Heater advice/recommendation

My fab Southern Burner heater has given up the ghost; no longer in business, parts unavailable, so I need a new backup heater in the greenhouse (Ohio, US) in case of main furnace failure or power outage. It must be around 25,000-30,000 BTU, work without electricity, ergo no fan. Needs to work from a standard thermostat connection or have its own thermostat, and come on automatically at the set temperature of ~55ºF. Any recommendations? The only thing I can find that looks like it might work is an unvented blu flame heater. Does anyone know if those are safe in an enclosed orchid greenhouse? There is a small 3" intake vent for the main heater, and currently an exhaust pipe from the Southern Burner which could be left as a fresh air vent as well if necessary. I would prefer a vented heater like the Southern Burner but can't seem to find one.

 

An electric heater will not be useful when the power is out. Our backup generator, as have the previous three, gave up the ghost after only two years and being used only two days. That's the single most crucial thing about this backup. no electricity required