Air Sensible Heat (1.08 Rule)

This is the water formula wearing a different hat: mass flow times specific heat times temperature change, with air's properties instead of water's. Standard air weighs 0.075 lb/ft³ (1.2014 kg/m³) and holds 0.240 BTU/(lb·°F), so 1.08 = 60 min/hr × 0.075 lb/ft³ × 0.240 BTU/(lb·°F). A furnace moving 1,000 cfm across a 20 °F rise delivers 1.08 × 1,000 × 20 = 21,600 BTU/hr, which this page returns as 6.33 kW.

Two traps. First, "standard air" means roughly 70 °F at sea level: at Denver's 5,300 ft the density drops about 18 %, so the constant falls to about 0.89 and a rooftop unit sized on 1.08 quietly under-delivers. Second, this is sensible heat only: it says nothing about the moisture the coil pulls out, which on a humid August afternoon can be a third of the total load. Balancing technicians run the formula backwards constantly: measure supply and return temperature at a known airflow, and you have the delivered capacity without opening a single panel.