What is a normal furnace temperature rise?

Most gas furnaces have a nameplate temperature rise range printed on the rating plate, typically a 30–40°F window somewhere between about 35°F and 85°F (for example, 45–75°F). You want the measured rise to land within that range, ideally near the middle.

How do you calculate furnace airflow (CFM)?

Heating CFM = BTU output ÷ (1.08 × temperature rise). The BTU output is the furnace input multiplied by its efficiency — for example, a 100,000 BTU/h input at 80% AFUE is 80,000 BTU/h output. The 1.08 factor is for standard air.

Do I use BTU input or output for the airflow formula?

Use BTU output — the heat actually delivered to the airstream. Output = input × efficiency. Using the input value would overstate the airflow because not all the input energy reaches the supply air.

What does it mean if the temperature rise is too high or too low?

A rise above the nameplate maximum means airflow is too low (dirty filter, undersized or closed ducts, slow blower) and can crack a heat exchanger. A rise below the minimum means airflow is too high (oversized blower, return leaks) and can cause condensation and short cycling.

Furnace Temperature Rise & Airflow Calculator

Free furnace temperature rise and airflow calculator. Find temperature rise from supply and return temps, and calculate heating CFM from furnace BTU output and the measured rise.

How the temperature rise & airflow calculation works

Temperature rise is how much the furnace heats the air as it passes through:

Temperature Rise (°F) = Supply Temperature − Return Temperature

Every furnace has a nameplate rise range (a roughly 30–40°F window, e.g. 45–75°F). The measured rise should land inside it — ideally near the middle.

Once you know the rise and the furnace’s heat output, you can solve for the airflow it’s actually moving:

Heating CFM = BTU Output ÷ (1.08 × Temperature Rise)

where BTU Output = Input × Efficiency (e.g. 100,000 BTU/h input × 0.80 AFUE = 80,000 BTU/h output). Always use output, not input — and the 1.08 factor assumes standard air.

Worked example

A furnace with 100,000 BTU/h input at 80% AFUE measures 130°F supply and 70°F return:

Rise = 130 − 70 = 60°F (within a typical range)
Output = 100,000 × 0.80 = 80,000 BTU/h
CFM = 80,000 ÷ (1.08 × 60) ≈ 1,235 CFM

Reading the rise

Rise vs. nameplate	Meaning
Below minimum	Airflow too high — oversized blower, return leaks, condensation risk
Within range	Airflow matched to the furnace
Above maximum	Airflow too low — dirty filter, restricted ducts, slow blower; can crack the heat exchanger

This is the heating-mode counterpart to the CFM & Tonnage Calculator. Confirm restrictions with the Static Pressure Calculator.

How FieldPad helps in the field

Run the rise and airflow here, then log them in FieldPad against the furnace and the client. Keep the readings on the same record as the maintenance job, flag a heat-exchanger concern, and turn it into a documented estimate — offline, on your iPhone.

Furnace Temperature Rise & Airflow Calculator

How the temperature rise & airflow calculation works

Worked example

Reading the rise

How FieldPad helps in the field

Stop Re-Running These Numbers by Hand

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