I can gladly chime in on this for you. I have a small greenhouse (8x15) and really wanted to go propane too. After much searching, if I wanted something vented, it seemed as though the purchase, installation and permits would have been three times what I paid for the greenhouse. (Nearly $1500) I found and opted for a setup using a "Mr Buddy" vent free unit along with (2) 20lb cylinders and the necessary lines and filters. It offered three heat settings at 4,000, 9,000 and 18,000 btu's. Well, within 6 nights at it's lowest setting, all buds blasted and a huge amount of my plants old growth turned yellow. Going in the greenhouse just made you sick. I returned everything to Lowes and opted to go electric. I ran a 240 volt line out to the greenhouse and got a 4000 kw wall pack that has worked wonderfully. I still need to contemplate a backup system.
Your big problem is ethylene which is a by-product of burning propane. Natural gas doesn't have the same problem so if you have the option, go with gas. If propane is still your choice, go vented! If you don't find a 10k btu unit, it doesn't hurt to go oversize on your heater and run it at a lower setting.
Last edited by Bradfo69; 12-11-2007 at 11:17 AM..
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