Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.SetPoints

Output setpoints for AHU control

Information

This package contains sequences generating setpoints for VAV AHU control.

Package Content

Name	Description
ExhaustDamper	Control of actuated exhaust air dampers without fans
OutsideAirFlow	Output the minimum outdoor airflow rate setpoint for systems with multiple zones
ReturnFanDirectPressure	Return fan control with direct building pressure control
VAVSupplyFan	Block to control multi zone VAV AHU supply fan
VAVSupplySignals	Multizone VAV AHU coil valve positions
VAVSupplyTemperature	Supply air temperature setpoint for multi zone system
Validation	Collection of validation models

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.SetPoints.ExhaustDamper

Control of actuated exhaust air dampers without fans

Information

Control sequence for actuated exhaust damper yExhDam without fans. It is implemented according to ASHRAE Guidline 35 (G36), PART5.N.8. (for multi zone VAV AHU), PART5.P.6 and PART3.2B.3 (for single zone VAV AHU).

Multi zone VAV AHU: Control of actuated exhaust dampers without fans (PART5.N.8)

1. The exhaust damper is enabled when the associated supply fan is proven on uFan = true, and disabled otherwise.
2. When enabled, a P-only control loop modulates the exhaust damper to maintain a building static pressure of dpBui, which is by default 12 Pa (0.05 inchWC).
3. When uFan = false, the damper is closed.

Parameters

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.SetPoints.OutsideAirFlow

Output the minimum outdoor airflow rate setpoint for systems with multiple zones

Information

This atomic sequence sets the minimum outdoor airflow setpoint for compliance with the ventilation rate procedure of ASHRAE 62.1-2013. The implementation is according to ASHRAE Guidline 36 (G36), PART5.N.3.a, PART5.B.2.b, PART3.1-D.2.a. The calculation is done using the steps below.

Step 1: Minimum breathing zone outdoor airflow required breZon

• The area component of the breathing zone outdoor airflow: breZonAre = AFlo*outAirPerAre.
• The population component of the breathing zone outdoor airflow: breZonPop = occCou*outAirPerPer.

The number of occupant occCou in each zone can be retrieved directly from occupancy sensor nOcc if the sensor exists (have_occSen=true), or using the default occupant density occDen to find it AFlo*occDen. The occupant density can be found from Table 6.2.2.1 in ASHRAE Standard 62.1-2013. For design purpose, use design zone population desZonPop to find out the minimum requirement at the ventilation-design condition.

Step 2: Zone air-distribution effectiveness zonDisEff

Table 6.2.2.2 in ASHRAE 62.1-2013 lists some typical values for setting the effectiveness. Depending on difference between zone space temperature TZon and discharge air temperature (after the reheat coil) TDis, Warm-air effectiveness zonDisEffHea or Cool-air effectiveness zonDisEffCoo should be applied.

Step 3: Minimum required zone outdoor airflow zonOutAirRate

For each zone in any mode other than occupied mode and for zones that have window switches and the window is open, zonOutAirRate shall be zero. Otherwise, the required zone outdoor airflow zonOutAirRate shall be calculated as follows:

• If discharge air temperature at the terminal unit is less than or equal to zone space temperature: zonOutAirRate = (breZonAre+breZonPop)/disEffCoo.
• If discharge air temperature at the terminal unit is greater than zone space temperature: zonOutAirRate = (breZonAre+breZonPop)/disEffHea

• If discharge air temperature at the terminal unit is less than or equal to zone space temperature: zonOutAirRate = breZonAre/disEffCoo
• If discharge air temperature at the terminal unit is greater than zone space temperature: zonOutAirRate = breZonAre/disEffHea

Step 4: Outdoor air fraction for each zone priOutAirFra

    priOutAirFra = zonOutAirRate/VBox_flow

    desZonPriOutAirRate = desZonOutAirRate/minZonFlo

Step 5: Occupancy diversity fractionoccDivFra

    occDivFra = peaSysPopulation/sum(desZonPopulation)

Step 6: Uncorrected outdoor airflow unCorOutAirInk, sysUncOutAir

    unCorOutAirInk = occDivFra*sum(breZonPop)+sum(breZonAre)

Step 7: System primary airflow sysPriAirRate

Step 8: Outdoor air fraction

    outAirFra = sysUncOutAir/sysPriAirRate

    aveOutAirFra = unCorOutAirInk/maxSysPriFlow

Step 9: Zone ventilation efficiency zonVenEff (for design purpose)

    zonVenEff[i] = 1 + aveOutAirFra + desZonPriOutAirRate[i]

Step 10: System ventilation efficiency

    sysVenEff = 1 + outAirFra + MAX(priOutAirFra[i])

    desSysVenEff = MIN(zonVenEff[i])

Step 11: Minimum required system outdoor air intake flow

    effMinOutAirInt = MIN(sysUncOutAir/sysVenEff, desOutAirInt)

    desOutAirInt = unCorOutAirInk/desSysVenEff

References

ANSI/ASHRAE Standard 62.1-2013, Ventilation for Acceptable Indoor Air Quality.

Stanke, D., 2010. Dynamic Reset for Multiple-Zone Systems. ASHRAE Journal, March 2010.

Parameters

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.SetPoints.ReturnFanDirectPressure

Return fan control with direct building pressure control

Information

Setpoint for return fan discharge pressure and exhaust air damper for a multi zone VAV AHU according to ASHRAE guideline G36, PART5.N.10 (return fan control with direct building pressure).

