This is what I did.
I installed a SPDT switch in the heating wire from the thermostat (W) to the common termanel of the switch.Then 1 wire to the (W) at the furnace and the third wire from the switch to the (G) at the furnace.With the switch in one position,I will call it gas furnace,the (W) from the thermostat on a call for heat goes through the switch to the (W) at the furnace to start the heating cycle.By changing the position of the switch to the other position,I will call this wood,The voltage now comes from(W)at the thermostat to (G) at the furnace ,and on a demand for heat the Indoor fan relay is energized and the fan will run until there is no more demand from the thermostat.Note that this setup does not give me the back up for the furnace should the OWB not make enough heat to keep the house warm,but I have been running this way for three years with no problems.In September when I start the OWB I turn off the gas valve to the water heater until May when the fire goes out in the OWB. I have not had the gas turned on to the furnace for three years. Another thing that I do is to remove the vent pipes for the water heater and the furnace and cap off the openings in the chimney to keep heat from going outside.