Furnace temperature rise calculator
Check furnace temperature rise against the nameplate range and estimate airflow (CFM) from input rating, efficiency and supply and return temperatures.
Imperial (US) Metric (SI) Furnace input ratingTemperature rise
50.0 °F
Within the nameplate range.
Estimated output
76,800 Btu/h
Estimated airflow
1,422 CFM
Assumes the burner is at full rated input. Clock the gas meter to confirm firing rate.
Estimates based on your inputs. Always verify against the manufacturer's data and the codes that apply where you work.
What this means
Temperature rise is supply air temperature minus return air temperature while the burner fires at steady state. Every furnace nameplate lists an allowed rise range. A rise above the range usually points to low airflow, and a rise below it to high airflow or an underfired burner. The airflow estimate assumes the furnace fires at its rated input, so clock the gas meter if you are not sure.
Formula used
- Temperature rise = supply air temperature − return air temperature
- Output (Btu/h) = input rating × efficiency ÷ 100
- Estimated airflow (CFM) = output Btu/h ÷ (1.08 × temperature rise °F)
- Metric inputs are converted: 1 kW = 3,412.14 Btu/h, 1 K = 1.8 °F, 1 CFM = 0.4719 L/s
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Ask Brixon, first question freeCommon questions
What is a normal furnace temperature rise?
Where should I measure supply and return temperatures?
What causes a high temperature rise?
What causes a low temperature rise?
How accurate is the airflow estimate?
Why does the formula use 1.08?
Can I use this on a two stage or modulating furnace?
Estimates based on your inputs. Always verify against the manufacturer's data and the codes that apply where you work.