Outside air damper, ventilation and mixed air temperature control (DmpOa13)

This application function operates an outside air damper.

The request signal for cooling (from the room controller) is transformed into mixed air temperature setpoint and is provided to its PID controller for cascaded mixed air temperature control. The damper position results from the output of the PID-controller.

There is an additional "economizer" functionality (free cooling when available).

Required elements and how to configure them:

Element	I/O	Signal type
DmpOaPos "Outside air damper position"	On-board output, Outside air damper position	0…10V or 3-position
TMx "Mixed air temperature"	On-board input, Mixed air temperature	any signal type

Function

The figure below shows BACnet objects associated with this application function. Primary signal flow is summarized as follows:

The request signal(s) are received (DmpOaVntReq / DmpOaCReq) and processed into the output signal for outside air damper position (DmpOaPos).

Basic function: Accept request signal from associated room controller and pass it through a device mode logic switch; Output the result as a command to the BACnet object that controls the device.

	Command or request (or related)
	Notification of condition or status, or availability
	Device mode
	Interlock (internal signal, not a BACnet object; see Interlocks section for additional information)

Interlocks

Room automation interlocks are internal signals that coordinate interaction between HVAC devices. They are not visible in ABT Site or web interface, but parameters associated with them are. See comment column for hints on parameters that affect interlock functionality.

Signal	Type	Direction	Description	Comment
AirFlSta	Boolean	In	Air flow status ▶ Interlock signal (from fan) that keeps the OA damper closed at equipment protection priority (PrPrio = 5) unless the fan is running. (not active unless EnMonAirFlSta = Yes)	Parameter EnMonAirFlSta must = Yes. See Configuration section for additional information.
AirFlCReq	Boolean	Out	Air flow cooling request ▶ Initiates air flow (fan turns on) to satisfy air flow support request for cooling or dehumidification. (See Economizer section for additional information.)	See Configuration section for parameters named "...AirFlCReq".
AirFlDhuReq	Boolean	In	Air flow dehumidification request ▶ Input signal from a cooling coil that needs air flow in order to dehumidify.	See "DmpOaPosDhu" in Configuration section.
AirFlVntReq	Boolean	Out	Air flow ventilation request ▶ Initiates air flow (fan turns on) to satisfy ventilation request. (See Ventilation section for additional information.)	See Configuration section for parameters named "...AirFlVntReq".

Economizer control

The economizer is enabled (DmpOaEcmSta = On) when the outside air is cold enough for cooling. The switch-on point is Room Temp minus (DiffTRTOaMinC + hysteresis).
Note
Configured value for DiffTRTOaMinC (default = 0) must be entered as a negative number. Hysteresis = 1K.

If AirFlDhuReq is active and dehumidification is occurring, the economizer is disabled (DmpOaEcmSta = Off) and the OA damper is placed at a configurable minimum position (DmpOaPosDhu) that results in more humid air from the zone being recycled through the cooling coil to remove excess humidity (see Configuration).

Mixed air temperature control

This AF maps the demand signal from the room temperature controller to a mixed air temperature setpoint, and performs a control loop to drive the mixed air temperature to setpoint. If the mixed air temperature sensor is not valid, temperature control is performed by the room temperature PID.

Equipment output limits

The maximum damper position is implemented using a reset function with configurable parameters:

X1TOa (default 6°C) and Y1DmpPos (default 20%)

X2TOa (default 18°C) and Y2DmpPos (default 100%)

When the outside air temperature is X1TOa, the maximum damper position is Y1DmpPos.
When the outside air temperature is X2TOa, the maximum damper position is Y2DmpPos.
These values can be adjusted depending on the internal loads of the space.

Ramp-up feature - A configurable rate limit prevents the damper from opening too fast if the air is extremely cold. It does not restrict closing, or manual commands.

The ramp-up function is enabled when the outside air temperature is less than the switch-on point for ramp-up damper (SwiOnPtRup) or the outside air temperature sensor is invalid.

By default, the value is set several degrees above the freezing point. User will typically not adjust it. This feature allows the damper setting to open to any required level, but it happens slowly enough that other functions in the system have a chance to respond to the cold air and prevent damage. In some installations, a low temperature detector shuts down the system to avoid freezing a coil. This rate limit avoids the inconvenience of shutting the system down.

Cooling available status - DmpOaAvlC is True if:

Device mode (DmpOaDevMod) is Control mode and Economizer is enabled (DmpOaEcmSta is True) and the damper position is open for cooling.

Ventilation available status - DmpOaAvlVnt is True if:

Device mode (DmpOaDevMod) is Control mode and the damper position is open for ventilation.

