If air cannot get to the furnace for combustion and if the spent combustion gases cannot escape out of the furnace, a problem will arise. Either of these problems can cause lack of oxygen to the flames. Here are some things that cause combustion air problems. Consequently, to ensure that the furnace operates properly, the furnace closet must be properly ventilated. If not, the furnace will soot and, under severe circumstances, create toxic carbon monoxide gas.
Many homeowners block their combustion air inlets to reduce noise or heat loss in the house.
This is a very dangerous condition that should be brought to the attention of the customer and repaired. Some homeowners may not allow you to unblock these vents. This should be noted on your invoice to show that you are not negligent. If air will flow vertically through an opening, it should have a free area of 1 sq in. If air will flow horizontally through the opening, it should have a free area of 2 sq in.