I've spent a lot of time over the past few years researching different ways to heat a greenhouse and getting the most bang for the buck. It's literally been five years in planning, or should I say dreaming, so finally building this greenhouse is like a dream come true for me. I spent a lot of time on thermal mass, and heat transfer with different mediums. When I came across GAHT I was impressed with the cost/ return aspect. I'm sure you can argue a better, more efficient way to heat but when you factor in cost, both operating and set up cost, I think GAHT is the way to go.
When I tell people about GAHT they say "oh yeah like geo thermal". It's not geo thermal it's exactly the opposite. In geo thermal you extract the existing heat from the ground and utilize it. In GAHT you heat the ground and store that energy for later use. It's a simple solar heat exchange using the ground as the thermal mass, all the parts being inside the footprint of the greenhouse.
Using a line fan and thermostat, you draw the hot daytime air from the top of the greenhouse and force it into the ground where the heat is transferred into the soil. Cooler dryer air returns to the greenhouse on the return side. This cycle continues till the temperature drops, and the same fan draws cool air into the warm ground then returns warm air to the greenhouse. I cant find any explanation on radiant heat coming from the floor, but it makes sense that the possibility also exists if the room temperature falls lower. I have read the lower the outside temperature the better the system works as far as heat differential, and that may be where radiant heat kicks in.
As far as I can tell the possibility of freezing inside the greenhouse in my zone is close to zero.