blockEnable

Multi zone VAV AHU economizer enable/disable switch

Information

This is a multi zone VAV AHU economizer enable/disable sequence based on the Section 5.16.7 of the ASHRAE Guideline 36, May 2020. Additional conditions included in the sequence are: freeze protection (freeze protection stage 0-3, see Section 5.16.12), supply fan status (on or off, see Section 5.16.5).

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 supply fan is off (u1SupFan = false),
the freeze protection stage Buildings.Controls.OBC.ASHRAE.G36.Types.FreezeProtectionStages is not stage0.

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, in order to prevent pressure fluctuations in the HVAC system:

The return damper gets fully opened (yRetDam_max = uRetDamPhy_max and yRetDam_min = uRetDamPhy_max) for retDamFulOpeTim time period, after which the return damper gets released to its minimum outdoor airflow control position (yRetDam_max = uRetDam_max and yRetDam_min = uRetDam_max).
The outdoor air damper is closed to its minimum outoor airflow control limit (yOutDam_max = uOutDam_min) after a disDel time delay.

Parameters

Type	Name	Default	Description
Conditional
Boolean	use_enthalpy	true	Set to true to evaluate outdoor air (OA) enthalpy in addition to temperature
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
Advanced › Delays at disable
Real	retDamFulOpeTim	180	Time period to keep RA damper fully open before releasing it for minimum outdoor airflow control at disable to avoid pressure fluctuations
Real	disDel	15	Short time delay before closing the OA damper at disable to avoid pressure fluctuations

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	TOutCut	OA temperature high limit cutoff. For differential dry bulb temperature condition use return air temperature measurement
Buildings.Controls.OBC.CDL.Interfaces.RealInput	hOutCut	OA enthalpy high limit cutoff. For differential enthalpy use return air enthalpy measurement
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uOutDam_min	Minimum outdoor air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uOutDam_max	Maximum outdoor air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uRetDam_max	Maximum return air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uRetDam_min	Minimum return air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInput	uRetDamPhy_max	Physical maximum return air damper position, output from damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.BooleanInput	u1SupFan	Supply fan proven on
Buildings.Controls.OBC.CDL.Interfaces.IntegerInput	uFreProSta	Freeze protection stage status signal
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	yOutDam_max	Maximum outdoor air damper position
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	yRetDam_min	Minimum return air damper position
Buildings.Controls.OBC.CDL.Interfaces.RealOutput	yRetDam_max	Maximum return air damper position

Components

Type	Name	Default	Description
Buildings.Controls.OBC.CDL.Logical.TrueFalseHold	truFalHol		Economizer should not be enabled or disabled within 10 minutes of change
Buildings.Controls.OBC.CDL.Logical.And	andEnaDis		Check 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.