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).
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).
At some point they realised and the newer ones have a dupex air duct so you can keep air pressure even
Huh, just like a balanced flue.