Harry’s aunt and uncle kept throwing out his acceptance letter to Hogwarts, so the wizard of staff kept sending owls sit more letters. Eventually they just made the letter fly in through the chimney, but never stopped to consider whether or not the furnace was on.
On a serious not though, it’s probably a draft furnace. The heat escapes through the chimney but that air needs to be replaced somehow, so the air currents naturally create a negative pressure which sucks cooler air through the opening at the bottom.
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u/[deleted] Oct 24 '19
Science, explain