1 thermostat for 2 Heaters?

[LogCabinFever]

So I have a Caddy Advanced furnace hooked up in parallel to my LP furnace that I use as back up. No other Heat or cooling.

I’d like to simplify my thermostat setup to just have one thermostat that is wired to both furnaces. I have a kill switch for the LP furnace along with manual baffles so there’s no risk of both furnaces running at the same time.

The Caddy comes with a Honeywell Pro 1000, which is a simple single stage thermostat. My question is, could I just tie both into the R and W terminals? Or is there a negative to potentially back feeding 24V to the not operating furnace? Both furnaces are just two wire setups.

I’ve done a bunch of research online but there’s not much written on this. The most info I can find seems to indicate that I can as long as the combined wattage doesn’t exceed a certain amount. Any advice?

 

[EbS-P]

Both are supplying 24v (I’m guessing). I would use a relay off the 24v of one to call for heat on the other so the 24v systems are separated

 

[LogCabinFever]

Yes both are supplying 24V

Could you be more specific on the relay location please? I’m not an electrical guy. Are you saying I pick one to put the relay on so that it only activates when the other is off? If so, what would that look like?

 

[EbS-P]

Yes both are supplying 24V

Could you be more specific on the relay location please? I’m not an electrical guy. Are you saying I pick one to put the relay on so that it only activates when the other is off? If so, what would that look like?

Put a 24v relay in series on Left unit, connected to the coil, so when the Tstat calls for heat the relay closes. Connect the other units (Right) wires to the relays normally open contacts. The relay now acts as a tstat sending the call for heat (only when the real tstat calls for heat).

Is this a two wire system? I still don’t really understand letter color for tstat wiring.

This keeps both systems isolated.

This makes sense in my head. Never done it. Someone with some experience might chime in.

 

[LogCabinFever]

Both furnaces are a two wire setup with Red and White wires. Normally you'd bring it to R/Rh and to W on the thermostat for a 2 wire heater. Everything else is for cooling on this particular thermostat.

I appreciate the diagram, but I'm not sure that would work. So with that relay, if the Tstat is not calling for heat, it defaults to the L furnace?

I'm wondering if a two stage thermostat is what I should be using, or at least one that has auxiliary terminals. Use the woodstove normally and hook up the LP to auxiliary. When the woodstove doesn't respond to the call for heat, maybe it would jump to the LP?