I am monitoring the greenhouse temps throughout the day and night and intend to do so when it goes down to zero or so the next 2 nights over the holiday. So far the heater with added fan is keeping the greenhouse 10-15 degrees cooler than the thermostat is set for. That tells me that when it gets even colder tonight and tomorrow night, the furnace will be that much less adequate...a good test for our climate. My gut feeling at this point is that I will need a larger furnace, which is what I thought in the beginning. Also, I need to discuss the cost of running radiant heat with gas vs. a non-radiant heat gas furnace.