My (London, UK) apartment building's hot water comes from a district heating system and is delivered to my apartment via a "Heat Interface Unit" within the property, owned and maintained by the energy company. Water from the main and the HIU is piped to the separate rooms in the property from a central closet.

While running a bath a couple of months ago, I noticed the hot water run out before the bath was even half full (previously the hot water supply seemed endless). A few weeks ago, I replaced a split shower hose in the same bathroom with one which happened to allow a higher flow rate. The next morning with the shower turned up to full flow I noticed it start to get cold after only a couple of minutes. Prior to this the shower would run seemingly endlessly with no loss of temperature.

I rang the energy company and they asked me to try my heating, which uses hot water from the same system. They said the HIU would only be at fault if the heating was also not working correctly – I tried it and it was fine, with all four of my radiators heating up quickly (worth noting that I normally never use the heating as the building's ambient temperature is high). I ran hot water in the bathroom sink and in the kitchen, and these seemed fine as well. Their policy is to only send an engineer if the fault can be demonstrated to be with the HIU, so I've sat on the issue for a bit.

On subsequent showers I found that if I kept the flow turned low – around 60%, the water would stay warm for the duration of my shower.

This evening I tried to run a bath and the same thing happened – the hot water ran cold (24ºC) after only a short time. The kitchen tap ran hot (46ºC) without issue. In the bathroom, I found that if I switched the bath tap off for a minute, then back on, it would run very hot very briefly, then back to cold. The tap over the sink seems unaffected – running hot for as long as I care to test. If I run the bath tap at the same time, both go cold. However if I switch off the bath tap, the sink will return to hot, but not the other way around. I note that the flow through the bath tap is much higher than the sink, in case this is relevant.

I am struggling to come up with a mental model of the plumbing that explains why the hot water supply (seemingly to these two appliances only) is being affected in this way, given all the hot water pipes seem to branch from the same point in the system. Can you think of anything that could cause this; is there any sensible further investigation I should do; could the HIU be at fault, or am I best off calling a plumber?

I attach summary photos of the submarine-control-panel-like HIU and the point of distribution of the water pipes. Also a floorplan of the apartment with the HIU closet marked with a red dot.

Thanks for your time and please let me know if I can clarify any of the above points!

Edited to clarify detail around the bathroom sink tap after further experimentation today.

Partial view of the HIU View of the distribution point for hot and cold water Apartment floorplan with HIU closet marked by red dot

  • where is the connection to the bathroom and to the kitchen?
    – jsotola
    Commented Feb 26, 2018 at 20:30
  • is there a manufacturer and model number on the HIU?
    – jsotola
    Commented Feb 26, 2018 at 20:46
  • @jsotola Thanks. The bathroom and kitchen connections are at the bottom of the second image, the blue and red pipe groups. Thermal Integration Ltd, no model number
    – BigglesZX
    Commented Feb 26, 2018 at 22:49
  • my guess is that the bathroom connections are on the right, since the bathroom is on the right. (please verify that by turning off the hot and cold) ... there is nothing at the HIU that limits the water flow as far as i can see. ... there must be something in the bathroom that limits the flow ...... is that gray cylinder, with the blue label, a pump? (at top right of bottom picture) ... do you get more hot water in bathroom when heating is on? ...
    – jsotola
    Commented Feb 26, 2018 at 23:06
  • @jsotola It is a pump – a Wilo ST 25/6-3. There are two there, close together. I have just done some further testing and the bathroom sink does not appear to be affected by this. It's just the shower and bath. The sink will run hot when the bath has "run out" and only turns cold when the bath tap is also running. I'll revise my question to include this detail.
    – BigglesZX
    Commented Feb 27, 2018 at 17:10

2 Answers 2


To me it sounds like an anti scauld valve is not working correctly. If you have a thermometer could you try just turning on the hot and see if the flow almost stops and what the water temp is. This could verify if the valve is the problem.

