Quote:
Originally Posted by MDSheds_SS
Most likely that register is for return air. Not a good idea to close it. The burner gets intake air from the PVC/outside. The actual air that gets heated and cooled is separate and has nothing to do with the PVC intake or exhaust. The more return air the better.
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Negative, this is literally a duct straight outside. Nice and drafty.
Furnace return for the closed system is different.