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

[MickeyFynn]

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 boiler through a second wiring centre. It's quite a distance between the boiler+timer and the airing cupboard, so there's no possibly of changing the wiring without major redecorating.

Yes, that does mean that running the UFH when the CH timer is off will pump cold water around the UFH loop. That's the way the house was when I bought it.

I want to upgrade to Nest (or another smart system). I'm not bothered about separate control of UFH because 95% of the time, the UFH warms the whole house. But I don't see how to control both CH and HW at the boiler and the valve+pump in the airing cupboard simultaneously.

Can one Nest be told to drive two heat links? I've looked online but can't find any documentation.

Ta

 

[RandomGrinch]

If you really wanted Nest, you could replace the Y plan controls with one, and keep the existing UFH thermostat - has it got a separate underfloor sensor?
Or, get two Nest.

 

[MickeyFynn]

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. I either have to run the pump run 24x7, or only have the UFH running when it would have run today (admittedly the Nest could turn off the boiler so the UFH doesn't get any hot water, but that doesn't seem like the right answer).

If I install two Nests, the UFH Nest will somehow have to tell the CH Nest to fire the boiler when the UFH wants heat. Can it do that, or can I run two heat links from one Nest?

 

[RandomGrinch]

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.

I see your edit.
You originally said just...

A wired thermostat is driving the UFH motorised valve and booster pump as well as calling for heat from the boiler

But additionally now say..

Yes, that does mean that running the UFH when the CH timer is off will pump cold water around the UFH loop.

So the UFH stat doesn't call for heat from the boiler?

 

[trojanhawrs]

Post a picture of your boiler wiring, wiring centres and existing stats/timers.

A wired thermostat is driving the UFH motorised valve and booster pump as well as calling for heat from the boiler through a second wiring centre. It's quite a distance between the boiler+timer and the airing cupboard, so there's no possibly of changing the wiring without major redecorating.

Yes, that does mean that running the UFH when the CH timer is off will pump cold water around the UFH loop. That's the way the house was when I bought it.

Why is the timer overriding the UFH call for heat?

 

[MickeyFynn]

Why is the timer overriding the UFH call for heat?

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.

 

[Stuckinarut]

Seems you need a wiring reconfiguration and or a pipe layout change. Your ufh motorised valve should drive open touch a microswitch and send a switch live to the boiler for that zone and the same with CH.

 

[RandomGrinch]

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.

Ok, got you.
And it sounds as if you're happy that if the UFH comes on, then that single thermostat would control the whole house - i.e. you'd have a single zone instead of two?
As the UFH thermostat does fire the boiler, there must be a link back to there - it should be possible to use this link in reverse, to enable the UFH pump from the main wiring centre.

 

[MickeyFynn]

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 other. I'm pretty sure the UFH was supposed to be a separate circuit, which is why (including the boiler) I have three pumps in a 4-bed house.

 

[RandomGrinch]

Thanks for the image, it also appears that you have 3 UFH stats, which brings me back to...

has it got a separate underfloor sensor?

Do your UFH stats have separate sensors?
These may make it more complicated to transition to a Nest.

However, your main problem seems to be that the UFH stats aren't triggering the 3 port valve.
I don't know about the others here, but I would attempt to tackle that, before thinking about the Nest.

 

[MickeyFynn]

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

 

[RandomGrinch]

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

Can you show us some pictures of the wiring in your wiring centres, we may possibly be able to work something out?

But as @Stuckinarut suggests..

Seems you need a wiring reconfiguration and or a pipe layout change.

Conversion to an S Plan+ system sounds like the ideal solution.

 

[MickeyFynn]

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 the thermostatic radiator valves to do most of the work, but here's not enough £ to be saved in energy bills to justify a significant amount of change.

UHF wiring is straightforward and I'm not going to remove the cover. The first three unlabelled LEDs come on for each of the three UFH sensor/actuator zones (fourth is unused). Boiler and motor lights come on when any of those three are calling for heat:

Meanwhile, this is the wiring centre in the airing cupboard. Possibly additionally complicated by our thermal solar, but I don't think that's the only issue here:

 

[MickeyFynn]

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.

 

[RandomGrinch]

Meanwhile, this is the wiring centre in the airing cupboard. Possibly additionally complicated by our thermal solar,

It looks like you have two 2 port valves, rather than a 3 port valve?
It looks more like an S Plan system, than

I have a conventional Y system (boiler, HW tank, CH).

That does make things (only very) slightly more straightforward.

In the wiring centre, does the flrst white flex from the right go to a pump?