I have two wet central heating loops: one for the radiators, one for the underfloor heating. One these loops (the UFH loop) is not getting hot.

I have found where the two loops split and checked the automatic valves, both work as expected. I have checked both move when switched.

I have checked the UFH pipes, and flushed them through with new water to prove no blockages. I have bled the air the UFH manifold as well as all the radiators. I have left it running for 48hrs to be sure it isn't something that just takes time to settle. I have checked the pump for the UFH, and that is spinning well. I have checked the UFH mixing valve, and that appears to work, and if I set it to fully cold, the flow rate can be seen on the manifold that water is being pumped round my UFH pipes.

I have run out of things to look at which could be the cause.

What else should I check?

UPDATE: I have replaced the mixing/bypass valve to ensure it isn't that which is broken. Still same behaviour.

  • "I have checked the UFH mixing valve, and that appears to work, and if I set it to fully cold, the flow rate can be seen on the manifold" - ...and if not set to fully cold?
    – Ecnerwal
    Commented Nov 12, 2023 at 13:14
  • Is there a bypass?
    – Solar Mike
    Commented Nov 12, 2023 at 13:36
  • @ecnerwal then there is no flow, which is what would be expected with no hot water feed.
    – Puffafish
    Commented Nov 12, 2023 at 13:52
  • @SolarMike not sure what you mean. Bypass to what? The valves can be manually switched on which I've already tried to no avail.
    – Puffafish
    Commented Nov 12, 2023 at 13:53
  • Often ufh loops are designed so that the flow is run through a bypass as the temperature control. You need to draw every pipe and make sure you know what is happening.
    – Solar Mike
    Commented Nov 12, 2023 at 13:55

2 Answers 2


This is not the answer people would want, but it is what happened.

After replacing the mixing valve, and experiencing the same behaviour, I looked for another option.

When running nearly, but not quite, fully cold, there would be flow round the UFH loops, and the pipe marked as RETURN would get very hot very quickly. Then the temperature could be set slightly higher, up to around 40C. The pipe marked FLOW would then very slowly heat up.

This hinted that the flow and return pipes were labelled the wrong way round. Luckily swapping the connections over was pretty straightforward, so I did that…. And then everything started working as it should. This also explains why the system could never run very hot, as it was reliant on scavenging some heat from the return port (which were plumbed into the flow of my heating circuit). Hopefully no major damage has been done to anything in the circuit after all this time being back fed.


Your symptoms point to the assessment of the mixing valve "appearing to work" being incorrect, as you have flow only when that is set to fully cold. If setting that to "other than fully cold" leads to "no flow in loop" then it appears to be not working correctly, from here.

That valve should take cold water returning from the loop and mix it with sufficient hot water to reach a target temperature to send it back to the loop.

  • I didn't claim the mixing valve worked, I was using that as much as a test of the UFH pipes and the pump rather than anything else. Is there any method to test/swap this valve without draining a good chunk of the system?
    – Puffafish
    Commented Nov 12, 2023 at 14:19
  • Not if the system lacks valves to isolate it for easier replacement, probably.
    – Ecnerwal
    Commented Nov 12, 2023 at 16:39
  • As updated in the question, I have swapped out the mixing valve, and I'm still getting the same behaviour: it's as though the hot feed from the thermal store is blocked or in some way isolated from the UFH manifold.
    – Puffafish
    Commented Nov 17, 2023 at 15:09

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