blockEnable

Multi zone VAV AHU economizer enable/disable switch
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Information

This is a multi zone VAV AHU economizer enable/disable sequence based on ASHRAE G36 PART 5.N.7 and PART 5.A.17. Additional conditions included in the sequence are: freeze protection (freeze protection stage 0-3, see PART 5.N.12), supply fan status (on or off, see PART 5.N.5).

The economizer is disabled whenever the outdoor air conditions exceed the economizer high limit setpoint. This sequence allows for all device types listed in ASHRAE 90.1-2013 and Title 24-2013.

In addition, the economizer gets disabled without a delay whenever any of the following is true:

The following state machine chart illustrates the transitions between enabling and disabling:

Image of economizer enable-disable state machine chart

After the disable signal is activated, the following procedure is applied, per PART 5.N.7.d, in order to prevent pressure fluctuations in the HVAC system:

  • The return damper gets fully opened (yRetDamPosMax = uRetDamPhyPosMax and yRetDamPosMin = uRetDamPhyPosMax) for retDamFulOpeTim time period, after which the return damper gets released to its minimum outdoor airflow control position (yRetDamPosMax = uRetDamPosMax and yRetDamPosMin = uRetDamPosMax).
  • The outdoor air damper is closed to its minimum outoor airflow control limit (yOutDamPosMax = uOutDamPosMin) after a disDel time delay.

This sequence also has an overwrite of the damper positions to track a minimum mixed air temperature of TFreSet, which is by default set to 4°C (39.2 F). This is implemented using a proportional controller with a default deadband of 1 K, which can be adjusted using the parameter kPFrePro.

Parameters

TypeNameDefaultDescription
Conditional
Booleanuse_enthalpytrueSet to true to evaluate outdoor air enthalpy in addition to temperature
Advanced › Hysteresis
RealdelTOutHis1Delta between the temperature hysteresis high and low limit
RealdelEntHis1000Delta between the enthalpy hysteresis high and low limits
Advanced › Delays at disable
RealretDamFulOpeTim180Time period to keep RA damper fully open before releasing it for minimum outdoor airflow control at disable to avoid pressure fluctuations
RealdisDel15Short time delay before closing the OA damper at disable to avoid pressure fluctuations

Connectors

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Interfaces.RealInputTOutOutdoor air temperature
Buildings.Controls.OBC.CDL.Interfaces.RealInputhOutOutdoor air enthalpy
Buildings.Controls.OBC.CDL.Interfaces.RealInputTOutCutOA temperature high limit cutoff. For differential dry bulb temeprature condition use return air temperature measurement
Buildings.Controls.OBC.CDL.Interfaces.RealInputhOutCutOA enthalpy high limit cutoff. For differential enthalpy use return air enthalpy measurement
Buildings.Controls.OBC.CDL.Interfaces.RealInputuOutDamPosMinMinimum outdoor air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInputuOutDamPosMaxMaximum outdoor air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInputuRetDamPosMaxMaximum return air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInputuRetDamPosMinMinimum return air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInputuRetDamPhyPosMaxPhysical maximum return air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.BooleanInputuSupFanSupply fan on/off status signal
Buildings.Controls.OBC.CDL.Interfaces.IntegerInputuFreProStaFreeze protection stage status signal
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyOutDamPosMaxMaximum outdoor air damper position
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyRetDamPosMinMinimum return air damper position
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyRetDamPosMaxMaximum return air damper position

Components

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Logical.TrueFalseHoldtruFalHol10 min on/off delay
Buildings.Controls.OBC.CDL.Logical.AndandEnaDisLogical and that checks freeze protection stage and zone state

Revisions

  • October 13, 2017, by Michael Wetter:
    Added freeze protection that tracks mixed air temperature.
  • August 3, 2017, by Michael Wetter:
    Removed unrequired input into block and2 as this input was always true if and2.u2 = true.
  • June 27, 2017, by Milica Grahovac:
    First implementation.