To ventilate or not?

I agree with Ray. (not that it matters but I frequently do) It would be hard to suffocate in there. I estimate all the cracks and crevices in my greenhouse are already equal to a square foot opening, ...at least... and this does not include the vent flews/flues/flooze for the heaters. I would have to have a hole in my head (its actually pretty windy in there too) to actually add more holes to the greenhouse.

You can calculate a good guess as to how often the volume of air in your greenhouse changes by calculating the air infiltration rate. The air infiltration rate is one part of the calculation for BTU requirements. Air infiltration and is based on the size and construction material of the structure , among other less tangible things like wind speed....

Al,

I got around the heater-related issues by buying a separated-combustion heater.

Combustion air is drawn in via a 3" flue from the outside. That actually runs inside of the 6" exhaust flue, which is sealed through the back GH wall. The cold, outside air is preheated by the exhaust, which increases fuel combustion efficiency.

All of the controls and the burner are in a sealed volume, so I don't suck the heat or humidity out!

But then there's the door....

That is a "fan" that runs 24/7 *G*