HVAC Equation for Duct Sizing
The relationship between Volume of air (CFM), the cross sectional Area of the duct (sq. ft), and the Velocity of the air (FPM) is defined by the basic continuity equation.
Frequently Asked Questions
CFM is calculated by multiplying the internal Cross Sectional Area of the duct (in square feet) by the Air Velocity (in Feet Per Minute).
Total cooling load (kW) = Sensible load + Latent load. Sensible load covers heat from: solar radiation through glass, heat conduction through walls/roofs, people (75W sensible each), lighting (watts × 3.41 BTU/hr), equipment, and ventilation air. Latent load covers moisture from occupants (55W/person) and ventilation. ASHRAE Handbook of Fundamentals and Manual J provide detailed calculation methods.
BTU (British Thermal Unit) measures heat energy; BTU/hr measures cooling/heating power. Conversion: 1 kW = 3,412 BTU/hr. A 5kW air conditioner equals ~17,060 BTU/hr. In the US, residential HVAC is rated in BTU/hr (range: 5,000–60,000 BTU/hr). Commercial systems are rated in tons of refrigeration: 1 ton = 12,000 BTU/hr = 3.517 kW.
A simplified rule of thumb: allow 150–200 BTU/hr (44–59W) per m² of floor area in a typical climate. For a 20m² room: 20 × 175 = 3,500W = 3.5 kW. However, accurate sizing requires Manual J or ASHRAE load calculations accounting for climate zone, insulation levels, window area and orientation, occupancy, and local solar intensity. Oversizing causes short-cycling and poor dehumidification.
COP (Coefficient of Performance) = useful heating or cooling output (kW) / electrical power input (kW). A COP of 3.5 means 3.5 kW of cooling for each 1 kW of electricity. SEER (Seasonal Energy Efficiency Ratio) = total seasonal cooling output (BTU) / total seasonal electrical input (Wh) — it accounts for varying conditions through the season. Minimum SEER ratings in the US are now 14–15 depending on climate region.
Sensible heat changes air temperature (measured by thermometer) — caused by solar gain, conduction, equipment, and lighting. Latent heat changes air moisture content (humidity) without changing temperature — caused by people perspiring, cooking, and humid outdoor air. In tropical climates, latent load can be 40–60% of total cooling load. Most packaged air conditioners handle both; in high-humidity climates, dedicated dehumidification or dedicated outdoor air systems (DOAS) may be needed.