Combustion requires oxygen, which furnaces get from the air. Ventilation of the combustion products from a draft hood appliance, such as a water heater or an older natural draft furnace, requires even more air. For theoretically perfect combustion you need 10 cubic feet of air for every cubic foot of natural gas that is burned. However, the burners in even the most modern and well designed furnaces are not perfect. Combustion appliances all introduce excess air to insure there is enough oxygen for safe combustion. Too little excess air will have the burners operating in an oxygen starved condition, creating high levels of carbon monoxide (CO). Too much excess air can also be bad. Too much excess air will cool the flame, and also produce high levels of CO. Typical older natural draft appliances with atmospheric burners use around 50% excess air, turning the 10 CF of combustion air to 15 cubic feet. Nearly all residential furnaces manufactured today are induced draft appliances with atmospheric burners. In these furnaces, the excess air is more typically 20% - 40%. Excess air can safely go as low as 10% for commercial power burners that do a better job of mixing the air and gas.
In general, excess air decreases efficiency by cooling the combustion process. For any furnace, the ideal amount of excess air would produce the highest combustion efficiency without introducing an excessive level of CO in the flue gas. In most cases, as you reduce excess air you will see both the efficiency and CO increase. If the amount of excess air is excessive, reducing the excess air may actually decrease the CO produced in the flue gas. You want to keep the air-free CO below 400 ppm, the ANSI standard. Many techs try to keep the air-free below 100 ppm. Older gas furnaces had primary air adjustments, making it possible to adjust the amount of air being mixed with the gas. Newer furnaces do not have any air adjustments. You can only adjust the amount of fuel by adjusting the manifold pressure or orifice size. Increasing the gas being burned has the effect of reducing the excess air because now more air is needed. However, you should NOT overfire the furnace in an attempt to improve efficiency. When making any adjustments to manifold pressure or orifice size, always check orifice sizes and manifold pressure against the manufacturers specifications and the heat content of the gas supplied by the local gas utility. To read more on how combustion efficiency and CO production are affected by excess air, check out the Combustion Guide from Tru-Tech Tools (it is a free download HERE).