blockEnable

Single zone VAV AHU economizer enable/disable switch

Information

This is a single zone VAV AHU economizer enable/disable sequence based on ASHRAE G36 PART 5.5 and PART 5.A.17. Additional conditions included in the sequence are:

Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.Types.FreezeProtectionStages (PART 5.9),
Supply fan status TSupFan (PART 5.4.d),
Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.Types.ZoneStates (PART 5.3.b).

The economizer is disabled whenever the outdoor air conditions exceed the economizer high limit cutoff setpoint plus the hysteresis low limit parameter (TOutHigLimCutLow or hOutHigLimCutLow) as shown in the figure. And the economizer is enabled whenever the outdoor air conditions are below the economizer high limit cutoff setpoint plus the hysteresis high limit parameter (TOutHigLimCutHig or hOutHigLimCutHig). This sequence allows for all device types listed in ASHRAE 90.1-2013 and Title 24-2013.

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

supply fan is off,
zone state is heating,
freeze protection stage is not 0.

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

Image of economizer enable-disable state machine chart

Parameters

Type	Name	Default	Description
Conditional
Boolean	use_enthalpy	true	Set to true to evaluate outdoor air (OA) enthalpy in addition to temperature
Boolean	use_fixed_plus_differential_drybulb	false	Set to true to evaluate fixed plus differential dry bulb temperature high limit cutoff; shall not be used with enthalpy
Advanced › Hysteresis
Real	delTOutHis	1	Delta between the temperature hysteresis high and low limit
Real	delEntHis	1000	Delta between the enthalpy hysteresis high and low limits
Commissioning › Physical damper position limits
Real	retDamPhyPosMax	1	Physically fixed maximum position of the return air damper
Real	retDamPhyPosMin	0	Physically fixed minimum position of the return air damper

Connectors

Type	Name	Description
Buildings.Controls.OBC.CDL.Interfaces.RealInput	TOut	Outdoor air temperature
Buildings.Controls.OBC.CDL.Interfaces.RealInput	hOut	Outdoor air enthalpy
Buildings.Controls.OBC.CDL.Interfaces.RealInput	TCut	OA temperature high limit cutoff. For differential dry bulb temeprature condition use return air temperature measurement
Buildings.Controls.OBC.CDL.Interfaces.RealInput	TRet	Used only for fixed plus differential dry bulb temperature high limit cutoff
Buildings.Controls.OBC.CDL.Interfaces.RealInput	hCut	OA enthalpy high limit cutoff. For differential enthalpy use return air enthalpy measurement
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uOutDamPosMin	Minimum outdoor air damper position, get from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uOutDamPosMax	Maximum outdoor air damper position, get from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.BooleanInput	uSupFan	Supply fan on/off status signal
Buildings.Controls.OBC.CDL.Interfaces.IntegerInput	uZonSta	Zone state status signal
Buildings.Controls.OBC.CDL.Interfaces.IntegerInput	uFreProSta	Freeze protection stage status signal
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	yOutDamPosMax	Maximum outdoor air damper position
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	yRetDamPosMin	Minimum return air damper position
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	yRetDamPosMax	Maximum return air damper position

Components

Type	Name	Description
Buildings.Obsolete.Controls.OBC.CDL.Logical.And3	andEnaDis	Logical and that checks freeze protection stage and zone state
Buildings.Controls.OBC.CDL.Logical.TrueFalseHold	truFalHol	10 min on/off delay
Buildings.Controls.OBC.CDL.Logical.Xor	xor	Either fixed+differential temperature cutoff or others

Revisions

July 30, 2019, by Kun Zhang:
Added the option to allow fixed plus differential dry bulb temperature cutoff.
July 06, 2017, by Milica Grahovac:
First implementation.