Overview

In this section, you will specify the layout and control of the air delivery and conditioning system encompassing zone return air, outdoor air intake, conditioning, filtering, humidification and fans. This might encompass one of the following systems or interconnected group of systems:

• Dedicated outdoor air system only (likely with zone-level conditioning specified later)
• Single-duct air-handling unit (AHU)
• Rooftop unit (RTU) or packaged split system
• Dual-duct AHU
• Dedicated outdoor air system delivering conditioned outdoor air to the outdoor air intake of single or dual-duct air-handlers

This is one of the most challenging modeling steps because it will likely involve intentional modeling decisions to simplify the building airside systems into a representative composite system that approximates the larger whole. In some cases this may not be possible and two instances of the BRS may be required to separately capture different kinds of predominate air-side systems.

It is recommended that the user perform a qualitative evaluation of the airside systems in the building to determine the prevalence of each type of system, and the most common setpoints and control strategies for those common systems. Ultimately, the various airside systems will have to be represented as a single system, using either the most common control strategy and setpoints, or a compromise strategy that approximates the collective operation of the airside systems in the building.

In-Depth

The specific type/configuration of system you want to model should first be addressed by indicating which components are present. It will not be possible to model every possible airside configuration, however close approximations are likely to be available for most real airside systems. The airside system diagram has a comprehensive set of components, some of which should be specified as present or not present in order to customize the airside system as desired. In order to specify the following sample systems, the following strategy can be employed:

• Dedicated outdoor air system only: The components in the DOAS system section of the airside diagram that are present in the actual DOAS system should be specified as present. Heat recovery is available for DOAS systems, but must be used upstream of all other components in the DOAS. All other components in the main AHU section and in the hot deck section should be specified as not present
• Single-duct AHUs and RTUs: All components in the DOAS system section should be specified as not present, with the possible exception of the heating coil, which can be used if there is an outdoor air preheat coil present. The components in the main AHU section should be specified in the correct order, noting that the supply fan has a single option for placement at the discharge side of the AHU. All hot deck components should be specified as ‘not present’.
• Dual-duct AHU: All components in the DOAS system section should be specified as not present, with the possible exception of the heating coil, which can be used if there is an outdoor air preheat coil present. All hot deck components should be specified as ‘present’ as well.
• Dedicated outdoor air system delivering conditioned outdoor air to the outdoor air intake of single or dual-duct air-handlers: The components in the DOAS system section of the airside diagram that are present in the actual DOAS system should be specified as present. The components in the main AHU section should be specified in the correct order, noting that the supply fan has a single option for placement at the discharge side of the AHU. All hot deck components should be specified as ‘present’ as well. Care should be taken in choosing an appropriate minimum outdoor air flow rate that captures the normal operating setpoint or sizing of the DOAS system. The economizer will need to be specified as ‘No economizer’.

Note that in addition to the visible components, up to two humidifiers can be specified; one within the DOAS section and one within the main AHU section. There are three available locations for each humidifier.

Once the appropriate components are specified, the next step is to define the operation of each component. Each component has a specific set of parameters that define its operation:

Heating and Cooling Coils:

• Setpoint type: Options are ‘constant’, ‘mixed air low limit’ (for DOAS section heating coils), and ‘maintain SAT setpoint’ (or ‘maintain DOAS SAT setpoint’).
  • ‘Constant’ setpoint operates the coil to maintain a constant temperature setpoint immediately downstream of the coil
  • ‘mixed air low limit’ operates a heating coil in the outdoor air section to maintain a temperature setpoint at the outlet of the mixing box.
  • ‘maintain DOAS SAT setpoint’ will control the coil to maintain the conditioned air setpoint at the discharge of the DOAS section. This setpoint control compensates for the operation of any downstream components such as fans (which create an associated air temperature rise during operation) and other coils (which may be leaking).
  • ‘maintain SAT setpoint: will control the coil to maintain the supply air temperature setpoint at the discharge of the main AHU/cold deck section. This setpoint control compensates for the operation of any downstream components such as fans (which create an associated air temperature rise during operation) and other coils (which may be leaking).
• Leaking coil: This is an optional feature that allows the user to approximate a leaking coil fault. The leaking coil accepts a constant temperature rise (positive value) for heating coils and constant temperature drop (negative value) for cooling coils. This temperature rise or drop will occur in the AHU section whenever the associated heating or cooling system is available and ON to serve other heating and cooling loads. If the user is modeling a specific airside system, BAS graphics can be used to estimate temperature differences across the coil when the valve is closed. If the user is modeling the whole building, a typical vale for leakage should be used.

