"....Are you saying that for the general use of a gas
heater to keep frost at bay in wintertime, and with a thermostatic control
on the heater then Venting for safety reasons should not be a worry -
assuming that I do not stay in the green house with the heater on and the
door shut - of course??? ........"

As long as the heater is burning with a Blue flame then you are OK, if the
flame turns yellow then a lack of Oxygen and it will give off Carbon
Monoxide.
If you are really worried then leave the door open half an inch when you are
in there.(But as you let the outside air in you'll also be letting the heat
out).
It's not as if the heater will be running on full flame much of the time
A small metal air vent (What are they 4inches by 8inches?) will be
sufficient and will still keep mice out.




--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk