What could cause my hot water to run out quickly... but only in the bath/shower?

Question

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.

Answer 1

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.

Answer 2

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?)