Anyone has dealt with old furnace with multiple zone control panel adding nest power connectors?

[wlibala]

So I've been trying to replace my Honeywell thermostat with the Nest Thermostat(2020), which lacks a C wire, and got the nest power connector ready.

It turns out my water-heat-only old furnace doesn't have a C wire available. It is on a 4-zone system, and I opened the panel but really can't find where I can wire the C wire to the 24v transformer. Has anybody dealt with this situation before? I would appreciate any recommendation. Please see the diagram and picture below, the top left is where the current thermostat cable coming out.

Oops, will try to upload the images when I get permission.

[Patrick_Caezza]

@wlibala 

You can upload a picture by clicking the little camera icon at the top of the "Replying to" window.

[wlibala]

[Patrick_Caezza]

@wlibala 

Thanks for the pictures.

Can you post a picture of the zone controller and its wiring

[wlibala]

@Patrick_Caezza Sorry for the late reply, I've been away for the holidays. Please see the pictures of the zone panel and boiler valve. I've tested the voltage and the top left T&T is currently wired to the thermostat but the voltage is 15V and it works fine with a circular Honeywell thermostat. The X&X shows 25V and is connected to the previous R845A box. The bottom left 1&2 is high voltage at 120V. 3&4 Seems to go to the valve actually to control the boiler valve. It seems I should follow the nest power connector in the pics, I've had a hard time determining where the C wire and transformer wire should go, I've tried connecting to X&X, and the relay coil seems constantly turned on and I disconnect it right after. Thanks!

[Patrick_Caezza]

@wlibala 

Do you have a room thermostat for each zone valve?

Is there a zone valve control board installed?

[wlibala]

@Patrick_Caezza 
Yes, the room thermostat was wired to the R845 through the T&T, with 4 thermostats for the 4 grey boxes.

There is no other zone valve control board, and the zone valve is controlled by the 1&4 terminal with 120 volts.