1. Return fan operates whenever associated supply fan is proven on and is off otherwise.

2. Return fan is controlled to maintain return fan discharge static pressure at setpoint dpBuiSet.

3. Exhaust damper is only enabled when the associated supply and return fans are proven on (uFan=true) and the minimum outdoor air damper is open (to be controlled in a separate sequence). The exhaust dampers is closed when the fan is disabled.

4. The building static pressure is time averaged with a sliding 5-minute window to dampen fluctuations. The averaged value shall be displayed and is used for control.

5. When the exhaust damper is enabled, a control loop modulates the exhaust damper in sequence with the return fan static pressure setpoint as shown in the figure below to maintain the building pressure equal to dpBuiSet, which is by default 12 Pa (0.05 inches).

The output signal of the building pressure control is as follows:

1. From 0 to 0.5, the building pressure control loop modulates the exhaust dampers from yExhDam = 0 (closed) to yExhDam = 1 (open).
2. From 0.5 to 1, the building pressure control loop resets the return fan discharge static pressure setpoint from dpDisMin to dpDisMax. The dpDisMin and dpDisMax are specified in Section G36 PART 3.2A.3.b.

Parameters

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.SetPoints.VAVSupplyFan

Block to control multi zone VAV AHU supply fan

Information

Supply fan control for a multi zone VAV AHU according to ASHRAE guideline G36, PART5.N.1 (Supply fan control).

Supply fan start/stop

• Supply fan shall run when system is in the Cool-down, Setup, or Occupied mode
• If there are any VAV-reheat boxes on perimeter zones, supply fan shall also run when system is in Setback or Warmup mode;
• If the AHU does not serve dual duct boxes (have_duaDucBox=true) or the AHU does not have airflow measurement station (have_airFloMeaSta=false), sum the current airflow rate from the VAV boxes and output to a software point.

Static pressure setpoint reset

Static pressure setpoint shall be reset using trim-respond logic using following parameters as a starting point:

Static pressure control

Supply fan speed is controlled with a PI controller to maintain duct static pressure at setpoint when the fan is proven on. The setpoint for the PI controller and the measured duct static pressure are normalized with the maximum design static presssure maxSet. Where the zone groups served by the system are small, provide multiple sets of gains that are used in the control loop as a function of a load indicator (such as supply fan airflow rate, the area of the zone groups that are occupied, etc.).

Parameters

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.SetPoints.VAVSupplySignals

Multizone VAV AHU coil valve positions

Information

Block that outputs the supply temperature control loop signal, and the coil valve postions for VAV system with multiple zones, implemented according to the ASHRAE Guideline G36, PART5.N.2 (Supply air temperature control).

The supply air temperature control loop signal uTSup is computed using a PI controller that tracks the supply air temperature setpoint TSetSup. If the fan is off, then uTSup = 0.

Heating valve control signal (or modulating electric heating coil if applicable) yHea and cooling valve control signal yCoo are sequenced based on the supply air temperature control loop signal uTSup. From uTSup = uHeaMax to uTSup = -1, yHea increases linearly from 0 to 1. Similarly, uTSup = uCooMin to uTSup = +1, yCoo increases linearly from 0 to 1.

The output uTSup can be used in a controller for the economizer.

Parameters

Connectors

Modelica definition

Buildings.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.SetPoints.VAVSupplyTemperature

Supply air temperature setpoint for multi zone system

Information

Block that outputs the supply air temperature setpoint and the coil valve control inputs for VAV system with multiple zones, implemented according to the ASHRAE Guideline G36, PART5.N.2 (Supply air temperature control).

The control loop is enabled when the supply air fan uSupFan is proven on, and disabled and the output set to Deadband otherwise.

The supply air temperature setpoint is computed as follows.

Setpoints for TSupMin, TSupMax, TSupDes, TOutMin, TOutMax

The default range of outdoor air temperature (TOutMin=16°C, TOutMax=21°C) used to reset the occupied mode TSetSup was chosen to maximize economizer hours. It may be preferable to use a lower range of outdoor air temperature (e.g. TOutMin=13°C, TOutMax=18°C) to minimize fan energy.

The TSupMin variable is used during warm weather when little reheat is expected to minimize fan energy. It should not be set too low or it may cause excessive chilled water temperature reset requests which will reduce chiller plant efficiency. It should be set no lower than the design coil leaving air temperature.

The TSupMax variable is typically 18 °C in mild and dry climate, 16 °C or lower in humid climates. It should not typically be greater than 18 °C since this may lead to excessive fan energy that can offset the mechanical cooling savings from economizer operation.

During occupied mode (uOpeMod=1)

The TSetSup shall be reset from TSupMin when the outdoor air temperature is TOutMax and above, proportionally up to TMax when the outdoor air temperature is TOutMin and below. The TMax shall be reset using trim and respond logic between TSupDes and TSupMax. Parameters suggested for the trim and respond logic are shown in the table below. They require adjustment during the commissioning and tuning phase.

During Setup and Cool-down modes (uOpeMod=2, uOpeMod=3)

Supply air temperature setpoint TSetSup shall be TSupMin.

During Setback and Warmup modes (uOpeMod=4, uOpeMod=5)

Supply air temperature setpoint TSetSup shall be 35°C.

Valves control

Supply air temperature shall be controlled to setpoint using a control loop whose output is mapped to sequence the hot water valve or modulating electric heating coil (if applicable) or chilled water valves.

Parameters

Connectors

Modelica definition