Ethylene has been shown to be a problem for orchids. Ethylene is produced by
the combustion of gases such as natural gas and propane. The ethylene
by-product of combustion has been proven to have a negative effect on many
plants. Harmless to humans, but not so with plants.

Venting of propane and other heaters is essential! Orchids, in particular,
need proper venting of the GH. One of the worst heaters for orchid-growing
greenhouses are the vent-free 99.9% efficient propane heaters. Since these
and other unvented heaters generate combustive gases, the ethylene goes
directly into the plant's atmosphere.

Any type of vented heater is better for use in greenhouses. I myself am
partial to the direct-vented models, in which the combustion chamber is
totally separate from the GH interior. Air from outside is brought in for
use in the combustion chamber, with the exhaust being vented out, both
usually in a concentric pipe configuration which partially warms the
incoming air prior to combustion.

In my opinion, the extra $300-$600 for such a heater as opposed to a
nonvented heater is well worth the improved health and appearance of our
flowers and vegetables.

Follow the link and read carefully. Best of gardening to you! :-)

Here is a link that might be useful:
http://www.ces.ncsu.edu/depts/hort/f...p_ethylene.htm