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Furnace Temperature Rise and Airflow Calculator

Estimate how much air a furnace blower is moving from the heat it puts out and the temperature rise you measure, and check the rise against the nameplate range.

Method reviewed October 25, 2026. Version 1.0.0. How it works

Your inputs

Unit system

AFUE is a seasonal figure; steady-state efficiency is usually close to it for this check.

Measure in the supply plenum out of line of sight of the heat exchanger, after the burners have run 10–15 minutes.

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

Estimated blower airflow
1,185CFM
Temperature rise
50°F

A 50 °F rise at 64,000 BTU/h output means about 1,185 CFM.

  • A 50 °F rise is within the 35–65 °F nameplate range.
Heat output used
64,000BTU/h
Show the working
  1. Heat output
    80,000 × 0.8 = 64,000 BTU/h
  2. Temperature rise = supply − return
    118 − 68 = 50 °F
  3. Airflow = output ÷ (1.08 × density ratio × rise)
    64,000 ÷ (1.08 × 1 × 50) = 1,185 CFM

The calculation

All of a furnace’s output heats the air passing through it, so the sensible heat equation can be turned around to find the airflow: CFM = output ÷ (1.08 × temperature rise). The 1.08 constant is for standard air and is scaled down for the thinner air at higher elevations.

Output can be entered directly, or worked out from the input rating — or better, a clocked input — and the furnace’s steady-state efficiency.

Measuring the rise

Measure the return air before the furnace and the supply air in the main trunk, far enough downstream that the probe cannot “see” the heat exchanger — radiant heat makes a supply reading too high. Let the furnace run 10–15 minutes on high fire first. The nameplate lists the allowed temperature-rise range; a rise above it means too little airflow, which overheats the heat exchanger.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Heat output from
Input rating × efficiency
Input rating (or clocked input)
80,000 BTU/h
Steady-state efficiency
80 %
Return-air temperature
68 °F
Supply-air temperature
118 °F
Nameplate rise range — minimum
35 °F
Nameplate rise range — maximum
65 °F
Site elevation
0 ft

Result

Estimated blower airflow
1,185 CFM
Temperature rise
50 °F
Heat output used
64,000 BTU/h

A 50 °F rise at 64,000 BTU/h output means about 1,185 CFM.

Assumptions

  • Sensible heat equation for standard air, Q = 1.08 × CFM × ΔT, scaled for elevation.

Limitations

  • The result is only as good as the output figure and the temperature readings. Radiant heat from the heat exchanger can make a supply reading too high.

Frequently asked questions

What is a normal furnace temperature rise?

Whatever falls within the range on the furnace’s rating plate, commonly something like 35–65 °F or 40–70 °F. Aim for the middle of the range unless the manufacturer says otherwise.

What causes a high temperature rise?

Too little airflow: a dirty filter, a closed or blocked supply or return, undersized ducts, a dirty blower wheel or a low blower speed. Over-firing can also raise it, which is why clocking the input first helps.

Is this as accurate as measuring airflow directly?

It is a reasonable field estimate if the output figure and temperatures are good. Flow hoods, traverses or a calibrated static-pressure method are more accurate when it matters.

Sources

  1. Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
  2. What is AFUE? — Carrier. AFUE is the share of fuel energy delivered as heat over a season.

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