Which type of air must be calculated for both horizontal and vertical ducts when providing combustion air?

• A. Filtered air
• B. Return air
• C. Outdoor air
• D. Total air

Answer: (C)

When providing combustion air, outdoor air must be calculated for both horizontal and vertical ducts. Outdoor air is necessary to ensure that appliances operate safely and efficiently, as it provides the required oxygen for combustion processes. Properly calculating the volume of outdoor air needed is crucial, especially in systems where air intake can be affected by the layout of ductwork, whether it runs horizontally or vertically.

Filtered air typically refers to air that has passed through a filtration system to remove particulates but does not account for the specific needs of combustion air. Return air refers to air that is cycled back into the system from the living space; it is not primarily intended for combustion. Total air does not specify the origin or suitability of the air for combustion purposes, which is why outdoor air is the correct focus in this context. Calculating outdoor air correctly ensures that there is sufficient ventilation and reduces the risk of hazards associated with combustion appliances, such as carbon monoxide buildup.