We have a very old Eltex propane heater in our greenhouse.
It's thermostatically controlled, and uses 1/3 to 1/2 a red 47kg propane cylinder per season (i.e. £20 to £30 worth). Because the summer is short in Scotland
, we use it to get everything started early in the greenhouse, which gives the plants a head start once they're planted out into the polytunnel.
No electricity needed, no wicks to trim (once it's set up, it can just be left), and no soot issues. Highly recommended
.
I did see plans once for a combined hen house and greenhouse. It was basically a potting shed, with the back half used as a hen house, the idea being that the heat from the hens kept everything from freezing at night.
I think you'd need lots of chooks to make that work though, since I often find the drinker inside our hen house has ice on it in the mornings (the hens are fine of course).