I have heated my 22' dome with propane for 18 years and have prolific harvests of everything from peas and tomatoes to Myer Lemons I light the furnace when the temp in Pagosa Springs, CO. hits mid teens and shut it off around March 15th. There are enough air leaks to offset the CO2 danger I believe. Temps during the day because of the automatic solar vents in the ceiling stay at around 75 to 80 degrees. Best 10,000 I ever spent since 2003. You can review them on Growingspaces.com. My only regret is that I didn't buy a 33 footer. At nearly 80 years old, it's a great place for my cat and I to sit out blizzards and in late Feb. watch the bees pollenate the sweet smelling Myer Lemon blooms
