Consult an HVAC contractor first
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Closing the heat register and door without sealing the return air duct
can actually increase cold air infiltration and cost you more than you
thought you'd save.
There are three good reasons to get an
HVAC contractor involved before you start closing off
heat vents, especially with today's high-efficiency furnaces
and well-balanced systems. First, it might actually
add to your heating bill. That's because with the
heat vent closed, the suction from the return air duct
can pull in cold air from the outside through any
cracks around windows, exterior doors or exterior wall
electrical boxes. Second, if the heat duct seams haven’t
been sealed properly, the extra pressure from closed-off
vents will force hot air through the leaks. That can be
as much as 15 percent of heated air into basements,
crawl spaces and floor cavities instead of into rooms.
Finally, if you have a well-designed, finely tuned heating
system, closing off too many rooms can damage
your furnace because it has to work too hard to distribute
the air. So, if you still want to seal off these rooms,
consider hiring an HVAC contractor for advice.