About this calculator This calculator determines the Air Sensible Heat Factor, which relates airflow (CFM), temperature difference (ΔT), and sensible heat load (BTU/hr) in the formula BTU/hr = CFM × Cs × ΔT. Cs accounts for both altitude (air density) and humidity ratio. At sea level with dry air (W=0), Cs = 1.08. At standard design conditions (W=0.01, ≈75°F / 50% RH), Cs ≈ 1.10. Altitude reduces Cs proportionally to air density — roughly 3% per 1,000 ft.
Use when You need to determine Cs for sensible heat load calculations at a given altitude and humidity ratio.
Formula Cs = 60 × 0.075 × (0.24 + 0.45 × W) × Density Factor
Variables
Altitude Elevation above sea level in feetHumidity Ratio W Mass of water vapor per mass of dry air (lb/lb)
Altitude (ft)
Installation altitude — autofilled from weather station
Humidity Ratio W (lb/lb)
W = 0.01 at approximately 75°F / 50% RH
Air Sensible Heat Factor 1.1002

What is the Air Sensible Heat Factor?

The Air Sensible Heat Factor is a coefficient used in the sensible heat equation BTU/hr = CFM × Cs × ΔT. It combines the effects of air density, specific heat of moist air, and unit conversion into a single factor. Cs is calculated as: Cs = 60 × 0.075 × (0.24 + 0.45 × W) × Density Factor, where 60 converts minutes to hours, 0.075 lb/ft³ is standard sea-level air density, (0.24 + 0.45 × W) is the specific heat of moist air (BTU/lb-°F), W is the humidity ratio in lb/lb, and the Density Factor corrects for altitude. At sea level with completely dry air (W=0), Cs = 1.08 — the classic constant used in simplified heating and cooling calculations. At W=0.01 (approximately 75°F / 50% RH, typical design conditions), Cs ≈ 1.10. At higher altitudes, Cs decreases proportionally to the reduction in air density, approximately 3% per 1,000 ft of elevation.

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