[xoxoxoBruce]

Originally Posted by BigV

I can see one potential problem here. Since the temperature difference between the air at the bottom of the room and the top of the room is likely to be less than the temperature difference between the bottom outside air and the top of the room, the vigor of the siphon could be less. I wonder though, if that would be offset by not having to heat the much colder air much more to make it "room temperature".

The "vigor" is governed by the amount of heat the system adds to the air in it. The bottom "suck", will be the same whether it's sucking inside or outside, so yes, sucking from the inside is much better unless you need fresh air.