Rads and UFH don't work together

9 Mar 2016
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United Kingdom
I have UFH downstairs and Rads upstairs. The CH out from the boiler goes through a Y Splitter and I then have a 2-port motorised valve on each CH circuit, one leads to the upstairs CH rads and is all controlled via one thermostat, the other goes downstairs where it meets the UFH manifold, blending valve and an additional pump for all the downstairs UFH, each room has its own room stat and a call for heat from any one of them opens the 2-port motorised valve.
If either the Rads OR the UFH is open, then that circuit is boiling hot, the boiler temp is running at 70°C and fires each time the temp drops as the water circulates and I can't touch the pipes near the boiler. If both are open, neither run hot, the boiler display shows the internal water temperature of around 30°C and the boiler doesn't fire to heat this.

Have you ever come across this before please and do you have any suggestions or recommendations as to what I can do to solve this?

Thank you all very much
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Is this fault or has it been like this since installation?

It sounds to me like a wrong wiring situation but you need to tell us more. How is hot water provided?

It would be a rather unusual fault but then sometimes unusual faults do occur.

I've only recently noticed this but don't know if it's always been the case - I noticed it as the house does warm, but only eventually, so whilst I can't say for sure it wasn't always like this, I seriously doubt it as this system is now 5 years old.

My boiler is a Baxi Duotech 28HE Combi Boiler and provides hot water with no issue. From my limited understanding of the system, I believe the divertor valve in the boiler decides which water runs through the heat exchanger, giving priority to hot water when the tap is on.
I would expect that it is a wiring problem rather than a fault. Regardless, the fault finding procedure will be the same to identify what is wrong.

Many installers virtually only fit combi boilers and seem to have difficulties on conventional systems like yours.

The normal procedure would be to call a competent heating engineer to come and diagnose the problem.

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The divertor valve was playing up. Have replaced it and all is working fine.

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