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) = 0.68 × CFM × (W_outdoor − W_indoor) when W is in grains per pound, or 4840 × CFM × ΔW when W is in lb/lb. 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 design day (peak dry bulb with mean coincident wet bulb), the dehumidification design day (peak dewpoint with mean coincident dry bulb), and the enthalpy design day (peak total enthalpy with mean coincident dry bulb). These are three independent statistical peaks — the cooling day is the hottest hour of the year, the dehumidification day is often cooler and much more humid, and the enthalpy day captures the hour with maximum total heat content. In humid climates, the dehumidification or enthalpy condition can produce a larger total cooling coil load than the cooling design day, because the latent portion grows faster than the sensible portion shrinks. Comparing all three prevents undersizing for latent-dominated conditions.