We have recently installed an outdoor woodburning furnace to help heat the greenhouse. It will blow warm air(not hot) for awhile and then the temperature of the air coming out of the furnace drops to almost "cool" and yet the thermostat doesn't shut down the blower. We're confused! All of this is happening with a very good fire in the firebox.
We need help!

Is the fan outracing the heat exchanger? Perhaps the blower is set to too high a speed. Just a guess, as I'm not familiar with this type unit.