Sorry...yes under some (well all common sense and safety) one would not vent into a standard -people walk into type greenhouse.
I did have in the early 90's have 4 tunnel greenhouses about 2m tall and 1m wide side on looked like a balloon (kids) 1/2 meter from the tied bottom was at first my Hydroponic's that later turned into Airoponic's and the 4 of them were not possible to enter so co2 was not a problem, the fire heated the water for the luttuce and between hot water and a warm flue air flow there were no bugs and better still dam fine lettuce in the dead of winter.
Earlier when my 36 year old son was 10 we had a major sprout farm in Brisbane (Queensland Australia) in the suburbs and it to was gassed every night, there I sent the flue contents through 2 200 liter water drums to scrub.
But if you were in a place that it would not likely that there would be trespass 1 good firing per week that heated a mass and vented into the space then a day later the air would be (with a small amount of venting) not a big worry.
The point is that if you use any sort of fire to for heating then a lockable green house would not mind (well the Plants) a dose of a good clean burn every now and then ...you may get a suprise.
Tony
WWW.tonycampton.com