  • Thanks – I can try and measure with a meat thermometer; the flow never seems to change from the hot tap or shower, just the temperature...
    – BigglesZX
    Commented Feb 26, 2018 at 21:57
  • OK so here's what I just observed. Shower ran from warm to cold with no discernible loss of flow. I then switched it off and ran the bath hot tap which quickly went cold, 24ºC. I then switched that off and ran the sink hot tap which soon reached 46ºC and kept that up for as long as I tested. Running the bath tap simultaneously produced a little hot water but eventually returned to cold – this made the sink tap go cold too, but when the bath tap was switched off the sink tap soon returned to hot. No loss of flow except when both taps were running. I hope this helps!
    – BigglesZX
    Commented Feb 27, 2018 at 17:06
  • This sounds like the anti scauld is set low or become fowled, depending on the model sometimes it can be turned up, I have had luck with making an adjustment that fixed the problem , but I have had to replace the entire valve also. I would try adjusting the hot limit higher to see if this clears the problem. What brand is the valve there are different ways to adjust the high limit depending on the valve. Also removing the cartridge and flushing the valve body out may help in case some lime scale or rust has prevented the valve from working as it did in the past.
    – Ed Beal
    Commented Feb 27, 2018 at 17:53
  • Thanks Ed – is that likely to be located near the distribution pipes in the closet? Any tips as to how to locate it would be much appreciated – meanwhile I'll have a look at Google Image Search!
    – BigglesZX
    Commented Feb 27, 2018 at 17:57
  • The valve is part of the shower valve assembly. In some cases there are plastic stops with a disk that limits the range pull the handle and try rotating the disk to see if this helps, other valves have flow adjusters for both hot and cold, on these I open them all the way flow water then adjust back down, if you can find the brand we may be able to provide better guidance hopefully it is just full of scale that an adjustment will let it through and have proper control again.
    – Ed Beal
    Commented Feb 27, 2018 at 18:52

I'm not too sure but I'm going to take a misguided stab at this...

Short answer: You only have a tank-full of hot water at any one time. If you use this all at once, the tank can't heat up as quickly as you use it, so your shower/bath water gets cold.

Longer: The water coming from the district heating isn't the same water that comes out of your taps. It runs into(bottom left, I think) a coil/heat-exchanger in your cylinder(that big white thing in your photo. This heats up the water in your cylinder and runs back out again slightly colder, but without ever making direct contact with the water in your tank.

Cold mains* water goes into your tank(filing your tank), gets warm then runs out(top right?) to your taps/shower.

The central heating...either runs through a similar heat-exchange coil through your tank or the tank is actually two tanks stacked on top of each other. Either way, colder water comes in bottom right, gets warm and through the pipe about 40cm directly above (helped by the circulation pump you can see on that line.

There is also an immersion heating element(grey circular thing sticking out, to the left and below the pump). This will heat the water also, but takes a long time and is expensive. Presumably there to provide hot water if DH fails(it would be a staggeringly inefficient way to heat a home).

Hopefully this explains the short answer. Obviously heat can only exchange at a certain rate. If say, it takes 40min for the whole tank of water to heat up from the heat exchanged from the district heating supply, but you empty the tank in 15min, then the water hasn't had long enough to heat back up and you get cold water.

This would also explain the question on the phone? Did you check the heating directly after taking a bath? From what I can see, your heating shouldn't work immediately after you've used all the hot water, but will once it's heated back up.

...Incidentally, I'm not so sure this is a "heat interface unit" as such, though it should do the job.

Any other thoughts on this? I'm very open to correction here...

*or pumped/gravity-fed from storage in your building

(incidentally, you sure you don't have five radiators?)

  • Thanks for the thorough answer – these are good points and your explanation of the system sounds spot-on. I guess my central issue is why the longevity of the hot water seems to have changed dramatically recently, with no real alterations to the plumbing. A neighbour with the same system said they have had to "top up the tank" before, so perhaps whatever component is responsible for doing that isn't working quite right. And yes, there is a 5th radiator after all – heated towel rail!
    – BigglesZX
    Commented Feb 27, 2018 at 10:43
  • I guess to summarize the above I should say your explanation makes sense if the hot water were becoming uniformly unavailable, but the seemingly unaffected heating is a confusing counter-example...
    – BigglesZX
    Commented Feb 27, 2018 at 10:44
  • I did some further experimentation today and have narrowed the issue down to bath + shower – the bathroom sink tap doesn't seem to be subject to the "running out" of hot water. See my comment on Ed Beal's answer below for more detail. Hope this helps and thanks again for your answer.
    – BigglesZX
    Commented Feb 27, 2018 at 17:17
  • @BigglesZX hmm...I'm wondering if this might be some kind of hybrid system. ie. that hot tap water is heated directly by a heat exchanger, but that hot water is also stored in the tank. It could be designed this way because the heat exchanger would only properly up to a certain flow rate, but uses stored water for higher flow uses. This would make sense as one thing your bath tap and shower have in common is that they have a much higher flow rate than a regular tap. Again, I'm doing a lot of guess-work here.
    – Niall
    Commented Feb 27, 2018 at 17:58
  • @BigglesZX RE: recency, two things that come to mind are that A: If your shower is working off stored water, then increasing its flow rate should use up your hot water more quickly - so it would make sense that this might happen only after getting a new hose. B: It's been pretty cold lately. The cold water coming into your house will be colder and so a lot more work has to be done to get it up to temperature. Additionally, with everyone in the district using more hot water, the hot water coming in might not be as hot?
    – Niall
    Commented Feb 27, 2018 at 18:00

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