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

In-Depth

This module provides a table that should be completed by the user to define zone thermostat controls as well as the VAV box (or other airflow delivery) controls. 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 AHU operation schedule, set in the ‘Define Building Schedules’ module. The AHU 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. This can be defined by clicking the humidity setback check-box at the top of the module.

Thermostat setpoints are defined on the left side of the table. The top row of the table allows the user to define a set of global setpoints for thermostats. Once global setpoints are defined, entries for defined global values for the rows below (each individual zone) become optional. Entries for individual zones can be made as exceptions to the global setpoints or left blank. If the global setpoints are not defined, however, an entry is expected for each thermostat setpoint for each zone in the table.

The left side of the table deals with airflow sizing and control at the zone level. 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. For single-duct AHUs and DOAS systems, there are five fields that need to be defined globally and/or for each zone. They are

• VAV Airflow Sizing Factor (1 indicates properly sized to meet the top 99.5% of zone cooling loads). Airflow can be oversized or undersized to each zone in order to capture observed dynamics. For example, a zone that has a damper normally open to 100% can be replicated through under-sizing the airflow, and vice versa
• Heating Capacity Sizing Factor (1 indicates properly sized heating coils to meet zone heating loads). Undersized or oversized heating capacity will affect recovery time for the zone to catch up to its occupied heating setpoints after a setback period. Greatly undersized heating capacities can be used to mimic zones that are underheated.
• Minimum Airflow fraction (cooling): The airflow setpoint in the thermostat deadband during cooling mode; as a fraction of the design cooling airflow
• Minimum Airflor fraction (heating): The airflow setpoint in the thermostat deadband during heating mode; as a fraction of the design cooling airflow
• VAV Maximum Heating Airflow Fraction: The maximum airflow setpoint in heating mode as a fraction of the design cooling airflow. For dual duct systems, 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

If dual duct systems are selected, only a single minimum airflow fraction is needed, and an additional filed of VAV maximum cooling airflow fraction is required. 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

For dual duct systems, the following 3 fields are needed to specify dual duct design and control:

• Ventilation Source: Do both ducts receive fresh outdoor air, or only on or the other
• Ventilation in Deadband: Which duct is used to supply ventilation air when the thermostat is in its deadband between the heating and cooling setpoint?
• Control Strategy: There are two choices; snap acting, where only one duct is permitted to be used at once and a “snap” from hot deck to cold deck and vice versa occurs as the thermal demands of the zone change. The other option is “mixing”, where there is a smoother transition from the use of one duct to the other with a mixing of air from the two ducts in that transition zone.

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. Note that this only applies to some form of supplemental cooling coil at the zone level as opposed to the cooling that is performed as a result of air delivery from the AHU.
• 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.