I have used propane heaters to heat the greenhouses for 25 years. I have small 18000 btu heaters that are about a foot tall and 8 inches wide. I have been told these are used by painters up north when it gets too cold that paint doesn't stick to the walls. One that size heats a 49x36x12 greenhouse with no problem, so you know you can do it for a lot less with a 12x12. A single tank of propane will last about 20 hours with this heater.
Hot air rises, but it also sinks as it gets cooler. It circulates on it's own, but if you want to bring that heat down faster than it will naturally, something like a ceiling fan pulling down the heat would help, but as I said, it is not needed. I just posted what I am heating a 12x12 greenhouse with in another thread. There are all sorts of ways to heat something that small. Dripping water from the house. Covering the floor with brick that will radiate heat during the night. Black barrels full of water that can be used as legs of a table that could hold plants above them. A pile of breaking down mulch ( I don't suggest this) will not only produce great but will also generate more Co2 as well. I bet you could get away with 1000 BTUs or maybe even slightly less, depending on the quality of the materials your greenhouse is made of and what lows you get and duration below freezing you experience. Tom
