Airflow / CFM
Estimate airflow using the heating temperature rise method. This is a field-friendly estimate—verify stable conditions, proper probe placement, and OEM procedures.
- Enter return air temperature.
- Enter supply air temperature.
- Enter heat input (BTU/hr or kW).
How to use (field notes) tap
- Use stable operating conditions (steady state).
- Ensure correct probe placement and avoid radiant pickup.
- ΔT is sensitive—small temperature errors can cause large CFM error when ΔT is small.
- Confirm equipment stage (low/high fire) and blower speed.
Saved calculationsAirflow / CFMtapSaved calculations (tap to expand).
This calculator estimates airflow (CFM) using a temperature rise method (commonly applied to gas furnaces). It’s meant for fast troubleshooting and sanity checks—use OEM data and proper test methods when precision matters.
Who it’s for
- Service technicians doing quick airflow sanity checks.
- Techs verifying blower setup changes (before/after).
- Apprentices/students learning what ΔT-based airflow implies.
When to use it
- You need a quick airflow estimate and can measure a stable temperature rise (ΔT).
- You’re checking blower setup (speed tap/ECM profile) and want a before/after number.
- You want to spot obvious airflow problems (filter, coil, duct restrictions).
Required inputs
- Delivered heat input/output (BTU/hr) consistent with your method (be explicit which you’re using).
- Required to compute results.
- Return air temperature and supply air temperature (for ΔT).
- Required to compute results.
- Stable operation (not cycling, not rapidly staging).
- Required to compute results.
Outputs
- Estimated airflow (CFM)
- Computed from BTU/hr and measured temperature rise. Treat as an estimate—especially when ΔT is small.
- Copy-ready summary
- A short text block you can paste into job notes (inputs + output + units) for repeatable documentation.
How to use (field notes)
- Run the equipment long enough to stabilize before recording temps.
- Measure true return and true supply (avoid stratified/spot readings).
- If ΔT is small, slow down and re-check—small temp errors can blow up the CFM estimate.
- Document whether you used input or output BTU/hr assumptions in your notes.
Assumptions & limitations tap
- Temperature measurements represent true mixed air (not a hot/cold streak).
- You’re using a reasonable BTU/hr value for the actual operating stage.
- The constant used (1.08) is an approximation tied to typical air properties; precision varies with conditions.
Why is my CFM estimate crazy high when ΔT is small? tap
Because ΔT is in the denominator. When ΔT is small, a 1–2°F measurement error can blow up the CFM result. Re-check probe placement, let temps stabilize, and verify you’re measuring true return and true supply (not near a heat source or a stratified area).
Where should I measure return and supply temperatures? tap
Measure where the air is well mixed. Avoid taking readings right at the heat exchanger, right at the coil face, or near a register with stratified air. If possible, use a method that averages across the airstream (or multiple points).
Do I use input BTU or output BTU? tap
Use the value that matches your method and assumptions. If you only have nameplate input, you may need an efficiency assumption to get output. For quick checks, consistency matters more than perfection—document what you used in your notes.
Is this method valid for cooling mode? tap
Not directly. Cooling airflow is usually tied to total capacity and enthalpy/temperature split methods, and it’s more sensitive to humidity/latent load. This page is focused on the heating temperature rise approach.