[OSSC] Optimum start stop control

Function block OSSC optimizes activation of HVAC equipment before start and end of room occupancy.

OSSC optimizes activation of HVAC equipment before start and end of room occupancy. OSSC is used to provide the desired room temperature at the beginning of occupancy at the lowest possible energy consumption (short duration of start phase). Additionally, energy supply (heating or cooling) is cut prior to the end of occupancy (longest possible stop phase).

Functioning

OSSC determines the start and stop phases based on the scheduler values: Present operating mode, next operating mode, and time to next operating mode. The outside and room temperature are also considered.

Adaptation is carried out after each plant start or stop.

The following functions are integrated in OSSC:

Optimum start stop control (OSSC) function for heating and cooling plants (not simultaneous)
Automatic adaptation of operating values

Start phase

The start phase comprises:

Step	Process
1	Prior to the start of occupancy ([OpModR] = Comfort or [OpModR] = Pre-Comf), the OSSC calculates the following values: Required duration of the start phase [TiSttEst]. Time [DTiStt] to start phase. Required room temperature setpoint [SpTRStt] in the start phase.
2	At the calculated time, the operating mode changes to [PrOpMod] = Start. The start phase begins.
3	In the start phase, the OSSC controls the room temperature via the room temperature setpoint [SpTRStt]. Control intends to ensure that the room temperature [TR] at the beginning of occupancy corresponds to the temperature setpoint [SpCmf] or [SpPcf].
4	After expiration of the start phase [TiSttEst], the operating mode is [PrOpMod] = Inactive. The start phase ends. Occupancy begins at this time.
5	During occupancy ([OpModR] = Comfort or [OpModR] = Pre-Comf), a start phase is not possible.

Stop phase

The stop phase comprises:

Step	Process
1	When [OpModR] = Comfort, OSSC calculates the following values: Required duration of the stop phase [TiSttEst]. Time [DTiStt] to stop phase. Required room temperature setpoint [SpTRStp] in the stop phase.
2	At the calculated time, the operating mode changes to [PrOpMod] = Stop. The stop phase begins.
3	In the stop phase, the OSSC controls the room temperature via the room temperature setpoint [SpTRStp]. Control intends to ensure that the room temperature [TR] at the end of occupancy corresponds to the temperature setpoint [SpPcf].
4	After expiration of the stop phase [TiSttEst], the operating mode is [PrOpMod] = Inactive. The stop phase ends. Occupancy ends at this time.

If the room temperature sensor is not used ([TRSen] = 0), OSSC disables stop optimization.

Start phase: Temperature reached too soon

If the temperature setpoint [SpCmf] in the start phase is reached prior to expiration of [TiSttEst], the operating mode changes to [PrOpMod] = Pre-Comf and the setpoint [SpTRStt] to [SpPcf]. Another start phase is impossible.

The time deviation to [TiSttEst] is indicated by [ErTiStt] > 0 and the temperature deviation [ErTRStt] is set to zero.

Start phase: Temperature reached too late

If the temperature setpoint [SpCmf] is reached later than after the expiration of [TiSttEst], the operating mode remains at [PrOpMod] = Start and start setpoint increased until the temperature setpoint is reached. The operating state changes to [OsscSta] = Late Start.

The time deviation to [TiSttEst] is indicated by [ErTiStt] < 0 and the temperature deviation by [ErTRStt] > 0.

Stop phase: Temperature reached too soon

If the temperature setpoint [SpPcf] in the stop phase is reached sooner than [TiStpEst], the operating mode changes to [PrOpMod] = Pre-Comf and the setpoint to [SpTRStp] = [SpPcf]. Another stop phase is impossible.
The time deviation to [TiStpEst] is indicated by [ErTiStp] > 0 and the temperature deviation [ErTRStp] is set to zero.

Stop phase: Temperature reached too late

If the temperature setpoint [SpPcf] in the stop phase is not reached after [TiStpEst], the operating mode still changes to [PrOpMod] = Inactive.
The time deviation to [TiStpEst] is indicated by [ErTiStp] = 0, the temperature deviation by [ErTRStp] < 0.

Special cases during room occupation

If the room operating mode changes to [OpModR] = Pre-Comf, the operating mode remains at [PrOpMod] = Inactive.

OSSC does not control the room temperature outside the start and stop phases. Outside the start/stop phases [PrOpMod] = Inactive and room temperature control must be taken over by other function blocks. For example, OSSC together with function block HCRV can be used for occupancy phases and sustained mode function for non-occupancy phases.
[HCRV] Heating curve

Determination of design start and stop times

The estimated time for start and stop phases requires configuration parameters. They are set as per the building properties and updated when adaptation to OSSC is enabled.

The parameters must be available on the associated BACnet object as extension property (Extension by persistence = Optimum start stop control).