Supply Fans (DOAS, main AHU and Hot Deck):

• Fan temperature rise: This is a constant temperature rise when the fan is running, and is typically in the range of 1-5°F. The fan temperature rise is a function of the pressure rise across the fan, the motor efficiency and the motor’s location in or outside of the airstream; it is relatively independent of airflow rate. An estimate of fan temperature rise can be obtained from BAS graphics, noting temperature sensor readings before and after the fan, or from trend data of those sensors. A suitable modeling choice, if unknown is 2°F.
• Fan efficiency: This is the efficiency of the fan in converting electric energy to the kinetic energy of the moving air. This variable is presented as a qualitative choice for the user based on the observable state of the fan. The default is “medium”, with “high” and “low” as alternate choices. Observe the apparent age and condition of the fans. Direct-drive fans are likely to be higher efficiency, but pay attention to noise and vibration. Belt-driven fans are more likely to be lower efficiency, particularly if the belts are loose or slipping or if there is significant vibration.
• Fan motor in airstream: For direct-drive fans, the motor is located in the airstream. Belt-driven fans have an external motor outside of the AHU section, in the mechanical room.
• Observed static pressure: This field is intended to be obtained from the BAS graphics for a sample air-handler at the same time that the fan temperature rise is observed. It is used to predict changes in fan temperature rise that accompany changes in duct static pressure.
• Total fan pressure rise: This is the total pressure rise that the fan must overcome, and includes the contributions to pressure rise from upstream components (heating coil, cooling coil, filter, return duct) as well as downstream components, which are controlled to a duct static pressure setpoint. The total pressure rise can be approximated as ½” w.c. for the return duct, plus ½” w.c. for each coil and ½” w.c. for each filter, plus the duct static pressure setpoint.

Return Fan:

• Control Type: Two control types are available for the return fan: “Constant offset”, which uses a fixed differential from the supply fan speed to control the return fan speed, or “offset reset with pressure”, which dynamically adjusts the return fan speed to attempt to maintain the building static pressure at a desired setpoint. This reset is linear, resetting between a maximum offset (in percentage terms, of return fan speed relative to supply fan speed) at minimum building pressure and a maximum offset at maximum building pressure.

Mixing Box:

• Minimum Outdoor Air Control type: Options include “min OA CFM”, which would apply to airflow control of the minimum outdoor air to meet a setpoint, “min OA fraction” which would be applied when there is no airflow sensor and the minimum airflow is based on a fixed damper position, “min OA fraction of design airflow” will control to a fixed airflow setpoint as a function of the design size of the air-handler. This is a good choice for simulating an entire building that uses airflow control. If unknown, a good starting value is 10%. A final choice is “DCV” for demand control ventilation, if that control strategy is used. Typically this will reset from a minimum damper command or airflow setpoint at a low CO₂ value resetting to higher outdoor airflow setpoints or damper commands at higher values. For constant speed systems, select “minimum outdoor air fraction” and choose 100%
• Minimum Outdoor Air Setpoint: This setpoint should be set to correspond to the choice made for the minimum outdoor air control type. If the control type is “min OA fraction” or “min OA fraction of design airflow”, a fractional value is expected for the setpoint. If the control type is “min OA CFM”, the total cfm setpoint for the represented fan system(s) is expected. If the control type is DCV, this field is not use.
• Economizer: Economizer control options include “No Economizer”, in which case, the minimum outdoor air setpoint is utilized at all times, “Differential Dry Bulb”, which activates the economizer for free cooling when the outdoor air temperature is cooler than the return air temperature”, “Differential Enthalpy” which activates the economizer when the outdoor air enthalpy is below the return air enthalpy, “Fixed dry bulb”, which activates the economizer when the outdoor air temperature is below a fixed economizer lockout threshold, and “Fixed Enthalpy”, which activates the economizer when the outdoor air enthalpy is below a fixed lockout threshold.
• Economizer high and low dry bulb temperature lockouts: Whether or not the economizer uses a fixed dry bulb strategy, an additional layer of lockouts on the economizer based on outdoor dry bulb can be added. A high dry bulb lockout prevents the use of the economizer when outdoor air temperatures are above the lockout setpoint and a low dry bulb lockout prevents the use of an economizer when the outdoor air temperatures are below the lockout setpoint.

Humidifiers:

• Fuel Source: Two fuel sources are available: electric or steam. Misting or ultrasonic humidification is not available, as there are typically health concerns with the use of these kinds of humidifiers in air-handlers.
• Humidifier location: There are 3 locations available for the use of a humidifier within the DOAS section of the AHU system (before the coils, after the coils, and before the mixing box). Likewise, there are 3 locations available for a humidifier within the AHU section (before the coils, after the coils, and after the return fan).
• Humidity sensor location: There are two options for the location of the sensor used to control the humidifier; in the return air plenum, or immediately downstream of the humidifier.
• Relative humidity setpoint: This is the setpoint that the humidifier is trying to maintain.

