Heating Performance

Gas heater output uses the sensible heat equation: Output BTU/hr = CFM × Cs × ΔT, where Cs is the Air Sensible Heat Factor and ΔT is the temperature rise (Discharge − Entering). Cs is the product of air density, the specific heat of moist air, and a unit conversion — roughly 1.08 at sea level and ~70°F, but it scales directly with air density. Cold air is denser, so a CFM specified at 0°F entering carries about 15% more mass than the same CFM at 70°F, which is why heat output rises in winter conditions. Air density also decreases with elevation, dropping roughly 3% per 1,000 ft, so a heater at 5,000 ft delivers about 15% less heat at the same CFM and ΔT. The direct-fired vs. indirect-fired toggle reflects CaptiveAire's design convention, not a combustion difference: direct-fired units are draw-through (the blower pulls heated air, so CFM is rated at discharge temperature), and indirect-fired units are blow-through (the blower pushes cold entering air, so CFM is rated at entering temperature). The toggle simply selects which temperature is used for the density correction. Input BTU/hr is fuel energy consumed: Input BTU/hr = Output BTU/hr ÷ Efficiency. Direct-fired units typically run near 92% because all combustion energy stays in the airstream; indirect-fired units run 80–85% because some energy leaves through the flue.