Parameters	Meaning
TiSttDs	Start time for design conditions For [ActgHC] = 0: [TiSttDs] is the start time required to heat up the room from 15°C to 20°C at an outside temperature of [TOaDsgn]. During start phase, the target room temperature setpoint ([SpCmf] or [SpPcf]) is increased by [DSpTRStt]. For [ActgHC] = 1: [TiSttDs] is the start time required to cool the room from 28°C to 24°C at a design outside temperature of [TOaDsgn]. During start phase, the target room temperature setpoint ([SpCmf] or [SpPcf]) is reduced by [DSpTRStt]. A change in [DSpTRStt] impacts all start parameters.
TiSttLld	Start time for low load conditions For [ActgHC] = 0: [TiSttLld] is the start time that is required to heat up the room an outside temperature of 15°C from 15°C to 20°C. During start phase, the target room temperature setpoint ([SpCmf] or [SpPcf]) is increased by [DSpTRStt]. For [ActgHC] = 1: [TiSttLld] is the start time required to cool the room from 28°C to 24°C at an outside temperature of 20°C. During start phase, the target room temperature setpoint ([SpCmf] or [SpPcf]) is reduced by [DSpTRStt]. A change in [DSpTRStt] impacts all start parameters.
TiSttWe	Start time for weekend conditions [TiSttWe] is the time that is added to the base start time at an design outside temperature [TOaDsgn], if the room is unoccupied over a longer period ([OpModR] = Protection or [OpModR] = Economy). After a weekend, the start phase can be started earlier with this parameter. [OpModFb] must be correctly generated and feedbacked to this parameter to influence the process.
TcnTRStp	Time constant of room temperature during stop time [TcnTRStp] is the time constant at which a room is cooled or heating when shutting down heating or cooling (constant outside temperature). Determination of time constant: Time constant is the time required to maintain 63% of the end value. After expiration of the time constant, a change to room temperature starts at 63% of the end temperature.

Typical interconnection to other function blocks

Connecting thread	Use
EnFnct	This input typically is used to indicate to OSSC if the scheduler R_MSCHED sets the room operating mode. If this is the case, OSSC can take effect (set [EnFnct] to 1) or not (set [EnFnct] to 0).
OpModRNxOpModRTnxOpMod	These inputs typically are interconnected to the R_MSCHED scheduler.
OpModFb	The winning (modified) changed room operating mode must be sent to this input. The modification is implemented as follows:
	Heating/cooling plant off	[OpModFb] = Inactive
	Else	[OpModFb] = Winning room operating mode.
	OSSC only improves start/stop times [TiSttDs], [TiSttLld], [DTiSttWe], [TiStt0], [TcnTRStp] if feedback to [OpModFb] meets expectations. It checks whether the start and stop phases occurred as provided by the block. This prevents adaptations to start/stop times based on manual intervention or faults.
	If [OsscSta] = Setback Early start Late start Early occupancy Pre-Comfort Comfort Stop Late occupancy	Expected by OSSC at [OpModFb]: Inactive, Economy, Protection, Purge, NightCool Start, Purge Start Pre-Comfort, Purge Pre-Comfort Comfort Stop, Inactive Pre-Comfort
PrOpMod	[PrOpMod] = Inactive	Other functional units must determine the room operating mode.
PrOpMod	[PrOpMod] <> Inactive	The OSSC block determines the room operating mode if no functional units with higher priority determine the room operating mode.
SpTRSttSpTRStp	If [PrOpMod] = Start	the corresponding room temperature setpoint [SpTRStt] is valid.
	If [PrOpMod] = Stop	the corresponding room temperature setpoint [SpTRStp] is valid.
	During the stop phases, the heating/cooling plant must be switched off to ensure that the setpoint is not active. However, [SpTRStp] is the room temperature the OSSC block is supposed to reach at the end of occupancy.
OsscSta	If an OSSC block is used both for heating and cooling, [OsscSta] can be used to coordinate the two blocks.

Reset parameters.

The configuration parameters can be reset to the defaults. This can occur via input [ResetPar].

Parameters	Meaning
None	Normal function (Default value).
StartPar	Parameters [TiSttDs], [TiSttLld], [TiSttWe], [TiStt0] have default values.
StopPar	Parameter [TcnTRStp] is set to the default value.

Room temperature sensors

Condition	Result
TRSen = Yes	The room temperature is measured and available at input [TR].
TRSen = No	If the room temperature is not available as a measured value, [TRSen] must be set to No. In this case, no stop phases are triggered and start times are not improved. Input [TR] must be set either with a fixed value or receive a value calculated by a logic outside OSSC.

