[HPsquared]

It's the same problem as those portable AC units: the exhaust (chimney in this case) draws large amounts of air in from outside which is at the wrong temperature (cold in this case).

[dietr1ch]

At some point they realised and the newer ones have a dupex air duct so you can keep air pressure even

[HPsquared]

Huh, just like a balanced flue.