Airflow (CFM) calculator

How it works

The tool uses the sensible heat equation for air:

BTU/h = 1.08 × CFM × ΔT, so CFM = BTU/h ÷ (1.08 × ΔT)

ΔT is the difference between supply and return air temperature. The tool uses the size of the difference, so it works for a furnace (rise) and a cooling coil (drop).

Where 1.08 comes from. It converts a volume flow per minute into a heat flow per hour for standard air:

60 min/h × 0.075 lb/ft³ × 0.24 Btu/lb·°F = 1.08

• 60 turns cubic feet per minute into cubic feet per hour.
• 0.075 lb/ft³ is standard air density, the value used in ASHRAE duct design.
• 0.24 Btu/lb·°F is the specific heat of air.

Elevation. The 1.08 only holds for air at standard density. Air at altitude is thinner, so each cubic foot carries less heat. The tool scales the factor by the ratio of standard-atmosphere air pressure at your elevation to sea-level pressure (14.696 psia). At 5,000 ft that pressure is about 12.23 psia, a ratio of 0.832, so the factor becomes 1.08 × 0.832 = 0.899. The same air pressure model drives the elevation field in the pressure converter.

The inputs:

• Sensible heat in BTU/h: furnace output, or the sensible part of a cooling coil’s capacity.
• Supply air temperature and return air temperature in °F.
• Elevation (optional, feet).

Worked example

The tool opens with 64,000 BTU/h, 125°F supply and 70°F return.

• ΔT = 125 − 70 = 55°F.
• CFM = 64,000 ÷ (1.08 × 55) = 64,000 ÷ 59.4 = 1,077 CFM.

64,000 BTU/h is what an 80,000 BTU/h input furnace puts out at 80% efficiency (80,000 × 0.80). Always use output, not the input rating.

The same furnace and temperatures at 5,000 ft:

• CFM = 64,000 ÷ (0.899 × 55) = 1,295 CFM. Thinner air needs more volume to carry the same heat.

Run the equation backwards to check a furnace’s temperature rise. If the blower is set for 1,200 CFM at sea level, the rise is 64,000 ÷ (1.08 × 1,200) = 49.4°F. Compare the measured rise with the range on the furnace rating plate. A rise above the range means too little airflow. A rise below it means more airflow than the design or less heat than rated.

Cooling. A coil delivering 24,000 BTU/h sensible with a 20°F air drop is moving 24,000 ÷ (1.08 × 20) = 1,111 CFM. Use the sensible capacity only. The equation does not account for moisture removed, so total capacity in it gives too much airflow.

Reading the result

• The answer is the airflow implied by the heat and the temperatures you entered. It is only as good as the BTU/h figure and the two readings.
• On a furnace, the rating plate rise range is the limit to check against. Stay inside it.
• On cooling, airflow affects the charge readings as well. Low indoor airflow lowers superheat on fixed-orifice systems, which can look like an overcharge. Check airflow before you charge with the target superheat calculator. CEC’s charge test also needs return air of 70°F dry bulb or warmer.
• If the result disagrees with the duct design, check the duct sizes against Manual D with the duct sizing calculator, and check external static pressure against the blower table. ACCA’s Manual D procedure starts from the airflow the blower makes at the system’s static pressure.

Common mistakes

• Furnace input instead of output. Multiply input by the efficiency first.
• Total cooling capacity instead of sensible. This overstates CFM.
• Readings too close to the heat source. A supply probe that sees the heat exchanger or coil, or sits in unmixed air, gives a false ΔT. Measure where the air is mixed.
• Small ΔT. Below 1°F the tool refuses to calculate. With small differences, a half-degree probe error makes a large CFM error.
• No elevation correction at high-altitude jobs. At 5,000 ft the uncorrected answer is about 17% low.
• Not letting the system settle. Take readings at steady operation. CEC uses a 15-minute run before measuring refrigerant charge.