Air Temperature Distribution - Three Node Displacement Ventilation

HVAC tab in model data

This model is applicable to spaces that are served by a low velocity floor-level displacement ventilation air distribution system. Furthermore, the dominant sources of heat gain should be from people and other localized sources located in the occupied part of the room. The model should be used with caution in zones which have large heat gains or losses through exterior walls or windows or which have considerable direct solar gain. The model predicts three temperatures in the room:

• A foot level temperature (T_FLOOR). The floor region is 0.2 meters deep and T_FLOOR represents the temperature at the mid-point of the region.
• An occupied sub-zone temperature (T_OC), representing the temperature in the region between the floor layer and the upper, mixed layer.
• An upper node representing the mixed-layer/outflow temperature (T_MX) essential for overall energy budget calculations and for modelling comfort effects of the upper layer temperature.

The following fields are used to define the Three node displacement ventilation room air temperature distribution.

Gain distribution schedule

Select the schedule defining the fractions of the convective portion of the internal gains in the occupied subzone that remain in the occupied subzone. The remainder of the convective portion of the internal gains in the occupied subzone enters the plumes and is carried to the upper subzone. The types of internal gains that are assumed to be located in the occupied subzone are:

• people
• task lights
• electric equipment
• gas equipment
• hot water equipment
• steam equipment
• other equipment
• baseboard heat

Types of internal gains that are assumed to be in the upper subzone are:

• general lights
• tubular daylighting devices
• high temperature radiant heaters

The schedule values should be between 0 and 1. A value of 1 means that all the convection gains from equipment, task lights and people are dispersed in the lower occupied subzone. Conversely a value of 0 puts all the lower subzone convective gains into the plumes rising into the upper well-mixed subzone.

Number of plumes per occupant

This field specifies number of plumes per occupant. Plumes are associated with localized sources of convective heat gain from people and equipment. For example, a value of 2.0 would be used if each occupant has a computer that generates a separate plume that does not merge with the plume from the occupant in the lower, occupied, subzone.

Thermostat height

This field is the height (in m or ft) of the thermostat/temperature control sensor above the floor.

Comfort height

The height (in m or ft) above the floor at which air temperature is calculated for comfort purposes. The air temperature at this height is used in calculating the available measures of comfort: Fanger, Pierce or KSU. The default is 1.1m.

Advanced

Temperature difference threshold for reporting

This field specifies a minimum temperature difference between the upper mixed subzone and the occupied subzone that will be used to trigger whether or not the displacement ventilation auxiliary outputs will be calculated. These outputs are Room Air Zone Transition Height, Room Air Zone Recommended Minimum Flow Fraction, Room Air Zone Average Temperature Gradient and Room Air Zone Maximum Temperature Gradient. They are set to negative values when the temperature difference is less than the threshold and the output Room Air Zone Is Mixed Status is set to 1.

The value should be greater than or equal to zero and is in units of Delta °C or Delta °F. The default value is 0.4°C.