Posted: 14/10/2015 at 12:11
Lorna lot of depends here, are you near an electric point, are you going to insulate the greenhouse, what are you going to do store plants or actually staring plants off?
if you can run electricity as I do then a fan heater with a thermostat and frost setting is ideal, if not then a burner of some kind which have problems, they cause condensation and cannot be finely adjusted, bottle gas is expensive and will have thermostatic controls parafin needs be well maintained, the wicks need trimming each time it is lit or you end up with a sooted greenhouse.
often the best way is to insulate part of the g house, a tent inside with shelving, just enough to house your plants and some blanket over the plants on frostie nights.
My way is the fan heater with frost guard and cover the plants at night, trying to heat the g house to grow plants is a costly and often useless exercise, I gave that up years ago, you will lose the odd plant though most are more hardy than we think. Good luck,