I completed building my greenhouse in Colfax about 9 months ago. Since it is small and tightly constructed, I decided to try using a small electric heater to start with. I quickly discovered that the electric heater could not maintain 50 degrees when the outside ambient temp is 20. I used an unvented propane heater to supplement on cold nights.

I was warned a long time ago that an unvented gas heater could cause problems, but I figured that it might not be too bad as a temporary supplement. Unfortunately, that was not the case as almost every bud in the greenhouse blasted within weeks – even on plants that, in the past, have been productive and bulletproof.

This link pretty much describes what have heard…
http://www.umass.edu/umext/floriculture/fact_sheets/greenhouse_management/jb_non_vented_heaters.htm

Today I met someone who deliberately vents a propane water heater directly into his greenhouse. His growing results are excellent although he did report a bit of flower malformation on one of his prize specimens.

I’m working on installing a small, vented propane heater, like I’ve used in my previous greenhouse, but I’m still baffled by this contradiction.