I have a 30,000btu gas heater, with a thermostate that keeps it above 50 degrees, that I converted to use a 20lb tank. I use it in zone 4, but only from April to May when our temps are probably like yours during the winter. I go through a tank every 3 days so you have an idea of what your usage will be. We're talking around 45 daytime and 35 at night.

Of course on sunny days, just the pilot light stays on.
My greenhouse is glass, not insulated.

Marquisella

It should be pointed out that unvented gas heaters are actually illegal for sale in some states. The reasons being carbon monoxide production, oxygen depletion, and seldom mentioned, ethylene production. Ethylene is definitely damaging, even in very small amounts, to vegetables such as tomato and pepper.

I'm also in Zone 7. I wrapped my entire greenhouse in bubble wrap. I bought it at u-haul, then took the doors off and started from the inside of the side panel, taped it with packing tape and went all around the greenhouse to the inside of the other side. I was only a foot wide, so after I put it on each side, I taped it to each panel. then just kept wraping all the way up. A few nights, it did get cold here this winter, so I used a space heater that had a blower and pluged it into a surger protector, I set the thermostadt, and it worked great. It didn't even run up my electric bill that much and I had fresh vegies and plants all winter.

Some small space heater(s) should be more than fine in your zone. I haven't done the math though. Two being the most you would need though.

Pretty general question however.

Dax

Unvented heaters when properly installed (include a fresh air vent) are a good way to heat your green house.
There have been many posts on this web site (Orchiddude I belive has used for quite some time) where people have used unvented gas and kerosene heaters.
I would bet there is a bigger risk by using an electric heater in a greenhouse that is not properly wired with GFI outlets.
I do not disagree there may be some risks if you place one of these heaters in a home, or bedroom,( or if you have a tightly sealed GH) however many of the heaters have been improved over the last few years with o2 sensors and have become some what safer. However proper installation is needed.
I used one for 2 years and my only problem was that they only make 30,000 btu and it was not large enough to heat my 16x 24 on a cold day. ALso they need to be maintained. Each year it is best to clean them so they burn cleanly ( blue flame). If I had a smaller greenhouse I would still be using.

In PA they are not illegal.

It may be good to hear from someone that has used one of these heaters for a long period.

Posted by plantman56 z6 PA (My Page) on Thu, Mar 15, 07 at 12:38

I would bet there is a bigger risk by using an electric heater in a greenhouse that is not properly wired with GFI outlets.
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The key word in your sentence is properly wired.
While that may be true no one other than a person with an IQ lower than outside temperature in January would install any electrical device in a greenhouse without a GFI so your statement has very little merit

I would like to see some test of what it cost to heat a well sealed GH and heated with electric as to using gas and venting to let air in that also lowers the temp and runs the heater more.
I pay 6 cents per kilowatt hour my well sealed and insulated GH heats real easy so I went with electric because it is cheap and easy to install and clean plus I don't have the hassle of changing tanks or worrying about gas leaks.
I have yet to hear of a electric heater asphyxiating the plants.
You can make a good argument for either type of heat but when both are done properly in a small GH I prefer electric heat if I had a big GH then I would have no choice I would be forced to use gas.