Furnace Airflow to Temperature Rise
The inverse of the temperature-rise tile: pick a blower airflow (a tap or a target CFM) and predict the rise it produces, to set the blower before measuring. rise = 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 and source
output = 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.
First-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 1.08 air factor and efficiency are editable.
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