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 header 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.

Unlike the ASHRAE cooling sizing option, Unitary DX can be used in conjunction with Natural ventilation.

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.