One of my furnace air intakes appears to be letting in fresh outside air. I have three intakes on the main floor and one in the basement. The basement one is the offender as far as I can tell.
When the furnace is off, I can feel air coming through enough to be noticeable. Today, it is very cold, so I can tell that it is outside air coming in. When the furnace turns on, air flow reverses and it seems to operate like a normal intake vent.
I believe my furnace uses outside air, but I thought that was just for combustion. I have attached a picture of the furnace, just in case it is helpful.
Any ideas?
If this is "normal", I was thinking I might get some thin paper to place on the back side of the intake such that it can flap open for intake and close when the furnace is off. It faces downward from the ceiling, so this is feasible.
Best Answer
When the furnace blower stops, the forced airflow stops. What you are feeling is the air remaining in the return ductwork as it cools and sinks. That's why you feel it at the lowest return register. Your idea of a one-way flapper valve made from paper is good. It will keep the calm cooling air from sinking. Be sure to minimize any resistance to air flow during normal operation.