VAV Cooling Sizing

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

Each zone has it's own VAV AHU with a fan (generating no pickup), a water-cooled AHU cooling coil, an outside air controller and a zone terminal box with a damper to reduce supply air flow. More specifically:

No economiser,
No heat recovery,
No AHU heating coil,
No reheat coil,
No preheat coil,
Zero pressure rise fan,
No humidification/dehumidification,
Standard zone terminal unit,
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, VAV can be used in conjunction with Natural ventilation.

Design supply temperature

The constant dry-bulb air temperature of the air supplied for cooling the zone

Design turndown ratio (supply air minimum flow fraction)

The Design turndown ratio is the fraction that the zone VAV box can reduce to based on the supply air maximum flow rate. It is normally specified to meet the minimum ventilation requirement for the occupants. A value of 1.0 means that the VAV box cannot reduce supply air at all (i.e. it is a constant volume system).

Turndown ratio is typically in range 0.3 - 0.5. Minimum is 0 and maximum is 1.