Pins

Input	Description	Data type	Default value Value range
EnFnct	Enable function Enables the function.	Boolean	1
EnFnct	0 - The function is disabled. 1 - The function is enabled.
OpModR	Operating mode for room Present operating mode for room	Integer	4: Pre-Comfort 1…9
OpModR	1: Inactive 2: Protection 3: Economy 4: Pre-Comfort 5: Comfort 6: NightCool. 7: Purge 8: Stop 9: Start
NxOpModR	Next operating mode for room Next operating mode for room, requested by the scheduler	Integer	2: Protection 1…9
NxOpModR	1: Inactive 2: Protection 3: Economy 4: Pre-Comfort 5: Comfort 6: NightCool. 7: Purge 8: Stop 9: Start
TiNxOpM	Time till next operating mode	Time	0 0…48h
SpPcf	Setpoint pre-comfort	Real	19.0 5.0…35.0 [°C]
SpCmf	Setpoint for comfort	Real	20.0 5…35 [°C]
TOa	Outside temperature	Real	20.0 [°C] 68.0 [°F] -100.0…100.0 [°C] -148.0…212.0 [°F]
ErTOa	Error for outside temperature Indicates if the outside air temperature [TOa] is faulty.	Boolean	0
TR	Room temperature Measures the (reference) room temperature.	Real	20.0 [°C] 68.0 [°F] 0.0…50.0 [°C] 32.0…122.0 [°F]
TRSen	Room temperature sensor	Boolean	1
ErTR	Error room temperature Indicates if the room temperature [TR] is faulty.	Boolean	0
OpModFb	Operating mode feedback	Integer	1 1…9
OpModFb	1: Inactive 2: Protection 3: Economy 4: Pre-Comfort 5: Comfort 6: NightCool. 7: Purge 8: Stop 9: Start
EnAda	Enable adaptation	Boolean	1
ActgHC	Control action for heating or cooling	Boolean	0
ActgHC	0 - Heating 1 - Cooling
TOaDsgn	Design outside temperature Defined as per heating plant planning data	Real	-11.0 [°C] 12.2 [°F] -50.0…10.0 [°C] -58.0…50.0 [°F]
TiSttMax	Maximum start time	Time	12h 0…72h
TiStpMax	Maximum stop time Maximum time (duration) for stop phase	Time	1h 0…12h
DSpTRStt	Delta setpoint room temperature upon start	Real	10.0
ResetPar	Reset parameters	Integer	1: None 1…3
ResetPar	1: None 2: Start parameter 3: Stop parameter
BaObjRef	BA-Object reference Link between function block and BACnet object.	Structure	---
BaObjRef > DeviceID	Device identifier [DeviceID] includes object type and object instance in one ID. Identifies the device object: local or remote. A local device object can also be identified by value 16#3FFFFF.	Double Word	16#3FFFFF
BaObjRef > ObjectID	Object identifier [ObjectID] includes object type and object instance in one ID. Identifies the object on a device.	Double Word	16#3FFFFF
BaObjRef > ItemID	Item identifier [ItemID] represents an index in a view node object that references the BACnet object. [ObjectID] and [ItemID] together define an indirect reference to the BACnet object. Value -1 indicates a direct interface.	Integer	-1

Output	Description	Data type	Default value Value range
PrOpMod	Present operating mode	Integer	1 1…9
PrOpMod	1: Inactive 2: Protection 3: Economy 4: Pre-Comfort 5: Comfort 6: NightCool. 7: Purge 8: Stop 9: Start
SpTRStt	Room temperature setpoint for start	Real	30.0 0.0…50.0 [°C]
SpTRStp	Room temperature setpoint for stop	Real	19.0 0.0…50.0 [°C]
OsscSta	OSSC state (optimum start stop control)	Integer	1: Setback 1…8
OsscSta	1: Setback 2: Early start 3: Late start 4: Early occupancy 5: Pre-Comfort 6: Comfort 7: Stop 8: Late occupancy
DTiStt	Delta to start time	Time	24h 0…72h
TiSttEst	Estimated start time	Time	0 ms 0…72h
DTiStp	Delta to stop time	Time	24h 0…72h
TiStpEst	Estimated stop time	Time	0 ms 0…12h
ErTiStt	Error start time	Time	0 ms
ErTRStt	Error room temperature at start time	Real	0.0 [K]
ErTiStp	Error stop time	Time	0 ms
ErTRStp	Error room temperature at stop time	Real	0.0 [K]
SysUnits	System of units Indicates the system of units used by the block.	Integer	1: International system of units (SI) (fallback) 1...5
SysUnits	1: International system of units (SI) (fallback) 2: International system of units (SI) 3: US system of units 4: Imperial system of units 5: Canadian system of units
ErrCode	Error code indication	Integer	0: No error
ErrCode	= 0 - No error > 0 - Error codes

Troubleshooting

Outside temperature error

If the outside air temperature is faulty, the OSSC block does not start the stop phase.

Room temperature error

If the room temperature is faulty, the OSSC block neither starts the start nor the stop phase.

Access to BACnet object

In the event of error accessing the BACnet object properties, start and stop optimization is performed with the default values.

No start and stop optimization is performed if the BACnet object reference is invalid.