UFH suddenly cold

[James Carmichael]

Hi,

I have an underfloor heating manifold with three zones. For about the last four years this has been working perfectly but on Saturday morning we woke up to find that all three floors were stone cold - and they have stayed that way since.

Things I know:
(1) The boiler is working fine - we have working radiators and got water. The supply pipe to the manifold is as hot as it always is. The return pipe is too.
(2) The two thermometers on the manifold are registering the same temperatures as they always do - 55 degrees on the flow side, 45 degrees on the return side.
(3a) The three flow meters are showing the expected level of flow to each zone - one at three litres per minute, two at two litres per minute.
(3b) And all three sets of plastic zone piping coming from the flow side of the manifold are as hot as you'd expect.
(3c) And all three sets of plastic piping returning to the manifold from each zone are as warm as you'd expect.
(4) Bleeding the system by flushing each zone with mains pressure water doesn't help.
(5) When I remove the manifold TRV the temperature on the flow side of the manifold initially drops to ~ 50 degrees, before rising as expected to boiler water temperature (~75 degrees).
(6) Pump is making nice quiet pumping noises, as it always has - no whining, rattling or anything else to suggest it has failed.
(7) Absolutely nothing has been changed. Absolutely nothing appears to have broken. This setup was working fine, as I wrote above, for quite a few years.
(8) The last few days have been on the colder side, but it has been colder and when it was those floors were toasty.
(9) One zone is an insulated concrete slab. The two smaller zones are suspended timber floor packed underneath with Celotex. As I wrote above, this is unchanged and all three zones have previously been just fine.
(10) All the pins on the various valves (TRV, manifold out, zone returns etc) move freely - no evidence anything is stuck.

Any ideas as to what this could be, or what I could try next?

Thanks

JC

 

[dilalio]

What roomstat(s) do you have?
Is the system demanding heat for long enough to warm up the thermal mass?
Could be that the timing is wrong or the stats faulty and is switching off too early.
Try setting it to constant. If nothing else has changed and you say all other components are operating as they should then only roomstats left out of above description!

 

[John D v2.0]

(2) The two thermometers on the manifold are registering the same temperatures as they always do - 55 degrees on the flow side, 45 degrees on the return side.
(3a) The three flow meters are showing the expected level of flow to each zone - one at three litres per minute, two at two litres per minute.
(3b) And all three sets of plastic zone piping coming from the flow side of the manifold are as hot as you'd expect.
(3c) And all three sets of plastic piping returning to the manifold from each zone are as warm as you'd expect.

This seems bizarre! So where is the 7 litres of 10c temperature drop going?
That's around 5kw going somewhere, you'd expect to feel it coming through the floor unless a sink hole has opened and your loops are dangling in the wind under your house
As above I'd just check to make sure the heating is on for long enough.