Installing a NEST Thermostat

[Alex123-4]

Hi Everyone,
Im planning to install a NEST Thermostat on my gas combi boiler. Its an oldish Halstead Finest Platinum. The current thermostat is a Danfoss RMT230 and its wired directly to the boiler with two wires.
Now heres my problem and NEST wont comment. I can wire up the live and neutral to the Nest Heatlink ok and also connect two wires to T1 and T2 directly to the NEST Thermostat but Im wondering will it be ok to connect the two wires currently in place (Call for Heat and common) to the Heat Link 3 and 4? I measured across the thermostat whilst it was off and I read 100vdc is that right for this type of boiler?
Has anyone else out there fitted this themselves it doesnt seem difficult but I dont want to go ahead unless Im dead certain. Ive read the NEST Installation and it appears staraightforward as long as you dont connect 230v directly to the thermostat!
I would appreciate advice on this. Thanks

 

[jackthom]

The HeatLink terminals you would use to fire up the boiler are

2) Common
3) Call For Heat.

There is no voltage across those two terminals as supplied, they are just the contacts of a relay, ie a switch. They become connected together internally when the boiler needs to be fired up.

Is the Heatlink going to be sited near to the boiler? It's normally recommended that it shares the same mains power feed as the boiler.

It should be possible to find a good point where you can sever the 2 core cable, which at present goes from the boiler to the Danfoss Thermostat, to connect the Heatlink terminals 2 & 3 to the boiler.

It's sometimes convenient to use the other section of isolated 2 core cable to carry the T1 & T2 connections to the Nest Thermostat, assuming you want to mount it in the same location as the old Danfoss.

 

[Alex123-4]

Hi Jackthom,
Many thanks for the useful information.
My boiler is fitted just inside the loft and ther power isolation switch is fitted directly below that. Im planning to fit the HeatLink next to the boiler approx 30-40cm distance as recommended. My old Danfoss is fitted in the inner hall on the wall so Im planning to fit the new Nest Thermostat in its place.

I will use a junction box to connect the heatlink to the mains power and thermostat/boiler so that will life easier.

Can I ask you sir are the contacts for the Call for heat polarity sensitive - I mean would the Nest still work if I got them wired the other way to how it is at present??
Apparently the NEST thermostat isnt polarity sensitive so that makes life even easier. I just wanted to confirm the Call for Heat connections like I said the boiler is an oldish Halstead Finest Platinum ( I believe Halstead have been bought over by Glenn Dimplex) and theres not many of these boilers about any more.

If I have any more questions I will fire them your way. Again many thanks for your reassurance. I see I got the NEST HEAT LINK connections wrong it is 2 and 3 NOT 3 and 4 haha

Kind regards
Alex

 

[jackthom]

You don't need to worry much about the polarity in this case because terminals 2&3 are simply working as a switch.

Generally the Com lead is made Live for 240V thermostat controls and when the thermostat is calling for heat the other contact also becomes Live ("Switched Live" ) telling the boiler to fire up. In this case it will work either way round.

 

[Alex123-4]

Hi again,
Thanks for the final update. I will be purchasing a NEST next week so Ill let you know how it goes. Again many thanks for all your assistance.

Kind regards
Alex

 

[Alex123-4]

Hello Jack,
. I appreciate your comments regarding the installation of a NEST thermostat and I plan to go ahead with this. Ive just seen an advert from Amazon re the new 3rd Generation Nest (available from the 8th Dec 2015) and I wondered (as one does) will this be wired the same as what you advised for the 2nd generation, ie. Live and Neutral for power to the HeatLink, Call for Heat and Common to Heatlink terminals 2 and 3 and then a two core from T1 and T2 on the Heatlink to the Thermostat?
Thanks
Alex

 

[jackthom]

Yes the 3rd gen Nest has a couple of extras added but the wiring terminals for mains power, controlling the boiler for CH and T1/T2 are the same.
There are some extra terminals if you want to use nest as a Hot Water programmer.