One Nest, two loads, two circuits.

chrismaki

My main living area has two separate baseboard heaters on two circuits. For years I’ve been running only one of them from a Honeywell R841C1169 which is an SPST 220VAC relay with a 24VAC coil. I ran the 24VAC side to W1 and RH of the Nest, this all worked fine for a long time except only one heater was connected. I finally got around to buying two Aube (now Honeywell) RC840’s and a White Rodgers 220 → 24VAC transformer. When I was trying to wire this new setup, I managed to burn out a terminal on the Nest baseplate, Nest is replacing this even though I fully admitted to it being my fault, great customer service. Right now I have both Aubes installed each inline on the load-side of either heater. I also tapped one of the circuits to provide 220VAC to the transformer. The 24VAC side of the transformer runs to my thermostat location, which right now I have temporarily installed an old (read: dumb) thermostat which is basically just a switch so when it engages, 24V from the transformer goes to the C and W connections of both relays, turning on both circuits/heaters. When my new Nest baseplate arrives, knowing that I only have two wires to my thermostat, how do I wire it? For the sake of simplicity, we can completely ignore the 220VAC side of the conversation, that’s all setup fine. I have the following connections that I need help mapping out:

* 24VAC Transformer:
* Blue/C →
* Yellow/24VAC →
* Aube1:
* C →
* W →
* Aube 2:
* C →
* W →
* Nest:
* W1 →
* RH →

I’m assuming any solution would have 24VAC running to the Nest at all times to keep it powered and online. I guess I don’t actually understand how with the old setup with the R841C1169 actually worked. I assume that when the Nest calls for heat, that it shorts the 24VAC leads and this feedback engages the relay, allowing 220VAC to flow to the heater.

Running a Common wire is not an option.

chrismaki

If I wire it like this, won't the Nest only get current for charging while the heat is activated?

ratio

Take a third wire from the same side of the transformer that's connected to the "C" terminal on the relays to the "C" terminal on the nest. Observe polarity on everything, i.e. same transformer terminal to all the "C" terminals.

Jamie Hall

I note that the OP states that running a common wire -- third wire -- is not an option. Well... the Nest, like all other wi-fi thermostats, is a power hog, and without a common wire it works somewhat erratically. Sorry about that...

ratio

Ah, I hadn't noticed that. It's possible that a power-stealing thermostat like the Nest will work, wire as per the diagram above & hope for the best.

chrismaki

Thanks for the help guys, I've determined the best course of action is to put the 24VAC transformer at the thermostat and not at the relays so that I can run a common (C) wire and just push 24VAC back to the relays over the existing copper pair. Just need to find a way to hide it behind the wall because yes, the Nest will have to cycle the heat periodically (even in the Summer) just to stay charged, not ideal.