Even if one uses Natural Gas there is still carbon monoxide to deal with. I would never consider a ventless gas heater of any sort of brand for use in a closed area. I have seen a ventless propane unit that needed as much free air coming from outside as it blew out of the heater. So what is the point then? Electric is still the safest, but the highest operational cost for a heater. Vented gas would be the cheapest to operate (but still not inexpensive).
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Ross
http://orchids-ross.blogspot.com/
I don't do most Dendros or large, "floofy orchids". For me it's minis (like Angraecoides), Masdies, Paphs, and others that crept in somewhere along the line. See my orchid list for complete collection (usually not current.)