Forced air furnace control wiring is not ultra complicated but you can screw things up if you randomly connect wires. The systems use a common code, First thing you need to do is go to Google or similar and punch in furance thermostat wiring or similar and study for 10-15 minites so the terms used makes sence.
Now, adding a second thermostat to control he fan only for the OWB does not require relays to keep the A/C from running nor do you have to throw breakers, pull fuses or do anything else that is out of the ordinary. It is not compilcated to install nor operate and it is not expensive. The key to all this is your OWB fan controll thermostat is installed in SERIES with the indoor furnace control circuit not in parallel. I have installed 4 now including my own and "fixed" 2 others that were installed wrong and all work perfectly. Here is how I do it.
First thing is your indoor furance thermostat must have a "fan on" feature, if it doesnt you must buy a new thermostat that does have that feature and install it. Last I bought was digital and $12 at Lowes. Again I am not going into the wiring here as it gets too long winded and depends on your furnace controller type. Nice instructions come wih the 'stats too. You do this prior to adding Thermostat number 2 then once you have your indoor furnace up and going with your new thermostat you are ready to proceed to the easy part, wiring up the 2nd thermostat to control the fan when running the OWB. Find the two wires that are connected when you sellect Fan On. One of them will have control voltage at all times, the other will have it only when you have selected Fan-On. Cut the wire that does not have voltage all the time and attach a wire to both ends you just cut that is long enough to reach your second thermostat. Your second thermostat is wired for heat only, again read the instrcuctions that come withthe stats and attach the two wires as instructed. This is what I meant earlier when I said that your OWB thermostat connected in seires not parallel.
To operate the system
Basicaly what you just did is break into the fan-on circuit with a second control, your OWB thermostat. The fan-on feature of your indoor furance thermostat should now be thought of as OWB ON or OFF. To operate you set fan-on switch on thermostat number 1 to ON. You then set the temperature with thermostat number 2. You can set your indoor furnace heat all the way down or set it a few degrees under the OWB thermostat so it will automaticaly heat the house if your OWB runs out of wood or pumps fail. In the summer you set the fan-on switch to OFF and use the indoor furance thermostat to controll the A/C just like nothing was changed. The only thing that happens out of the ordinary with this set up is when the second thermostat calls for heat the fan is going to run no matter if there is heat in the loop or not. If you wish and dont mind the added complication you can add a switch that monitors the loop temps and wire it in (also in series) with your OWB thermostat. That way the fan will not run until the loop temps reach "X" temp.
It is always hard for me to make this sound as simple as it realy is. Hope I have helped and not hindered.