Furnace Airflow from Temperature Rise
Estimate furnace airflow from the rating plate and a supply/return temperature split, and check the rise against the rated range.
How it works
Output = input BTU/h × AFUE
CFM = output ÷ (1.08 × temperature rise)
1.08 is the sensible heat factor for standard air (60 minutes × 0.075 lb/ft³ × 0.24 BTU/lb·°F). The answer is an estimate: it assumes the furnace is firing at its rated input. Clock the gas meter or check manifold pressure if you suspect it isn't.
Taking the temperatures
- Let the furnace run on high fire for 10–15 minutes first.
- Measure supply air out of sight of the heat exchanger, so radiant heat doesn't skew the probe.
- Measure return air before the filter and any humidifier bypass.
- The acceptable rise range is printed on the furnace rating plate.
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Results are estimates for field reference. Always confirm against the equipment manufacturer's charging chart, installation instructions and local code. Found a problem with this tool? Tell us.