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Discuss Unsure if wet UFH is working - advice on diagnostics needed please. in the Water Underfloor Heating Installations area at PlumbersForums.net

I happen to have an IR / laser gun thermometer - I am getting between 28 and 30 degrees C on the brightwork below (B), the upper manifold (under the flow gauges) and the exterior of the copper infeed pipe to the upper manifold.
The obvious answer is that the overheat thermostat or the thermostatic mixing valve (TMV) has failed. The overheat stat operates to prevent water of more than 55 degrees C entering the UFH system - but the flow temperature limit setting is adjustable at the controller so could be significantly lower.

The TMV limits flow temperature by blending the incoming flow temperature.

However, from the description of your first post you seem to be getting flow across the manifold when the UFH pump is off - you said that the top branch of the manifold is hot but the lower branch is cold. That seems to indicate that the control valve is passing and that you are getting a secondary reverse flow of unblended boiler flow water. If that is the case you could get water at a higher temperature than 55 degrees (or whatever the overheat stat is set at) reaching the overheat thermostat, which could / would trigger it to operate.

The control logic for your system to operate correctly is that the control valve on the lower right of the manifold opens to allow the boiler flow to enter, it passes through the thermostatic mixing valve with the external temperature sensor and blends with the return water from the UFH, it reaches the pump, is pumped and passes though the overheat thermostat (limited by its setting - see above) then enters the UFH system - which can be seen by the balanced flow valves, after the last balanced flow valve has been satisfied the flow becomes the return, which should be direct, back to the boiler.

In essence if you are getting seepage from the control valve, with the Wilo pump not running, the lower part of the manifold should be hotter than the upper part.

In your case there appears to be an installation configuration issue that may be allowing a secondary reverse flow when the UFH pump is not operating

If everything is operating correctly, when the Wilo UFH pump is off ( after a reasonable period of time) the top and bottom of the manifold should be cool.

You have probably not got complete hydraulic separation between the two pumps - replacing the thermostatic mixing valve and reconfiguring using an injector valve would resolve that.

Hope this all makes sense
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Hi Brambles, thank you so much for taking the time to post all the replies - I have never really understood the flow and now I do (OK, it took me a couple of reads, but it's crystal now and what you've suggested regarding seepage also makes complete sense).

I have tested with the hi limit stat bypassed; the Wilo energised and we got good flow. The external temps on all pipework did not exceed 32 degrees during the 10 minute test, and the pressure gauge remained within 10-15% of the pointer's original position. I have since reverted the connections so the stat is back inline with the Wilo, and as expected the pump / flow has ceased.

I've ordered a new stat this morning as swapping it out seems the easiest / quickest way to see if it is actually faulty.

If the new stat allows the pump to run then the plan currently is to run it for now, but keep eye and still get a specialist / engineer in to check for the issues you suspect / do a general check or service later in the year (when hopefully circumstances are a bit better). If the new stat still prevents the pump from running then clearly the issue is more urgent and we will have to decide whether we try to find someone to look at it right away, or manage without the underfloor for a bit.

A big thank you to everyone that replied, it's a huge help and aside from the issue I am much more at ease just having a far better understanding of how it should function and what the various bits do in the system. Everything south of the controller was a bit of a mystery until yesterday!!

Reply to Unsure if wet UFH is working - advice on diagnostics needed please. in the Water Underfloor Heating Installations area at PlumbersForums.net

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