I'm hooking up an outdoor woodburner as my main heat source (hot water w/radiator in existing ductwork) and using my existing forced air oil furnace as a backup. 2 thermostats. How do I wire up the blower motor so that both thermostats don't "kick" on at once and give the motor 220V? Is there a solinoid/relay that could accomplish this? (either one or the other thermostat--not both)

With the oil forced air, the thermostat switches 24 volts from the red wire to the white wire that runs back to the furnace. At the furnace, it connects to what ever relays are needed start the oil pump, the ignition, and the blower. The blower also has a limit switch that doesn't start it until the air in the furnace is warm, and keeps going until the air cools off after the oil goes off. What you need to do is to buy another 24 volt coil relay with 120 volt contacts rated for the power the blower draws. Most heating and cooling places have them, maybe hardwares, and if nowhere else, Radio Shack. Connect the power feed from the house to one contact, and the blower motor to the other. Connect the white wire and the common from its transformer to the coil terminals.

I am not sure how you have wired the second thermostat to activate the hot water, but you should have run a red wire to the R terminal of the thermostat, an a white one to the W terminal. If the hot water circulates all the time, the white wire only needs to go to the new relay.

If this isn't clear, or I don't understand you system, post back.

Thinking about this some more. You don't need a second thermostat or relay. Just a switch that switches the white wire away from all the other components in the oil furnace except the blower. If you need power to a pump for the hot water, us a double pole switch. Wire the wire from the thermostat to the common and the blower. Wire the NCC contact to the water pump or whatever. Wire the NOC to all the other oil components. Flip the switch, and you are back to oil.

Would this work if I weren't at home and the fire in the outdoor burner went out? I was told that I would have to have 2 separate blowers to make the system work. (which wouldn't be bad--there is an add-on woodburner w/blower already inside the house--once my new system is working, it'll be put in storage for later use in the shop. If this wouldn't be an "automatic" way for the blower to work on the oil... what are my other options.
Thanks

If you go with 2 thermostats, and left the oil furnace one set a little lower than the wood burner, then if the wood burner wasn't supplying enough heat, the oil furnace would kick on. Wouldn't hurt anything for the single blower to already be on. Many thermostats have a fan on position where the blower runs all the time. Click on Heating and air conditioning here and see how many questions there are about the blower staying on when the relay or limit switch sticks.