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The sensible heat formula in HVAC
The sensible heat equation Q = 1.1 x CFM x DT is the most used formula in air-side HVAC design. It calculates how much heating or cooling an airstream delivers based on how much air is moved and how big the temperature change is. Every supply air CFM calculation, every coil sizing check, and every duct heat gain estimate uses this formula in some form.
Where 1.1 comes from
The 1.1 factor is not arbitrary. It comes from: air density at standard conditions (0.075 lb/ft3) times specific heat of air (0.24 BTU/lb.F) times 60 minutes per hour = 0.075 x 0.24 x 60 = 1.08, rounded to 1.1. At higher altitudes or temperatures, air density drops and this factor decreases. At Denver (5,280 ft elevation), the correct factor is about 0.93. This calculator applies the correction automatically when you enter altitude and air temperature.
Common uses of this formula
Supply air CFM sizing: if a room needs 12,000 BTU/hr of cooling and the supply air is 20°F cooler than the room, CFM = 12,000 / (1.1 x 20) = 545 CFM. Coil capacity check: 1,200 CFM of air entering at 80°F and leaving at 55°F removes 1,200 x 1.1 x 25 = 33,000 BTU/hr of sensible heat. See the SHR calculator to split total cooling load into sensible and latent components. Use the latent heat calculator for the moisture side of the calculation.
Frequently Asked Questions
In metric units: Q(kW) = 1.23 x L/s x DT(C). The factor 1.23 comes from air density (1.2 kg/m3) times specific heat (1.006 kJ/kg.K) = 1.207, rounded to 1.23. So 100 L/s of air with a 10C temperature difference transfers 1.23 x 100 x 10 = 1,230 W = 1.23 kW of sensible heat. This calculator handles both unit systems automatically.
Air density decreases with altitude, reducing the 1.1 factor. The correction is approximately: factor = 1.1 x (1 - 0.0000226 x altitude_ft)^5.256. At 5,000 ft (Calgary, Denver), the factor is about 0.92. This means a duct system carrying 1,000 CFM at 5,000 ft delivers about 16% less sensible heat capacity than the same system at sea level. Always apply the altitude correction for installations above 2,000 ft. Use this calculator with the altitude field to get the corrected factor automatically.