Furnace Temperature Rise Calculator
Verify furnace airflow CFM and supply-to-return temperature rise against manufacturer nameplate rating plates.
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How Furnace Temperature Rise and CFM Are Calculated
Temperature rise ($Delta T$) is the temperature difference between the return air entering the furnace and the heated supply air exiting the plenum. Because the specific heat of standard air is $0.24 ext{ BTU}/( ext{lb}cdot^circ ext{F})$ and density is $0.075 ext{ lb}/ ext{cu ft}$, the thermodynamic constant is $1.08$:
Output BTU/hr = Input BTU/hr × (AFUE % / 100)
Actual CFM = Output BTU/hr / (1.08 × Temperature Rise °F)
Frequently Asked Questions
Where should supply temperature be measured?
Measure around an elbow or at least 2 feet above the heat exchanger out of direct radiant line-of-sight of the hot metal or evaporator coil to prevent radiation errors.
What happens if temperature rise is too low?
If temperature rise is below minimum, the furnace is moving excessive airflow or is under-fired. High-efficiency condensing furnaces will suffer from cold draft complaints and poor combustion efficiency.
How do I adjust temperature rise on a furnace?
Adjust the blower motor speed tap (or dip switches on ECM / variable-speed motors) for heating. Increasing blower speed lowers temperature rise; decreasing blower speed increases rise.