Nest 3rd Gen + Heat Only Setup

[tmannion87]

Hi all,

What information do I need to confirm whether a Nest 3rd Gen Thermostat is compatible with my 2-wire, heat-only gas boiler setup? From what I understand, the main question is whether the thermostat can power itself from the existing wiring or if I’ll need the Nest Power Connector. I’d like to be certain before purchasing to avoid any return issues.

My current system uses an old Honeywell 2-wire thermostat (R and W only) and a Weil-McLain CGA Gold natural gas boiler. The Nest compatibility tool suggests it should work, but I’d appreciate confirmation from anyone with a similar setup before moving forward. Any insight would be greatly appreciated. Thanks!

https://imgur.com/a/xRoqHDL

[CoolingWizard]

@tmannion87 ,

The good news is that the CGa boiler usrs a 24VAC control system. The bad news is you need three wires or Google Nest Power Adapter to get common to the thermostat unless you install a new thermostat cable. 

You can find the thermostat connection in the lower right.  There ate 5-wires.  Left most black wire is Power Common.  The second wire is green and goes to chassis ground. The third wire is blue and is for DHW control. The fourth and fifth wire are black and go to your thermostat. The fifth wire is R and the fourth is W1. 

The Cooling Wizard

 

[tmannion87]

Thanks so much for taking the time to reply! Just to confirm, this would be the correct wiring diagram for my setup with the power connector:

[CoolingWizard]

@tmannion87 ,

Yes indeed that will work. The R will be the black wire in connector position pin 5, W is Pin 4 and Common is Pin 1.  

The Cooling Wizard