So I've usually left my flue open during the winter months on the off-chance that I'll start a fire and forget to open said flue. My wife apparently just realized this and complained of cold air coming down the chimney tonight. So I closed it with the intention on opening it and leaving it open when she goes to bed.
But what's the physics there? Does your heated air escape out the top? Does cold air come down the chimney to make the house colder? My house is 60 years old and already drafty as $#@!.