DuroZone SMZ-2SW Common Wire

[homeowner123]

Hello.

First time house owner here, I have some notions of electronics, but never dealt with HVAC before. I have the following setup. Carrier heater connected to a DuroDyne DuroZone SMZ-2SW. No AC, just this heater. There are two zones setup and a damper for each one. I’m guessing that the heater itself is acting as the fan for both zones. I installed Nests everywhere before I had a chance to learn about the whole common wire fiasco, and of course I do not have common wires. I have done a bit of research but nothing matches my situation specifically and I don’t want to break something.

The master zone goes to a bedroom and there’s 3 wires connected: R, G, W. Surprisingly the master zone has worked pretty well so far both summer and winter, although I’ve heard that a Nest as a master thermostat is not a good idea.

The other zone goes to a room and has only R & W connected. G is floating (because there’s only one fan I guess?). After working well throughout the summer while heating was off, the Nest now keeps running out of battery. I am guessing it’s because it can’t do power stealing between R and W as the heating is on and there’s no G wire, because the other one works well with 3 wires.

So I looked at the spreadsheet for the SMZ-2SW and there’s no dedicate slot for a common wire. However I see many points where I could tap some 24V AC that the Nest could use to charge.



So here are my questions:
1. Can I connect the C terminal of the Nest in zone 2 to the Terminal 1 of zone 2 on the board? The spec says 24vac common. If not, why?
2. Do I need an Add-a-wire system instead? What does it do exactly?
3. If I need to change the board to get one that’s supports common wires, which one would you recommend and why?
4. Why do people say here and there that it’s a bad idea to have a next as master thermostat?

Here’s a link to the spec for the SMZ-2SW: http://www.durodyne.com/durozone/SMZ-SWLR.pdf

Here are a few pictures of the wiring on the board:



Thanks a lot for your help!