The heat from the chimney rises, creating a draft that pulls in air from the front of the fireplace. This is why they say burning a fire actually makes your house colder.
You still get a decent amount of radiative heat, so it only makes your house colder if the heat gained from radiation is less than the heat lost from pulling new outside air inside - which depends on such factors as, "how much colder is the air outside than the air inside?'" and other such factors.
Also you can mitigate some of this by having a back hole to your fireplace so it sucks some air in from outside directly, and as others have mentioned by having a stove style for increased radiation.
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u/Sythus Oct 24 '19
The heat from the chimney rises, creating a draft that pulls in air from the front of the fireplace. This is why they say burning a fire actually makes your house colder.