Is my multizone S plan wiring correct for UFH please?

[scotster]

I'm currently looking at modifying the install of an S plan with CH Zone, HW Zone and single UFH zone off a vaillant combi. The wiring diagram is attached, mine are the wonky lines! The UFH diagram is partially complete but the missing wires are hopefully obvious. I have read that the UFH manifold pump is fed directly from the UFH thermostat as I have shown here which surprised me as I thought it would be fed by the valve switched live when open but now I see it makes little actual difference.

The only question I have is that this means in a situation where the UFH TRV does not need any hot water (UFH is running at correct temp) and is closed and the HW is off and the CH ZV is off (My CH will come on later in the morning) BUT the UFH thermostat IS demanding heat the boiler pump may well run which will force flow either through the internal boiler Auto by pass or an external fitted auto bypass. It seems from what I have read that this is OK? I thought the bypass was kind of for 'emergencies only' in case of some type of failure but here it may be used much more frequently. Can you advise on what bypass pressure should be set at on the external one please or how I can figure it out? Thanks.

 

[oldbutnotdead]

The reason it's been wired that way is the installer didn't want to fit a relay.
The UFH controller/thermostat has to do 2 jobs- call for heat from the boiler and power the UFH pump. If installer had been shoddy
and parallelled them then the ufh pump would end up latching on and running when there was dhw or Ch demand.
But your actual question makes no sense. If the UFH setup has reached temperature then the stat will be off, no call for heat to boiler, all good. If UFH has not reached temperature then heat is required so the boiler pump will be running to move heat to the ufh system, again all good

 

[scotster]

Thanks for the reply. So I just want to check the there is no scenario when the room thermostat will be at for example 17C demanding heat on a cold winter morning and the UFH heating water will have been running for 45 mins and will have reached for example 40C so the UFH TRV will have stopped bringing in HW from the boiler BUT the room has yet to warm up... So it follows the UFH Zone valve will be still be open and there is demand on boiler as the room thermostat is still demanding heat. In that case the boiler is pumping against closed Zone valves for CH and HW (not yet on from programmer) and against a UFH TRV that doesn't want more HW? Any help welcome thanks.

 

[oldbutnotdead]

Best way will be a DPCO relay to separate the electrical feeds to the ufh pump and the boiler. Detail to follow or search in this forum. All v straightforward

 

[SpecialK]

I had to plumb in my bathroom rad as a bypass due to the combi not liking the pressure of the UFH system and bypassed straight away. This caused the boiler to go into a cycle protection every time and the water temp at the UFH manifold to vary too much...