Calculate the CFM of a 96% efficient natural-gas downflow furnace, with a nameplate input rating of 80,000 BTU/h. Measured return air 66°F, measured supply air 115°F.

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Airflow needed comes from how much heat the furnace can deliver to the air and how much that air heats up as it moves through the system. The delivered heat equals the input rating times efficiency, and for most HVAC problems the relationship between heat per hour, airflow, and temperature rise is Q = 1.08 × CFM × ΔT, where Q is BTU/h, ΔT is the rise in °F, and 1.08 is a constant that comes from air properties and unit conversions.

First find the actual heat the furnace can deliver: 80,000 BTU/h input × 0.96 efficiency = 76,800 BTU/h.

Next, the air heats from return to supply by ΔT = 115°F − 66°F = 49°F.

Now solve for CFM: CFM = Q / (1.08 × ΔT) = 76,800 / (1.08 × 49) ≈ 76,800 / 52.92 ≈ 1,451.2 CFM.

So the required airflow is about 1,451 CFM.