Several additional controls are available for the AHU system, including Optimal Start, Outdoor Air Temperature-based lockouts on heating and cooling, Supply air temperature control/reset and Duct static pressure control/reset.

• Optimal Start: Optimal start is available to dynamically choose a custom start-up time each morning based on a desired ‘latest start time’ on weekdays, Saturdays and Sundays. A maximum early start parameter defines the maximum number of hours, relative to the latest start time each day that the AHU can start up. The final parameter is an assumed recovery rate, or in other words, the rate of change of zone temperatures per hour (higher in the winter and colder in the summer) when the AHU starts up. 1.5-2°F is a good starting assumption.
• Outdoor Air Temperature-based lockouts on heating and cooling: A global setpoint can be defined for all building heating coils and cooling coils (not just in the AHU system, but also for all zone equipment) that will lock them out based on the outdoor air temperature. A cooling coil lockout setpoint will lockout the cooling coils when the outdoor air temperature falls below the lockout setpoint. The heating coil lockout setpoint will lock out the heating coils above the lockout setpoint.
• Supply air temperature control/reset. Supply air temperature control can be defined in three places, depending on the presence of components in the respective sections: At the conditioned air outlet of the DOAS section, at the discharge of the main AHU (single duct) or cold deck (dual-duct configuration), and the discharge of the hot deck (dual-duct configurations). Supply air temperature can be controlled to a fixed setpoint, or it can be reset using either a linear reset between user-defined minimum and maximum setpoints based on a user-defined range for a selected feedback variable (sensor), or a trim-and-respond logic can be used, in which a desired value for a feedback variable (sensor) is defined, and the supply air temperature is updated at each timestep to attempt to target that desired value. Parameters for linear resets include:
  • Feedback variable, e.g. outdoor air temperature, zone heating or cooling demand (0-100 value based on deviation from setpoint), fan speed, average zone damper, maximum zone damper, etc.
  • Minimum value for feedback variable
  • Setpoint at minimum value of feedback variable (if the feedback variable goes lower than the minimum, the setpoint will remain at this limit)
  • Maximum value for feedback variable
  • Setpoint at maximum value of feedback variable (if the feedback variable goes higher than the maximum, the setpoint will remain at this limit)
  Parameters for trim-and-respond resets include:
  • Feedback variable maintenance setpoint
  • Change rate: this is the maximum rate of change per hour in the setpoint based on deviation of the feedback variable from its maintenance setpoint.
  • Minimum SAT: This is the minimum limit for the supply air temperature setpoint
  • Maximum SAT: This is the maximum limit for the supply air temperature setpoint
  • Variable SAT limits: This option can be used to dynamically adjust the minimum and maximum SAT setpoints based on outdoor air temperature. Both setpoints can be reset linearly. For example, the minimum AHU SAT setpoint could be set to 60°F at 30°F outdoor air temperature and reset down to 55°F at 60°F outdoor air temperature. This could prevent excessively cool SAT setpoints during cold weather. A similar strategy can be used for the maximum AHU SAT setpoint to prevent excessively warm SAT setpoints during hot weather.
• Duct static pressure control/reset. Duct static pressure control can be defined in two places, depending on the presence of components in the respective sections: at the discharge of the main AHU (single duct) or cold deck (dual-duct configuration), and the discharge of the hot deck (dual-duct configurations). Duct static pressure can be controlled to a fixed setpoint, can be scheduled to reset based on time of day, or can be reset using either a linear reset between user-defined minimum and maximum setpoints based on a user-defined range for a selected feedback variable (sensor), or a trim-and-respond logic can be used, in which a desired value for a feedback variable (sensor) is defined, and the duct static pressure setpoint is updated at each timestep to attempt to target that desired value. Parameters for linear resets include
  • Feedback variable, e.g. outdoor air temperature, zone heating or cooling demand (0-100 value based on deviation from setpoint), fan speed, average zone damper, maximum zone damper, etc.
  • Minimum value for feedback variable
  • Setpoint at minimum value of feedback variable (if the feedback variable goes lower than the minimum, the setpoint will remain at this limit)
  • Maximum value for feedback variable
  • Setpoint at maximum value of feedback variable (if the feedback variable goes higher than the maximum, the setpoint will remain at this limit)
  Parameters for trim-and-respond resets include
  • Feedback variable maintenance setpoint
  • Change rate: this is the maximum rate of change per hour in the setpoint based on deviation of the feedback variable from its maintenance setpoint.
  • Minimum SAT: This is the minimum limit for the supply air temperature setpoint
  • Maximum SAT: This is the maximum limit for the supply air temperature setpoint