Outdoor Air Load Calculator

Outdoor air load is the heating or cooling energy required to condition ventilation air from outdoor conditions to the desired indoor conditions. It has both sensible and latent components. The sensible outdoor air load is calculated as: Sensible Load (BTU/hr) = 1.08 × CFM × (T_outdoor − T_indoor). The latent outdoor air load is: Latent Load (BTU/hr) = 0.69 × CFM × (W_outdoor − W_indoor) when W is in grains per pound, or 4840 × CFM × ΔW when W is in lb/lb. Both loads are multiplied by a density factor for the site's barometric pressure, so they drop at altitude. The outdoor air fraction required by ASHRAE Standard 62.1 for acceptable indoor air quality is the primary driver of this load. Energy recovery ventilators (ERVs) and demand-controlled ventilation (DCV) are the two most common strategies for reducing outdoor air load in commercial buildings. ASHRAE publishes three summer design conditions for each weather station: the cooling condition (0.4% dry bulb with mean coincident wet bulb), the dehumidification condition (0.4% dew point with mean coincident dry bulb), and the enthalpy condition (0.4% total enthalpy with mean coincident dry bulb). Each is exceeded only 0.4% of the year, about 35 hours, and they are independent statistics — the cooling condition is the hottest, the dehumidification condition is often cooler and much more humid, and the enthalpy condition has the highest total heat content. In humid climates, the dehumidification or enthalpy condition can produce a larger total cooling coil load than the cooling design condition, because the latent portion grows faster than the sensible portion shrinks. Comparing all three prevents undersizing for latent-dominated conditions.