Winter cold water and your TMVs: what London buildings should check

A complaint you'll often hear in December: "The showers have gone lukewarm." Nothing's broken, nobody's touched anything, and yet residents in flat 3 are queuing to tell the managing agent their morning wash is tepid. More often than not, the answer is sitting in the cold water main.

What winter does to London's cold water

The cold water coming into a London building isn't a fixed temperature. It follows the seasons. In a warm summer it can creep up towards 20°C, which brings its own Legionella headaches. In winter it drops a long way, and in a cold snap it can fall well into single figures.

A thermostatic mixing valve doesn't care what the weather's doing. Its job is to deliver the same mixed temperature, say 41°C at a shower, whatever goes in. So when the cold supply gets colder, the valve has to let in more hot water and less cold to hit its set point. Usually it copes. Sometimes it can't.

Why showers go lukewarm in December

A few things tend to be behind winter temperature complaints in London buildings:

• The hot supply is marginal. If the hot water reaching the valve is only just hot enough in summer, it won't be in winter, because the valve needs more of it. The TMV runs out of headroom and the outlet drops below its set point. This is worth flagging to whoever looks after your Legionella control too, because hot water should be reaching outlets at 50°C or more within a minute (55°C in healthcare buildings) under HSG274.
• Scale and debris are restricting the hot side. London's hard water leaves scale in strainers and check valves. A partly blocked hot strainer might be fine when the valve only needs a little hot water, and not fine when it needs a lot.
• The cartridge is sluggish. A scaled thermostatic element responds slowly, so temperatures wander around more than they should, especially when demand jumps at 7.30am.
• The boiler or plant is struggling. In mansion blocks and other buildings with communal hot water, winter demand peaks are when an undersized or poorly maintained plant shows itself. Every TMV in the building gets the blame for a problem in the basement.

The tempting fix is to turn the TMV up. Please don't, or at least not without finding out why first. A valve that's been wound up to cope in January can deliver water that's too hot once the cold mains warms up again in spring. That's precisely the scalding risk the valve is there to prevent.

The Christmas shutdown problem

Winter brings a second issue that has nothing to do with the cold. A lot of London buildings go quiet over the festive break. Schools close for two weeks or more. Offices empty out. Some clinics shut between Christmas and New Year. Even in residential blocks, plenty of flats sit empty while people visit family.

Water that sits still in pipework is a Legionella concern, and the warm mixed water between a TMV and its outlet is the part of the system bacteria like best. So the shutdown needs a plan:

• Make sure little-used outlets are on your flushing regime over the break, if someone is on site.
• Flush outlets properly before the building fills up again in January, following your written scheme.
• Check hot and cold temperatures once the system is back in normal use, because it can take a while to settle.

If your TMVs are due a service around this time, booking it for the first week back is a sensible move. The valves get cleaned and checked just as normal use resumes, and any temperature problems get picked up early.

What to check this winter

Here's the short list we'd suggest for a London building heading into the cold months:

1. Look at your last service records. When were the valves last serviced, and were any marked as heavily scaled or drifting? Those are the ones likely to struggle first.
2. Check the hot supply temperature at a few outlets at a busy time of day. If it's marginal, talk to whoever maintains your hot water plant.
3. Listen to complaints. Note which flats or rooms report problems. A pattern tells you a lot.
4. Don't adjust set points to compensate. Get the cause found and fixed instead.
5. Plan for the shutdown if your building empties out over Christmas.
6. Protect exposed pipework. Cold water tanks in lofts and pipes in unheated voids can freeze in a hard frost. A frozen cold supply to a TMV is effectively a fail-safe test you didn't plan for.

A typical winter scenario

Imagine a 1930s mansion block in West London with communal hot water and a TMV on every bath. In early December, a handful of top-floor flats start reporting tepid baths. The building manager's first instinct is to get the valves turned up.

On a visit like that, we'd usually start by checking the hot supply at the affected flats and at the plant. If the hot water arriving on the top floor is well short of where it should be, the TMVs aren't the problem. They're doing their job with what they're given. We'd record the readings, service the valves while we're there (in London, they'll almost certainly have scaled strainers worth cleaning), and pass the supply readings to the building's plant engineer and Legionella assessor. Set points stay where they are.

The takeaway

Winter is when weak spots in a hot water system show up. Lukewarm TMVs are usually a symptom, not the disease. Find the real cause, keep the valves serviced and descaled, and resist the urge to turn them up. Your residents get warm baths now, and nobody gets scalded in April.

If you'd like your valves checked before the cold really bites, have a look at our TMV servicing page or our TMV compliance guide for the background on temperatures.