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Hot air rises. Cold air falls.
Furnaces and heat pumps work with more efficiently when there is a greater differential of air temperature coming in to the furnace/heat pump
In hot weather - where does heat come from?
In cold weather - where is heat lost to?
Pressure equalizes - if you take air away from a place - other air will flow in to equalize the pressure (see osmosis and arbitrage).
If you have a house with a single heating and cooling system then it needs to be adjusted to flow differently when it is hot versus when it is cold.
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