Furnace Airflow to Temperature Rise
Pick a blower airflow (a tap or a target CFM) and predict the rise it produces, to set the blower before measuring.
Example
You enter
- Furnace input (BTU/hr) 100000
- Efficiency (%) 80
- Blower airflow (CFM) 1200
- Return-air temp (°F) 70
- Plate min rise (°F) 40
- Plate max rise (°F) 70
You get
- Predicted temperature rise 61.7 F
- Supply-air temperature 131.7 F
Checked against published worked examples from: Shearer Supply.
Details, formula, and sources
Scoperise = output / (1.08 x CFM), output = input x efficiency, supply = return + rise. A 100,000 BTU/hr furnace at 80% on 1,200 CFM (70 F return) gives a 62 F rise and 132 F supply - in the 40-70 F band; drop to a lower tap and the rise climbs. A lower airflow raises the rise (overheat risk); a higher airflow lowers it. The rating-plate rise range governs.
Formula or tableoutput = input x efficiency / 100; delta_T = output / (1.08 x CFM); supply_air = return_air + delta_T. Verdict checks delta_T against the rating-plate rise range.
Source and editionFirst-principles sensible-heat relation Qs = 1.08 x CFM x delta-T solved for the rise, the inverse of the temperature-rise tile (public); the efficiency is editable; the 1.08 sea-level air factor is fixed.
Free accessThe sensible-heat relation is public physics; the rating-plate rise range governs.
What governsEstimate. AHJ and licensed professional govern.
API field namesinput_btuh, efficiency_pct, cfm, return_air_F, rise_min_F, rise_max_F, delta_T_F, supply_air_F
- Rise from airflow delta_T = output / (1.08 x CFM), output = input x efficiencyfirst principles
- Air factor 1.08 BTU/hr per CFM per F is sea-level standard air; at altitude or high humidity it falls (air-density-correction gives the corrected factor)first principles
- Rise range default 40 to 70 F; the rating plate's stamped range is the governing limitrating plate