Overview

In this section, you will define the HVAC equipment and control at the zone level. This includes any zone conditioning equipment such as VAV Boxes, baseboard heaters, fan-coil/induction units, radiant heating/cooling panels, etc. Many of these components the BRS will model in a similar way. For example, if a zone has a VAV box that provides supply air, it doesn’t matter from an energy perspective whether the heating at the zone level is performed using a reheat coil in the VAV box, a baseboard heater, or a radiant heating panel (assuming in this case, each device is fed from the same hot water loop). Zone-level equipment can be defined separately for perimeter and interior zones. For example, perimeter zones can be defined with both supplemental heating and cooling coils, and interior zones with only supplemental cooling coils.

If the zone has VAV boxes (either single-duct or dual-duct), an airflow response curve needs to be defined. In the case of single-duct boxes, this entails a definition of the airflow response to the thermostat deadband mode, as well as to increases in heating and cooling demands. For dual-duct boxes, this also includes a definition of how the two airflows mix (or flip), and which stream is used for ventilation and in zone deadband.

Finally, the zone details tab involves setting the zone thermostat occupied setpoints and unoccupied setback setpoints. These setpoints can be set globally or zone-by-zone.

In-Depth

Zone thermostats: Zone thermostats may be set via global setpoints (single set of setpoints applied to all zones), or via zone-by-zone setpoint definitions. In either case, there are four parameters needed to define the thermostat; occupied heating setpoint, occupied cooling setpoint, unoccupied heating setpoint and unoccupied cooling setpoint. The switch from occupied to unoccupied setpoints is mediated by the previously-defined HVAC operation schedule. The HVAC operation schedule keeps the airside system off when the HVAC operation schedule is 0, except when one of the zone thermostats exceeds the bounds of the setback setpoints. An additional humidity setback can be defined (especially for humid climates), that will also turn the AHU back on when the HVAC operation schedule is 0 if any zone relative humidity readings exceed a maximum relative humidity setpoint.

Zone Heating and Cooling Equipment: Zone heating and cooling equipment (including VAV reheat coils) should be defined as present or absent as a function of zone orientation (perimeter versus interior). Six parameters are used to define the zone-level conditioning equipment (3 each for perimeter zones and interior zones:

• Presence of zone-level heating equipment (yes/no), including VAV reheat coils, baseboard heater, space heaters, radiant heating panels, fan-coil units, etc.
• Presence of zone-level cooling equipment (yes/no), including fan-coils, radiant cooling panels, etc.
• Does the primary air (from the main air-side system) stay off during after-hours heating cycles? Note that for zones that have independent heating equipment such as baseboard heat, or parallel fan-powered VAV boxes that can recirculate and heat zone air, running the main AHU or other airside fan is not required to maintain setback, but often is anyway. If the setback operational strategy cannot be determined, a strategy that involves the main AHU should be considered the most likely.

VAV Airflow Control:

• Single-Duct VAV Boxes or Constant Speed boxes: Single-duct VAV boxes include standard (non-fan-powered) VAV boxes with or without reheat, as well as series or parallel-configured fan-powered boxes. All types can be modeled with a single framework that considers the flow setpoints from the primary air source (the air-handling unit). Constant speed air terminals (e.g. diffusers) can also be modeled by setting all airflow setpoints equal to 100%. For VAV boxes, there are typically two types of control: single-maximum control and dual-maximum control Single maximum control uses a maximum airflow setpoint that is active in peak cooling mode, and a minimum airflow setpoint that is active both in thermostat deadband mode and in heating mode. As the cooling demand increases around the cooling setpoint, the airflow setpoint linearly ramps up towards the maximum airflow setpoint. Dual-maximum control has a different maximum airflow setpoint in heating mode versus cooling mode, with a lower minimum airflow setpoint in deadband mode. Heating maximum airflow rates and minimum airflow rates should be specified in terms of a percentage of the cooling maximum airflow rate. Take a large sample of VAV airflow setpoints and note the variation in setpoint and how the minimum setpoint fractions change or don’t change as a function of space use type and zone location. The user can choose a simple approach that uses the same airflow setpoint percentages for all zones (average among the sample), or the user can use any patterns determined in the sample to specify different setpoints zone-by-zone. This helps also for providing some diversity in zones from a modeling perspective. The following set of parameters are available to change for each zone, or globally for all VAV boxes:
  • VAV Maximum Heating Airflow Setpoint: (set to 100% for constant speed air terminals and the same as the minimum airflow setpoint for single-maximum VAV control)
  • VAV Minimum Heating Airflow Setpoint: (set to 100% for constant speed air terminals. Set equal to VAV minimum Cooling Airflow setpoint, unless two minimum setpoints are used)
  • VAV Minimum Cooling Airflow Setpoint: (set to 100% for constant speed air terminals. Set equal to VAV minimum Heating Airflow setpoint, unless two minimum setpoints are used)
  • Heating Deviation for Maximum Airflow: This is the deviation from the heating thermostat setpoint associated with the specified maximum heating airflow rate. Default is 0.5 degrees below the heating setpoint.
  • Heating Deviation for Minimum Airflow: This is the deviation from the heating thermostat setpoint associated with the specified heating minimum airflow setpoint. Default is 0.2 degrees above the heating setpoint.
  • Cooling Deviation for Minimum Airflow: This is the deviation from the cooling thermostat setpoint associated with the specified cooling minimum airflow setpoint. Default is 0.2 degrees below the cooling setpoint.
  • Cooling Deviation for Maximum Airflow: This is the deviation from the cooling thermostat setpoint associated with the maximum cooling airflow rate (autosized for the zone). Default is 0.5 degrees above the cooling setpoint.
• Dual-Duct VAV Boxes or Constant Speed boxes: Dual-duct zone mixing boxes can be either VAV or constant speed. Both types of boxes can be modeled with a single framework that considers the airflow setpoints. For constant speed boxes, all airflow setpoints should be set to 100%. Several parameters are used to define Dual-duct boxes, and these include:
  • Maximum Heating Airflow: If the maximum heating airflow rate is larger than the maximum cooling airflow rate, this should be set to 100%, otherwise, this should be set as a percentage of the maximum cooling airflow rate. Set to 100% for constant speed mixing boxes
  • Minimum Airflow Rate: This should be set to 100% for constant speed mixing boxes, or otherwise to a percentage of the larger of the maximum cooler or heating airflow setpoints.
  • Maximum Cooling Airflow: If the maximum cooling airflow rate is larger than the maximum heating airflow rate, this should be set to 100%, otherwise, this should be set as a percentage of the maximum heating airflow rate. Set to 100% for constant speed mixing boxes
  • Ventilation Source: Determine through the upstream AHU configuration which of the two ducts (or both) provide ventilation air via an outdoor air mixing box or 100% outdoor air configuration
  • Ventilation in Deadband: In the event that both the hot and cold duct provide ventilation air, specify which duct should be used when the zone thermostat is in its deadband. This can be determined through investigation of zones in deadband mode and their corresponding hot deck and cold deck VAV damper commands via the BAS
  • Control Strategy: Specify how the control switches from the hot deck to the cold deck and back. Mixing control will gradually mix the two airstreams when appropriate to achieve the desired zone temperature, whereas snap-acting control will use only one duct at a time, unless a minimum fraction from the other duct is required for ventilation.
  • Heating Deviation for Maximum Airflow: This is the deviation from the heating thermostat setpoint associated with the specified maximum heating airflow rate. Default is 1 degree below the heating setpoint.
  • Cooling Deviation for Maximum Airflow: This is the deviation from the cooling thermostat setpoint associated with the maximum cooling airflow rate. Default is 1 degree above the cooling setpoint.