Heater advice/recommendation

[tenman]

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.