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.


3 Answers 3


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.


Many homes have what is called “makeup air” this is always on the intake side and is usually a small duct 4” or less connected to outside air in some way some pull from the attic so the air is not so cold it is rare to pull from a crawl space because that air is usually musty or damp.

I normally install a damper on the make up air ducts I install so if you run your fan 24-7 the amount of air pulled in is reduced this reduces the heating cost.

Why do we install make up air? As we started to make houses tighter we found in some cases the air inside was becoming unhealthy so a small amount is brought inside to maintain a 20.8 O2 level and keep CO2 at a trace level.

Don’t let CO2 scare you every time you exhale about 5% is CO2 your body used up about 5% O2, so with some very tight homes the CO2 levels high enough to give you headaches and other issues and the O2 levels were reduced.

A small amount of air is drawn in on the intake and the excess pressure is vented through bathroom, kitchen and if gas appliances their exhaust stacks. When your furnace is running more frequently the make up air can be reduced. You might pull the filter and look for a opening that may have a damper or adjustable flap and close it a bit. This will reduce the amount of make up air but I know I dampen mine to 50% in the winter because I normally have the fan running in the air handler to circulate the heat in the house (wood stove).

If you close it down more than 50% watch for headache and lethargic feeling even ringing in the ears and rapid breathing can be caused by high Co2 levels but the most common issue is headaches. If you sleep with a window open you can close the make up air off the air movement will normally keeper the fresh air from the window mixed well enough.


It is replacing air that is leaking out of the house somewhere else. If you block it , the outside air will leak in from other places ; around doors, windows, exhaust fans , etc. The air pressure in your house is going to "try" very hard to match the outside pressure. The outside combustion air improves heating efficiency and comfort in the house. If outside air is not used for combustion , then warm, humid inside air is used . And cold DRY outside air will be drawn into the house. Inside humidity is very important for comfort , more is needed as the outside temperature drops. I added my first outside combustion air duct in 1964, been doing it ever since.

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