I am quite new to this server, but I do know that commercial green-
houses use propane heaters in order to heat their greenhouses during the
winter. The products of a full propane burn are carbon dioxide and water, two
things that are quite good for plants. The only problem with such a thing
would be a possible fire hazard (different heaters may not pose as much of a
hazard as other and that should be looked in to) and an improper burn would
produce ethelyne (which causes defoliation). An improper burn is rare, but
like I said, I would look in to that.
The only other thing I can think of at the moment would be small water
tanks heated by compost. I have forgotten how much compost is actually needed
to produce what amounts of heat, but we have a compost pile at our Horticulture
farm on campus that is normally about 130 degrees F. Putting small containers,
say Zip-loc bags, filled with water in the middle of a couple of layers of
compost and seeding it with bacteria might produce the heat you need with a
place to contain and slowly reradiate the heat (the water) into your chamber.
If you don't understand what I mean by this, let me know and I will send you
a representation of what I mean. Good luck.
-Matthew