Estimate furnace airflow in CFM from delivered heat and supply and return air temperatures, with an optional nameplate temperature-rise check.
Advanced options
Table of contents
How to use our Furnace Airflow from Temperature Rise Calculator
- Choose Heat rating you have. Use output rating when the furnace label lists delivered output; otherwise use input rating and efficiency.
- Enter the rating from the furnace plate, manual, or data sheet. Keep input BTU/hr separate from output BTU/hr.
- After steady heating operation, enter Supply air temperature and Return air temperature from the furnace area.
- Optionally open Advanced options and enter both nameplate temperature-rise limits from your specific furnace.
- Check that the measured rise and delivered heat shown match your entries, then use the CFM estimate as a starting point for troubleshooting.
Definitions
CFM: Cubic feet per minute, a measure of how much air the blower moves.
Temperature rise: Supply air temperature minus return air temperature during heating. Furnace instructions commonly specify an allowed rise range for a particular model. [1]
Delivered heat: Heat transferred to the air, measured in BTU/hr. It is the furnace output rating, not the unadjusted fuel input rating.
Furnace efficiency: Delivered heat divided by fuel input, shown as a percent for the input-rating method.
Nameplate temperature-rise range: The minimum and maximum rise printed for a specific furnace model; it is not a universal target.
Common mistakes and quick fixes
Mistake: Entering the fuel input number in Furnace output rating .
Fix: Use the delivered output number for Furnace output rating, or choose the input-and-efficiency method.
Mistake: Measuring Supply air temperature at a distant room register.
Fix: Measure in the supply plenum after heating has become steady, away from direct heat-exchanger line of sight.
Mistake: Entering a return reading equal to or warmer than Supply air temperature .
Fix: Recheck both locations; supply air must be warmer for a heating airflow calculation.
Mistake: Filling in only Nameplate minimum temperature rise (deg F) .
Fix: Enter both nameplate limits, or leave both blank to calculate airflow without the range comparison.
Mistake: Treating a high or low Temperature rise offset from nameplate range as a diagnosis.
Fix: Verify readings and the furnace instructions, then contact a qualified technician if the result remains outside the listed range.
Limitations & Key Assumptions / Boundary Conditions
- The CFM result uses the rounded standard-air factor of 1.08. It does not adjust for altitude, air density, humidity, or duct leakage.
- Supply and return temperatures must be measured after steady heating operation at appropriate furnace-area locations. Register readings or readings near the heat exchanger can give a different rise.
- Using Furnace efficiency to convert input BTU/hr to delivered heat is an estimate. An AFUE value is not necessarily a direct steady-state output measurement.
- A temperature rise within the entered nameplate range does not prove that airflow, combustion, ductwork, controls, or safety systems are working correctly.
- This tool applies to heating operation with supply air warmer than return air. It does not calculate cooling airflow or replace a full airflow test.
Methodology
Calculation method
The calculator first finds the measured furnace temperature rise from two readings. Supply air must be warmer than return air.
temperature rise (deg F) = supply air temperature - return air temperature
For the direct method, delivered heat is the entered Furnace output rating. For the alternate method, it estimates delivered heat from Furnace input rating and Furnace efficiency.
delivered heat (BTU/hr) = input rating (BTU/hr) x efficiency (percent) / 100
The airflow estimate divides delivered heat by the standard-air sensible-heat factor and the measured temperature rise. The 1.08 factor is a conventional rounded relationship for CFM, sensible heat, and deg F temperature difference. [2]
airflow (CFM) = delivered heat (BTU/hr) / (1.08 x temperature rise (deg F))
Example:
60,000 BTU/hr of delivered heat with 130 deg F supply air and 70 deg F return air gives a 60 deg F rise. The estimate is 60,000 / (1.08 x 60) = 925.9 CFM.
If both nameplate limits are entered, the calculator reports zero when the rise is inside or exactly on the listed range. Otherwise, it reports the number of deg F below the minimum or above the maximum.
range offset (deg F) = rise - minimum if below; 0 if within; rise - maximum if above
Calculation choices
The result is a standard-air estimate. A fuel input rating without efficiency is not used as delivered heat, and an AFUE-based efficiency entry should be treated as an approximation.