How to wire Nest to both a boiler and a second pump

[MickeyFynn]

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

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 CH valve and HW valve. So now I'm complete confused about how the CH thermostat fires the valve but the UFH doesn't.

 

[RandomGrinch]

So now I'm complete confused about how the CH thermostat fires the valve but the UFH doesn't.

It sounds like we may be getting there, but you're looking at it the wrong way around. The C/H thermostat doesn't connect to the orange wire.
The C/H thermostat connects to the brown wire of the C/H valve - this drives the valve open. When it is open, the orange wire becomes live (fed from the grey, it is just a switch).
When the orange wire is live, the thick black wire seems to be a switched live that will fire the boiler.
The other black wire, if it is the switched live from the UFH, bypasses the valve; it turns on the boiler, without moving the valve.

If you are completely confident that the thin black wire is the switched live from the UFH, simply connect it to the brown wire of the C/H zone valve (along with the brown wire from the rad thermostat).
When the UFH calls for heat, the zone valve should open, and when it is fully opened, the boiler should be fed a switched live.

 

[Mister Banks]

It's a secondary return so the bronze pump returns DHW to the tank so you always have hot water at the tap.

I know that. Why would you have a radiator/towel rail on the HW circuit?

 

[Mister Banks]

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 that circuit.

That sounds odd to me. If the towel rail is on the secondary return then it will only heat up when a hot tap is turned on.

As you say off topic so enough said.

 

[MickeyFynn]

If you are completely confident that the thin black wire is the switched live from the UFH, simply connect it to the brown wire of the C/H zone valve (along with the brown wire from the rad thermostat).

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 for heat and not otherwise. I couldn't run up and down stairs fast enough to see whether it went live as soon as a UFH thermostat switched, or only after the motorised valve hit the microswitch. There relevance of which being:

If I connect that thin black wire to the connector above, the UFH doesn't run. Motor doesn't start, valve doesn't open, LED lights on the wiring centre don't light (neither pump nor boiler).

Want to take a look at the UFH wiring too?

 

[RandomGrinch]

This is the one, right?

Nope!
We're looking for the brown wire connected to the C/H zone valve - which I believe is this one...

 

[MickeyFynn]

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 your address and I'll send you a bottle of something.

I still have the question about Nest etc, but now it's not as a workaround for a mis-installed system. I'll get back to that and ask it again later.

 

[RandomGrinch]

Brilliant!
Well done, and thanks for sticking with it.
I hope it gets you by for the moment - I will keep thinking about a way forward for the smart stat.
But @trojanhawrs idea may be along the right tracks.

Some thoughts for another day...
Firstly, we now know that the black wire is the call to heat from the UFH - if you can trace the cable it comes from, how many wires are in that cable that we can potentially use to control the UFH.
Secondly, your UFH wiring centre has a timer link here....

It may be possible that a smart control bridging this link could add some more control over your UFH, without switching the whole lot on and off as Trojan was concerned about.