Fresh Air Supply Vent Video
If you have a central vacuum system, it pulls a lot of air out of the house and exhausts it outside. Bathroom exhaust fans, clothes dryers, gas water heater and other household items also pull air out of the house. Modern homes are very air tight. How is this air replaced in the house?
If you don't have adequate inflow, you can get a situation called back drafting. There is a special air vent intake to help replace the make-up air. It looks like an oversize dryer vent. However, it has two vent holes. One supplies air directly into your furnace. The other vent supplies combustion air for your appliances.
Back drafting can pull carbon monoxide back into the house. This special fresh air vent allows the appliances to pull in the air required and avoid back drafting.