ASHRAE Cooling Sizing

In the ASHRAE cooling sizing method the cooling design load calculations are carried out using the standard ASHRAE Heat Balance method implemented in EnergyPlus using the 'ZoneHVAC:IdealLoadsAirSystem' system type. You can define the conditions of the supply air delivered to cooled zones using Design supply temperature and Supply air humidity ratio. The total (sensible + latent) load is calculated as the enthalpy change between the zone return air and the specified supply air condition for the current load.

The ASHRAE system is modelled assuming:

• No economiser,
• No heat recovery,
• No heating,
• Zero pressure rise fan,
• No humidification/dehumidification,
• Zone plenums will not 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.

Supply air conditions

Supply air delivery temperature

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

Supply air humidity ratio

The constant humidity ratio (mass of water per mass of dry air) of the cold supply air to be delivered when cooling is required. The default humidity ratio is 0.008. You should be careful to enter a value that is a valid condition, i.e. it is under the 100% saturation line on the Psychrometric Chart.

Caution: The Ideal Loads Air System component as implemented in DesignBuilder does not control zone humidity levels. The Ideal Loads Air System component total cooling load is simply the enthalpy change between the return air and the user-specified supply air condition for the current load. The latent load is the total load less the sensible load and may be positive or negative.