Fitting Google nest thermostat, looks as if I replace the existing thermostat with heat link but only live, neutral and earth - can it be this simple?

[ThomasAnderson]

Hi,

Just a brief one. I've got a Google Nest learning thermostat, which claims you cant power it with greater than 12v. The heat link appears to replace the old thermostat, an OpenTherm ESRTP4+. However, there are only three cables, standard live, neutral, Earth. There are wiring diagrams in the manual, but from what I can understand, is it possible I just put those three wires into the heat link, and run the thermostat itself wirelessly?

 

[ericmark]

No. With Nest e it needs two wires, Nest Gen three needs two wires to power it, earth only required if taking 12 volt from heat link, but to control boiler between 2 and 5 wires depending on how wired on top of the supply wires.

OT used 0-48 volt from memory, if boiler is OT enabled. Other wires 6 terminals volt free with com N/O and N/C for both CH and DHW which may go to boiler or may go to motorised valve.

The N/C CH is rarely used.

 

[RandomGrinch]

Not that simple I'm afraid.
Your current stat is a two wire stat (and isn't utilising the OT capability) - the brown wire may be a live and the black wire may be a switched live. It won't be a neutral.
It is also possible that your boiler is using (extra) low voltage switching, which means the brown may not even be a live at all!
Telling us what boiler you have may give us a clue as to which.
Usually the Heatlink is mounted next to the boiler, this allows the existing stat wiring to be reused to provide 12V power to the Nest thermostat - providing the existing stat was placed in a convenient location.
If you did want to mount the Heatlink here, there may be a grey wire hiding behind the ESI that could be used as a neutral, providing it is connected up correctly at the other end.
Alternatively, the 2 wire, battery operated Heat link of the Nest thermostat E could be a simple swap in replacement for the ESI.

 

[RandomGrinch]

the thermostat itself wirelessly?

Just to add, the thermostat itself need not be directly wired up to the Heat link, it can run off the Nest thermostat stand...

 

[ThomasAnderson]

Not that simple I'm afraid.
Your current stat is a two wire stat (and isn't utilising the OT capability) - the brown wire may be a live and the black wire may be a switched live. It won't be a neutral.
It is also possible that your boiler is using (extra) low voltage switching, which means the brown may not even be a live at all!
Telling us what boiler you have may give us a clue as to which.
Usually the Heatlink is mounted next to the boiler, this allows the existing stat wiring to be reused to provide 12V power to the Nest thermostat - providing the existing stat was placed in a convenient location.
If you did want to mount the Heatlink here, there may be a grey wire hiding behind the ESI that could be used as a neutral, providing it is connected up correctly at the other end.
Alternatively, the 2 wire, battery operated Heat link of the Nest thermostat E could be a simple swap in replacement for the ESI.

Hi, thanks for the reply. Yes, I should have said - it's a Logic ESP1 30 combi boiler

 

[RandomGrinch]

Hi, thanks for the reply. Yes, I should have said - it's a Logic ESP1 30 combi boiler

I believe your boiler would be mains switching, but it can also support OT.

Mounting the Heat link near the boiler and connecting to the OpenTherm terminals with a new cable would probably be sensible - the Nest could better control the modulation of the boiler, with the potential for some efficiency improvements.

As mentioned before, the existing cabling to the ESI could then be reused to power the 12V Nest thermostat, via terminals T1 and T2 on the Heatlink.

However, in Ideals wisdom, I'm afraid that in order to change any wiring within your boiler requires the front cover to be removed - this effects the room sealing of the unit and would require the boiler to be checked for safety afterwards.