HVAC tab in model data
This model is applicable to exterior spaces that are served by an underfloor air distribution system. The dominant sources of heat gain should be from people, equipment, and other localized sources located in the occupied part of the room, as well as convective gain coming from a warm window. The model predicts two temperatures in the room:
The following fields are used to define the Under Floor Air Distribution (UFAD) Exterior data.
The total number of diffusers in this zone. This field can allowed to Autocalculate (in which case it is set to the design occupancy level; i.e., number of people). If the design occupancy is low or zero but there are still heat sources that could generate buoyancy driven plumes, the user should input a value based on the design supply air flow rate of the zone and the design flow rate of an individual diffuser. In the absence of any other information, divide the zone area by 100 ft2. The default for this field is Autocalculate.
The power in watts incorporated in a buoyancy driven thermal plume. Normally we assume all the loads of a workstation create a single plume so that this represents the convective heat gain from a workstation – 1 person, 1 computer terminal, plus any task lighting. A typical value would be 220 W. However, the model assumes an “equivalent” plume derived from the zone extraction rate normalized to the number of workstations/occupants. This field should normally be allowed to default – the program will calculate a value based upon the occupancy and the extraction rate. The default is Autocalculate.
This is the design air flow opening area in square meters of a single diffuser. The default value depends on the diffuser type. For swirl diffusers it is 0075 m2, for variable area diffusers 0.035 m2, for horizontal swirl diffusers 0.006 m2, and for linear bar grilles 0.03 m2. The default is Autocalculate..
This input describes the angle at which air emerges from the diffusers. It should be the angle in degrees between the vertical and the diffuser slots. The default is Autocalculate in which case the setting depends on the diffuser type: for swirl diffusers it is 28°, for variable area diffusers 45°, for DV diffusers 73°, and for linear bar grilles 15°.
The choices for this alpha field are:
The swirl and displacement diffusers are fixed area diffusers. The variable area diffusers maintain an approximately constant exit velocity. Linear bar grilles are normally used in exterior zones and are fixed area diffusers. Custom is used to signify that the user intends to input the coefficients A – E (see below) rather than let the program set the coefficients based on diffuser type. The default is 1-Swirl.
When the diffuser type is set to 5-Custom the coefficients A to E should be entered to define the performance of the diffuser. The coefficients are used in the equation:
Kc = A*Gamma**B + C + D*Gamma + E*Gamma**2.
Gamma is a variable that characterizes the amount of stratification in a UFAD zone. Kc is the fraction of the total internal convective heat gain that is assigned to the lower (occupied) subzone. The coefficients in the formula are defaulted based upon diffuser type.
The coefficient A in the above formula.
The coefficient B in the above formula.
The coefficient C in the above formula.
The coefficient D in the above formula.
The coefficient E in the above formula.
This field is the height (in m or ft) of the thermostat/temperature control sensor above the floor. The default is 1.2m.
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.
If you would like to define the Transition height (below) then check this check box.
An optional field to allow the transition height (in m or ft above floor) to be defined rather than have the program calculate it. The default is 1.7m.
This field specifies a minimum temperature difference between the upper subzone and the occupied subzone that will be used to trigger whether or not the UFAD auxiliary outputs will be calculated. These outputs are Room Air Zone Transition Height and Room Air Zone Average Temperature Gradient. They are set to zero 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.