Safe and efficient combustion in a furnace requires a proper air-to-gas ratio. This is dependent on the availability of air(combustion inlet) and the ability to discharge oxygen-depleted air (flue gas venting). There is a potential for incomplete combustion whenever there is a malfunction of either the inlet or outlet system. This can result in carbon monoxide (CO) gases.
Residential gas heating systems with non-dedicated combustion air inlets are inside the living space without direct communication to the outside air. Many of the older homes have furnaces that draw air from the occupies space. As the air is pulled from the space, it is replenished by air infiltrating the house through the window, doors, and other obvious opening. This can create a potentially dangerous situation.
Modification done to a structure can inadvertently inhibit this flow of combustion air, causing the furnace or any other gas appliance to malfunction. Other problems in the house can turn an improperly vented furnace into a CO-generating machine.
Most residential furnaces use natural draft to drive the combustion process. Even 80%-AFUE furnaces with combustion blowers are natural-draft furnaces.The combustion blower assists the combustion gases, but the flue still operates like a natural-draft furnace. The natural pressure differential created by heating the combustion gases in the furnace is what pushes the flue gases up and out of the flue.
Combustion gases expand as they are heated which makes them less dense than the colder air entering the furnace, so they weigh less. They also exist at a lower pressure than that of the colder inlet air.
Since the combustion gases are at a lower pressure, the higher-pressure inlet gases displace them, pushing them up and out of the flue. The inlet air must always be at a higher pressure than the combustion gases so that this pressure differential can be maintained in order for the flue to operate properly. Air stops drafting through the furnace when this pressure differential is diminished by the flue pressure becoming too positive or the combustion air inlet becoming too negative. This is when CO is created and if the furnace is not drafting, the CO ends up staying in the residence.
The pressure differential needed to drive the flue gases up and out of the furnace is somewhere between 1 and 5 Pascals depending on the outdoor temperature. This means as the flue gases leave the furnace, they must be at -1 to -5 Pascals relative to atmospheric pressure. The pressure differential needed to be maintained between the inlet and outlet of the furnace’s combustion air inlet system is very delicate and can be easily disturbed. Anything that will raise the pressure in the flue or lower the pressure in the combustion air inlet system can interrupt the flow of air through the furnace and create carbon monoxide.