Nest G4CVZ wiring issue (replace old mercury thermostat with 3 wires for boiler)

[XMNest]

I have a mercury thermostat with 3 wires (R, G and W) thermostat control. Between the R & W wires, I tested 24V AC. The Aquastat relay for the boiler is L8148 E 1257 and I want to replace my old thermostat with a Google Nest Thermostat (G4CVZ). I would like to know how to wire and do I need to buy a C-wire adapter?

[Ryan_G]

Hi XMNest,

Thank you for reaching out and we're sorry for the delayed response.

The Nest Thermostat is compatible with a 24V HVAC system like yours. The same wire will be connected to the base terminal on Nest Thermostat with the same label. The common wire is recommended for the Google Nest Thermostat (G4CVZ), but if you don't have an existing wire for the common wire, it is more convenient and affordable to have the Nest Power Connector.

You could also reply with the photo of your current wiring for us to better assist you on the setup.

Regards,

Ryan

[XMNest]

Hi Ryan,

Thanks for replying to my question about the Nest G4CVZ, maybe the 3 wires system is not suitable for the Nest GrCVZ thermostat. All details and photos are attached in a PDF file (I don't know how to insert photo and PDF firl here, so I will email to you), may be it is the world class problems, I hope experts here can solve it.

Thanks in advance!

XMNest

On my old mercury thermostat I measured (It works well but the Changes after call for heating  is very weird ??):

No call for heat:	RG=24v, ∞ ohms;	RW=24v, ∞ ohms;	GW=0v, 0 ohms;
Call for heat:	RG=24v, ∞ ohms;	RW=0v, 0 ohms;	GW=24v, ∞ ohms;

I tried three ways of wiring like follows but they are all not work:

(On our Aquastat relay (L8148 E 1265), TV=W, W or Z=R, T=G, see the photo above)

1. The following connection shows an "N253" error, indicating that W is not powered. Nest seems works but can't control my boiler (my boiler does not heat up whether the Nest is “heating” or not).

R -----> TV (my White) 

W-----> T (my Green)

C----> W or Z (my Red) 

 2. I swap the position of the G and W line, as shown below, but it shows "no power" and "wifi disconnected", and it doesn't work at all.

R -----> TV (my Green)

W-----> T (my White)

C----> W or Z (my Red)

 3. I measured a 24v AC between my R and W, so I wired like as follows, Nest seems works but can't control my boiler (my boiler always heats up whether the Nest is “heating” or not):

R -----> T (my White))

W-----> TV (my Green

C----> W or Z (my Red)

[XMNest]

Sorry my mistakes, the third wiring method should be as following: 

3. I measured a 24v AC between my G and W, so I wired like as follows, Nest seems works but can't control my boiler (my boiler always heats up whether the Nest is “heating” or not):

R -----> T (my White)

W-----> TV (my Red)

C----> W or Z (my Green)