Nest for underfloor heating - replacing existing Thermostat

[RowLarry]

Good Morning all

Apologies for posting another Nest thread, but I've searched and can't seem to find the answer. Thank you in advance for taking the time to read.

Brief intro: Combi Boiler (Vaillant EcoTec Plus 937) and recently replaced the rubbish Honeywell wireless thermostat solution originally installed (BDR91) with a 3rd Gen Nest for the main CH control in the house. A straight swap so nice and straightforward.

We also have wet UFH in our kitchen extension on the ground floor. This has it's own Thermostat (Reliance touchscreen jobby) and is one zone I believe. This can call for heat independently from the rest of the house and often does so.
The UFH has the pump, manifold and two active valves that I can see.

Problem is that the Reliance has now broken, and I want another like a hole in the head.
So I have bought another Nest 3rd Gen to replace it.
I'm aware it's not totally ideal, but the app smarts and the fact I have another one sealed it.

However, I am struggling to get my head around how the Heat Link will be wired.
Nest is in essence a switch, and I'm replacing the existing wired 230v Thermostat, which has 3 wires going to it: L,N,L (3A) (Please see photo)


Am I right in thinking that the current wiring from the (messy) junction box to the wired Thermostat should be replaced as follows and I don't even need to touch anything there?

From:
JN Box > Existing Thermostat
To:
JN Box > Heatlink > Nest

I have attached a diagram with the assumed wiring, does this look right?
I'm aware the temperature sensor will be surplus to requirements.


Any help gratefully received. I'll say at this point that if this is wrong or I need to open up the junction box I will employ a friendly local tradesperson ASAP. Thanks!

 

[ianmcd]

Yes that will work, would be good practoce to put brown or red sleeving over the green wire or some red or brown tape to identify it as a live wire, will work without it just good practice

 

[RowLarry]

Many thanks Ian, your input is much appreciated.
Yes I'm aware that the wire is the wrong colour (the same bodge has been used upstairs also for the previous BRD91) and I will ensure the wire gets labelled.

The only thing I really wasn't sure about was the need for the jumper wire from the Live to 2 on the Heat Link?

 

[ianmcd]

Many thanks Ian, your input is much appreciated.
Yes I'm aware that the wire is the wrong colour (the same bodge has been used upstairs also for the previous BRD91) and I will ensure the wire gets labelled.

The only thing I really wasn't sure about was the need for the jumper wire from the Live to 2 on the Heat Link?

yes you need that link for your setup, other setups can be used but that is how yours was originally wired

 

[ianmcd]

I take it you are aware that the existing UFH temp sensor will no longer work with the nest

 

[RowLarry]

Yes I am, thanks. As I see it it's 6 of one 1/2 dozen of the other - the UFH temp of the substrate doesn't bear any resemblance to the temp in the room so when setting the old Thermostat you had to set it as such. Hopefully the Nest will learn what heating is needed to get the room to the desired temperature, not the floor!

 

[ianmcd]

Yes I am, thanks. As I see it it's 6 of one 1/2 dozen of the other - the UFH temp of the substrate doesn't bear any resemblance to the temp in the room so when setting the old Thermostat you had to set it as such. Hopefully the Nest will learn what heating is needed to get the room to the desired temperature, not the floor!

yes it will do, not sure as I have never fitted one on UFH but you might , not saying you will, have to go into the settings menu and tell it you have UFH, really dont know for sure, will be interested to hear how you get on

 

[RowLarry]

That’s true Ian, definitely something I’ll look into. Thanks again.

Any other persons experience of nest for UFH would be welcome.

 

[Lower]

I have a nest with underfloor heating and it works well. You have to tell the nest you have underfloor heating but other than that it is simple set up.

I have the underfloor heating schedule set to be on all the time and let the nest manage the temperature in the room and after a week or so of overshooting it learnt the system lag time and works perfectly. Its quite a well insulated room, so the nest turns the underfloor heating on between 4am till around 8am and then it remains at temperature for the remainder of the day unless its really cold.

The only time we have a problem is in the spring when sun starts to add signficant solar gain and the room overheats, but thats not the nest's fault. The nest is getting the room up to temperature at the right time and turning off the heating. Its the solar gain that then taking the room over temperature.

 

[RowLarry]

I have a nest with underfloor heating and it works well. You have to tell the nest you have underfloor heating but other than that it is simple set up.

Hi Lower, thanks for the reply. Just out of interest, how did you tell the Nest that it was controlling UFH? I have set one up so far for the main house central heating and don't remember a setting which did this. Many thanks.

 

[Lower]

Hi Lower, thanks for the reply. Just out of interest, how did you tell the Nest that it was controlling UFH? I have set one up so far for the main house central heating and don't remember a setting which did this. Many thanks.

My nests are around 3 years old, so i would imagine that its a bit different now, but i had to tell my nest that the heat source was gas and that it was controlling in-floor radiant heating.

 

[ericmark]

How are you connecting Nest? There are different ways to use a wall thermostat. But Nest as far as I am aware can only monitor one room, you can"t link multiple OpenTherm thermostats together. You can link multiple mark/space together, and Nest has both options.

Wall thermostats in parallel can ensure the boiler fires when required, but can't stop a room over heating. In the same way thermostatic radiator valves in parallel can ensure a room does not over heat, but can't turn on the boiler.

So to get full control you need both thermostats, one to stop over heating the other stops under heating. Both can be simple or programmable.

However some careful thought and you can limit the number of wall thermostats, in some homes one is enough, the idea has been for years to put the wall thermostat in the coolest room with no door to outside, on entrance floor and no alternative heating, this ensures boiler runs when required, and all other rooms are controlled by TRV heads.

A TRV head is the same as having a wall thermostat connected to a motorised valve, the difference is a motorised valve is on/off only not anaguloge and it has to be hard wired, where the TRV if connected to boiler electrically is normally connected using wifi. Of course in both cases they don't have to be electrically connected to boiler.

In the main UFH is not used as only form of heating, it is too slow to react, but it could be used, and there is nothing to stop having a system which controls the UFH water temperature just like OpenTherm controls boiler output water temperature, and just because I have not seen it done, it does not mean it can't be done.

 

[ianmcd]

But Nest as far as I am aware can only monitor one room

He is only using it for one room

you can"t link multiple OpenTherm thermostats together. You can link multiple mark/space together, and Nest has both options.

Who said anything about opentherm ?

 

[RowLarry]

Hi all, thanks for your responses so far.
Especially thanks to Ian, all is wired up and working fine.
I had to tell the Nest it was connected to underfloor heading, so we’ll see how it goes over the next few weeks.

 

[dwag]

Sorry for dragging up an old thread.

I am in a similar position to RowLarry, where I already have a Nest/Heat Link connected directly to the boiler which controls the central heating radiators, though have just built an extension where I've laid wet UFH, I'm yet to fully install it, though just wanted clarification and perhaps insight into how to wire it all up if I purchase another Nest Thermostat for the UFH in the kitchen.

There's no existing thermostat like RowLarry's so I'll be starting fresh..

Am I right in thinking I could potentially copy RowLarry's diagram from the UFH junction box to the Heat Link, and then connect cables from the Heat Link to the Nest Thermostat? (What cables would those be also?)

Appreciate any replies.

Thanks