Unitary DX Cooling Sizing

When using the Unitary DX cooling sizing method the Cooling design calculation is made through a detailed EnergyPlus simulation using Compact HVAC Single zone Unitary DX equipment. A diagram of the system layout used is shown in the Unitary Single Zone topic.

Each zone has it's own Unitary system with a fan (generating no pickup), a DX cooling coil, an outside air controller and a damper to reduce supply air flow. More specifically:

• No economiser,
• No heat recovery,
• No heating coil,
• Zero pressure rise fan,
• No humidification/dehumidification,
• Zone plenums will be used if specified,
• Outside airflow rate and operation schedule defined by the data under the Mechanical Ventilation and Natural ventilation headers on the HVAC tab at the zone level,
• Cooling coil availability defined by the schedule under the Cooling header on the HVAC tab at the zone level,
• Zone design cooling temperature setpoints are defined by the data under Environmental Control on the Activity tab.

Note: Unitary DX will tend to calculate higher latent loads than the other 3 cooling sizing methods and the zone air humidity will generally be lower. This is because DX coils run colder than water-cooled coils and will condense out more water from the supply air. Unitary DX can often be used to provide a conservative calculation for the latent cooling loads.