Furnace Temperature Rise and Derived Airflow
The supply-minus-return temperature rise checked against the rating-plate range (default 40 to 70 F).
Example
You enter
- Return-air temp (°F) 70
- Supply-air temp (°F) 120
- Furnace input (BTU/hr) 100000
- Efficiency (%) 80
- Plate min rise (°F) 40
- Plate max rise (°F) 70
You get
- Temperature rise 50.0 F
- Heat output 80,000 BTU/hr
- Derived airflow 1,481 CFM
Checked against published worked examples from: Shearer Supply.
Details, formula, and sources
ScopeThe supply-minus-return temperature rise checked against the rating-plate range (default 40 to 70 F), and the airflow derived from the heat output via Qs = 1.08 x CFM x delta-T: from return / supply temperatures, furnace input, and efficiency, the rise, the output, and the CFM, with a rise-low / in-range / rise-high reading. The rating plate governs.
Formula or tabledelta_T = supply_air - return_air; output = input x efficiency / 100; CFM = output / (1.08 x 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 (public); the 1.08 air factor is fixed; the efficiency (default 80%) is editable.
Free accessThe sensible-heat relation is public physics; the rating-plate rise range governs.
What governsEstimate. AHJ and licensed professional govern.
API field namesreturn_air_F, supply_air_F, input_btuh, efficiency_pct, rise_min_F, rise_max_F, delta_T_F, output_btuh, cfm
- 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
- Efficiency default 80% steady-state / thermal efficiency, an editable nameplate valuerating plate
- Rise range default 40 to 70 F; the rating plate's stamped range is the governing limitrating plate