between supplies, which messes with temperatures elsewhere in the building and isn't great for water quality either.
The thermostatic element
This is the clever bit. It expands and contracts with temperature and moves a shuttle that adjusts the mix of hot and cold. Coat it in scale and it gets sluggish. It responds slower to changes in supply, so outlet temperatures wobble, and in the worst cases it can stick.
The shuttle and mixing chamber
Scale on the moving parts adds friction and narrows the gaps water flows through. The valve has to work harder for the same result, and it gets less precise.
Why it matters for safety
The point of a TMV is the fail-safe. If the cold supply fails, the valve should shut off the flow quickly so nobody gets a blast of very hot water. A scaled-up valve may react too slowly, or not shut down fully. You won't know until the day it happens, which is why the fail-safe test is part of every proper service.
There's a Legionella angle too. Scale gives bacteria somewhere to settle and makes it harder for the system to stay clean. Keeping valves and strainers free of scale is part of good water hygiene, not just good plumbing.
Signs your TMVs are scaling up
- Showers or basins that take longer to reach temperature, or wander between warm and cool
- Reduced flow at outlets fed by a TMV, especially on the hot side
- Complaints that come and go with demand, such as fine at midday and poor at 7.30am
- Service records noting "heavily scaled" more than once in a row
- Valves that fail or only just pass the fail-safe test
- Visible scale at the outlet: crusty shower heads and spouts are a clue about what's happening upstream
What a proper London service looks like
In soft water areas, a service can sometimes be a quick clean and check. In London, descaling is the job. On every valve we:
- Record the temperatures before we touch anything, so you can see the before and after
- Remove and clean the strainers, and replace them if the mesh is damaged
- Strip and descale the check valves, or replace them if they won't seat
- Descale the thermostatic cartridge, or replace it if it's beyond saving
- Reassemble, set the temperature for the outlet type and carry out the fail-safe test
- Record how scaled the valve was, so we can judge whether the interval's right
That last step is the important one. Our TMV servicing visits don't just clean the valve; they build up a picture of how fast scale builds on your site.
Getting the service interval right
HTM 04-01 sets out an approach where the interval between services depends on how much the valve drifts between tests. In hard water, drift and scale tend to go hand in hand, so London sites often end up on shorter intervals than the same building would need elsewhere.
As a rough guide, a busy shower block or a care home in hard water will often need six-monthly servicing. A lightly used basin valve in a flat with a softened supply might reasonably go annually. Your written scheme should set the interval, based on the records. If every service finds valves heavily scaled, the interval is too long.
A typical scenario: imagine an HMO in Hackney with five en-suites, serviced annually. Every year the strainers come out packed and two of the valves only scrape through the fail-safe test. That's a site telling you it wants six-monthly visits. The extra visit costs less than replacing cartridges every year, and it's a lot safer.
Can you stop the scale in the first place?
Partly. Some buildings use water softeners or scale inhibitors on the supply to the hot water system, and that can make a real difference to how quickly TMVs scale up. It's a decision for the whole building, not just the valves, and it needs to fit with your drinking water arrangements and your Legionella risk assessment. If you have treatment in place, tell us, because it changes what we'd expect to find.
Upgrading old valves helps too. Modern valves are easier to strip and clean, and having isolation valves and strainers in an accessible spot makes regular servicing quicker and cheaper. If your valves are tiled in or the parts are no longer made, have a look at our installation and replacement page.
The short version
London's water will scale up your TMVs. That's not a failing on anyone's part, it's geology. What matters is that someone takes the valves apart regularly, cleans them properly, checks they still fail safe, and uses what they find to set the right interval. Do that and hard water becomes a maintenance item, not a safety problem.
Got TMVs in London that need a look?
Tell us roughly what you've got and where. We'll give you a fixed quote and a date that works around your building.