Configuration

Objects

Description	Object	Type	Default value
Outside air damper position for dehumidification ▶ When dehumidification is required, set the Outside air damper position to a value that is at least equal to minimum ventilation position (e.g., 20%). Default command must be configured on priority level 17.	DmpOaPosDhu	APrcVal	0 [%]
Mixed air temperature setpoint for cooling ▶ When cooling is required, setpoint for the PID controller for mixed air temperature control.	SpTMxC	ACnfVal	12 [°C] 53.6 [°F]

Parameters

Description	Parameter	Default value
Economizer settings
Minimum difference room temperature/outside air temperature for cooling ▶ The differential between room temperature and outside air temperature used to calculate the economizer switch-off point.	DiffTRTOaMinC	0 [K] 0.0 [°F]
Output limitations
Output limitation characteristics for outside air temperature X1 ▶ Minimum outside air temperature that provides cooling from cold outside air that resets the outdoor air damper position to minimum position (i.e., several degrees above the freezing point).	X1LmOutTOa	6 [°C] 42.8 [°F]
Output limitation characteristics for damper position Y1 ▶ Minimum outside air damper position (in percent) that provides cooling when the outside air temperature is at minimum (X1LmOutTOa).	Y1LmOutDmpPos	20 [%]
Output limitation characteristics for outside air temperature X2 ▶ Maximum outside air temperature that provides cooling from the outside air that resets the outdoor air damper position to maximum position (i.e., full open).	X2LmOutTOa	18 [°C] 64.4 [°F]
Output limitation characteristics for damper position Y2 ▶ Maximum outside air damper position (in percent) that provides cooling when the outside air temperature is at maximum (X2LmOutTOa).	Y2LmOutDmpPos	100 [%]
Interlock
Enable monitoring for air volume flow state ▶ Enable monitoring of the fan interlock signal (from fan) that keeps the OA damper closed unless the fan is running. 0:No 1:Yes	EnMonAirFlSta	0:No
Switch-on point for air volume flow cooling request ▶ When the outside air damper cooling request (in percent) is above this value, the air volume flow cooling request goes to True.	SwiOnAirFlCReq	4 [%]
Hysteresis for air volume flow cooling request ▶ When the outside air damper cooling request (in percent) is below this value, the air volume flow cooling request goes to False.	HysAirFlCReq	2 [%]
Switch-on delay for air volume flow cooling request ▶ The time after the switch-on point for air volume flow cooling request is True that air volume flow cooling request goes from False to True.	DlyOnAirFlCReq	30 [s]
Switch-on point for air volume flow ventilation request ▶ When the outside air damper ventilation request (in percent) above which the air volume flow ventilation request goes to True.	SwiOnAflVntReq	4 [%]
Hysteresis for air volume flow ventilation request ▶ When the outside air damper ventilation request (in percent) below which the air volume flow ventilation request goes to False.	HysAirFlVntReq	2 [%]
Switch-on delay for air volume flow ventilation request ▶ The time after the switch-on point for air volume flow ventilation request is True that air volume flow ventilation request goes from False to True.	DlyOnAflVntReq	30 [s]
Ramp-up function
Switch-on point for ramp-up function ▶ When the outside air temperature falls below the switch-on point, the outdoor air damper ramp-up function is activated. (i.e., several degrees above the freezing point).	SwiOnPtRup	5 [°C] 41.0 [°F]
Ramp-up time for outside air damper ▶ The time it takes the outside air damper to slowly open in cold weather to the required level.	TiRupDmpOa	2 [min]
Internal settings – do not change
Temperature unit ▶ Defined by selected engineering unit - do not change.	TUnit	[°C] [°F]
Temperature difference unit ▶ Defined by selected engineering unit - do not change.	TDiffUnit	[K] [°F]

Interface

Interface	Description	Type	Ref.	Owned by
DmpOaPos	Outside air damper position	AO	◉	Room segment / Device
DmpOaDevMod	Outside air damper device mode 1:Off 2:Control mode 3:Fully open 4:Maximum allowed	MPrcVal	●	-
DmpOaTMxCtrC	Mixed air temperature controller cooling for outside air damper	Controller	●	-
DmpOaEcmSta	Outside air damper economizer state 0:Off 1:On	BPrcVal	●	-
DmpOaCReq	Outside air damper cooling request	ACalcVal	●	-
DmpOaVntReq	Outside air damper ventilation request	ACalcVal	●	-
DmpOaAvlC	Outside air damper available for cooling 0:No 1:Yes	BCalcVal	●	-
DmpOaAvlVnt	Outside air damper available for ventilation 0:No 1:Yes	BCalcVal	●	-
DmpOaPosDhu	Outside air damper position for dehumidification	APrcVal	●	-
SpTMxC	Mixed air temperature setpoint for cooling	ACnfVal	●	-

Engineering and commissioning

Check switching points for economizer.

Check operation of economizer with failed temperature data.

Verify that damper closes when fan is off.

Verify that damper satisfies max of cooling and vent demand.

Verify fan request signal(s).