Thanks to all for the bit of education. I didn't provide all the pertinent information at the beginning of the thread because at the time I didn't know it was relevant. But now I do. The operation of the furnace I mentioned was happening when the greenhouse was at 40° F, It had only been running for two days this season. The temperature was turned up to 55° F because new plants had just arrived. The owner wasn't trying to "Grow" the plants, just hold them for a couple days till they're transplanted. It's a battle in the greenhouse business to balance how fast the plants grow with the heating bill. I've read that inverted temperature to light grows a really nice plant (like 80° ant night and 75° during the day) but you could never make any money.
As for the fan cycling, it may not be doing it now that the greenhouse is warmer, I haven't been back there in days. I was there to install a low temperature alarm system in case of furnace failure while unattended.