COMBUSTION AIR INFILTRATION
When a structure has a properly designed combustion air inlet system, the furnace enclosure has two dedicated vents communicating with the outside air, eliminating the need to obtain combustion inlet air from or through the living space. Residences with a non-dedicated combustion air inlet systems are the homes that are at risk. As a result of modernization, these homes may have worked fine since they were built, but can develop problems. This includes new high-efficiency window, weather stripping, and general sealing of the structure.. Anything that disturbs the pressure in the space can affect the operation of the furnace. The filtration rate may be diminished if a house is tied up for energy conservation. The house pressure drops as the furnace draws combustion air from the space and eventually the house pressure can drop enough to reduce the combustion air inlet pressure to a point that the flue will quit drafting.
Other things that can reduce combustion inlet air pressure can include the furnace blower return air drawing from the furnace enclosure, which should always be well sealed. Ductwork supply leaks can cause pressure in the structure, and the house pressure will drop, if the duct system pulls more air out of the house than it returns due to supply leaks. Closing the doors to rooms that have no dedicated return air grille can aggravate supply duct leaks. Exhaust fans drawing on the house can also reduce combustion inlet air pressure. This includes clothes dryers, bathroom fans, whole-house fans, range hoods, other gas appliances, and fireplaces. And finally, customers sometimes block furnace door vents to reduce noise, but whether a blockage of a furnace enclosure vent is intentional or unintentional, this can also cause the reduction of combustion inlet air pressure.