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Aha. That might be the ticket. At the very least, a hardware switch would make it easy to turn the IGU off for a couple of hours when there's no-one home. Because nobody would bother to go to all the thermostats. A light switch would do that. And a simple WiFi light switch would me to work it...

Yep, that did it. UFH now energises the CH pump and the valve whatever the state of the CH thermostat. Massive win. You have no idea how many plumbers I've asked about this over the last fifteen years, and none of them had a clue. I owe you big time. Anything I can do? Feel free to send me...

OK, I understand that now. This is the one, right? I know the wire is black, but it's got a brown sleeve, and that brown wire coming in from the top is definitely the brown wire from the CH valve. I double-checked with a multimeter that the thin brown wire was live when the UFH was calling...

And the results are. The two black wires a different guages. This might be a clue as the wires that run long distances seem to be thinner. The thick wire is live when the CH thermostat is calling for heat The thin wire is live when the UFH is calling for heat The red wires, BTW, lead from the...

We're off-=topic now, but my DHW circuit is actually a circuit with it's own pump (not gravity fed). The secondary return pump (a special bronze one that doesn't corrode in fresh water) circulates hot water around the circuit so there's instant hot water when you run a tap. Towel rails on on...

Yes, I have a multimeter and I'm not afraid to use it. Hold my coffee.

I'd assumed that one powered the UFH valve/pump, and the other went to the boiler (not the wiring centre we're discussing with RandomGrinch). But I'm hoping that it might go through the wiring centre. Watch this space.

Yes, that's what I have now. The UFH doesn't get flow unless the CH thermostat calls for heat. I think you diagram is how it was supposed to be installed. There is a UFH valve where you drew it - I missed that off my diagram. There is one radiator (in the 4th bedroom) that is hot whenever the...

Folks, this is awesome. I've been trying to get a grip on this problem for literally 15 years. There are two black wires. So, are you saying that those two black wires are the call for heat from the UFH and the CH thermostat? If so, we're close to the key to my problem + The CH thermostat...

Maybe in a more competently built house. I met the plumber who did mine (after it was completed) and he was very proud that he'd thought to plumb the towel rails into the secondary return so they would come on with the HW circuit instead of the CH. Not realising that the towel rail's he'd...

The other white cables appear to be a bronze pump for a totally unnecessary secondary return on the hot water and the cylinder thermostat. That secondary return has been a pain. The bathroom radiators are chrome plated. They all rusted from the inside from being on a freshwater circuit and...

Yes, to the pump in my diagram. And the black cable next to it is the CH motorised valve And the black cable on the left to a motorised valve for the hot water. So S Plan it is. Sorry for the confusion.

Other thing maybe worth noting is that I have a Vaillant EcoTec boiler with a built-in overrun shunt. There isn't a shunt otherwise.

I hadn't considered that posting pictures of wiring would be helpful, for reasons that will become obvious. But clearly a wiring change is preferable to a pipe layout change. Currently I leave the CH motorised valve locked open during the winter an everything kind of work works. I am relying on...

Well, if anyone here covers southwest London, it looks like this is more than I can handle on a DIY basis.

Something like this. I haven' shown the three separate underfloor loops and actuators, but you get the idea. Nobody would have designed it this way. I think the folks who build the house contracted the underfloor heating separately from the rest of the plumbing and they didn't talk to each...

Sorry, maybe I wasn't clear. The UFH can call for heat and the boiler does run, but unless the CH wired thermostat is also calling for heat, the CH motorised valve doesn't open and the UFH doesn't get any flow. I'll draw a picture.

Not sure I get that. My main priority is to control the UFH which warms the whole house 95% of the time. And then UFH loop only warms the house when the CH circuit is also running. If I only replace the controls at the boiler, the Nest can fire the boiler but can't control the UFH pump/valve...

Hi I have a conventional Y system (boiler, HW tank, CH). But my underfloor heating is an extra loop on the CH circuit with its own motorised valve and pump in the airing cupboard. A wired thermostat is driving the UFH motorised valve and booster pump as well as calling for heat from the...