Here's the installation manual for that thermostat:
http://www.emersonclimate.com/Documents/White-Rodgers/instruction_sheets/0037-1558.pdf
You're right about L1 and L2, and about it not mattering which is which inside the thermostat housing. It is a single throw switch so its terminals are interchangeable.
The yellow triangle labeled "2" in the diagram is any device you want to turn off when the temperature gets too high. The installation manual uses an oil burner as an example of such a device. In your case you'd leave that switch unwired, unless you want to rig up a solenoid valve to douse your wood fire. :)
You can use any heat/cool thermostat on the market to do this. All is required that you run 2 separate 18/4 wires from each system to the thermostat.
R is what carries continuous 24v power to the thermostat. RH is continuous power for heating, and RC is contentious power for cooling. As long as you have constant 24v to these terminals, your system will work. Many modern thermostats have these built in to one terminal on the thermostats (will only show R). All the thermostat does is split the 24v continuous power to RC and RH at there thermostats control board.
From Furnace:
Connect R to R - If your thermostat has RH and RC you can either keep the jumper in, or attach it to just RH for heating. This does not matter as long as you have 24v constant from the unit.
Connect W to W For Heating
Connect G to G for the Fan
From A/c Air handler:
Connect R to R - you can use both R wires at same connection on thermostat if there is only an R terminal. If there is RH and RC like above, you can either leave the jumper in, or connect it to RC.
Connect Y to Y for Cooling
Connect G to G for the Fan
The reason for connecting the G terminal for only the air handler is because you will only really want to have the fan continuously running for the cooling season. Additionally, depending on the type of air handler you have, some control boards require the Y and G terminals to be energized together to run the Cooling and the Fan speed on high. If older units with older style control boards have just Y energized on a call for cooling, the unit will only run the outdoor condenser, which will cause issues with the system. Almost all thermostats will energize G and Y together to avoid this happening if there thermostat is used on a older system.
Best Answer
You can certainly connect and use the fan-only wire for the AC.
Whether you can also drive the heating unit's fan-only input depends a great deal on your configuration and is probably not a good idea.
A breaker popping for one fan is not a concern for the thermostat system as the controls operate via 24V relays. As long as the appropriate 24V transformer is still powered, the other fan/system will operate normally. No additional harm will accrue to the dead fan other than what caused the breaker to trip in the first place.
First you must determine your exact control configuration, especially if you have one 24V transformer or two.
R,Rc, andRh. Is there one wire connected to these or two wires?C) terminal or 2? With 0, 1, or 2 wires?G.The biggest problem with one thermostat for 2 separate residential systems is usually the power supplies. (2 different 24 VAC systems tied together often leads to one 0 VAC system and possibly smoke or a blown fuse.)
Your thermostat is designed to keep these 2 supplies separate; trying to drive both fans at once can tie them back together (and release the aforementioned smoke).
If you have 2 or more power wires and/or 2 or more common wires,
then you have 2 separate 24V transformers (this is the normal configuration) and driving both fan wires will do one of three things:
In this scenario, you would need to bust out your Electronics-engineer skills and add a low voltage relay to safely drive the heater-fan relay (and not blow any transformers). That's beyond the scope of this question and there might be some commercially available solution an HVAC pro could hook you up with, maybe.
If you have only one power wire, and zero or one common wire(s),
Then the last installer might have driven everything off one transformer . And it might be a third transformer installed just for that purpose.
In that case, it might be okay to connect both fan wires to the same thermostat
Gterminal, see below.The next question becomes whether the thermostat and transformer can supply enough current for the fan relays on both systems.
The thermostat can probably handle it, but the transformer might not supply enough current.
It would be prudent to:
Locate the 24V transformer(s). Each air handler should have one, and one or both should be disconnected (in this 2-wire scenario).
There may also be a third transformer installed to meet the demands of the combined system. In this case, you're probably good to go.
If labeled/possible, note the current or wattage the transformer is rated for.
Test the furnace fan control. Unplug the thermostat and jumper the power wire to the heater's fan(only) wire.
Does the heater fan come on right away and the air stay cool after 10 minutes?
~~~ Do not attempt to run both fans just yet. ~~~
If the fan failed to blow cool air in step 3, then your heater may not really have a fan-only mode.
If you can, measure the current across the jumper in step 3 for both the heater fan relay and the AC fan relay (separately). The total of both currents should be less than 95% of the transformer's rated capacity.
If the conditions of step 3 and 5 are met, then you should be able to connect both fan wires to the appropriate (usually
G) terminal of the thermostat. There is a small chance that it will burn out the thermostat, but it probably won't.