STANDARD DDC SEQUENCES OF OPERATION
[Guideline: Direct digital control (DDC) systems have been applied since early 1990s as the preferred method for defining controlling HVAC systems. Schematic control diagrams and narrative sequences of operation are commonly used by HVAC designers to define the design intent for control and operation of HVAC systems.
The sequences of operation provided here have been developed and tested over the past 20+ years, modified from time-to-time to comply with ever-changing mechanical and energy conservation building codes and new standards developed by ASHRAE, et al. These sequences comply with the draft of BSR/ASHRAE Guideline 36P, High Performance Sequences of Operation for HVAC Systems, published April 2015.
The control sequences are presented in “narrative” format since experience has proven that this format is more well understood method by which HVAC designers can describe their design intent, for owners and contractors to understand how each HVAC system is to operate, and for control vendors to determine DDC system requirements.
Basic sequences of operation requirements are specified in Section 230993. Review/edit this section and associated sequences below as required for the project.
Control sequences of operation are presented in these guidelines as individual sequences for each HVAC system “element.” It is anticipated that the HVAC designer will assemble these “element” control sequences to define the complete control requirements for each specific HVAC system as follows:
HVAC System/Application / Sequence(s) of Operation /All Projects / All applicable Section 1 and Section 2 Sequences
Air-Handling System (All) / 4.00, 4.01, 4.05, 4.06, 4.07, and 4.08
Air-Handling System (CHW/HW, Multiple Zone, VAV) / AHU: 4.20, 4.21, 4.22, 4.23, 4.24, 4.25, and 4.26
TUs (Single Duct): 4.27.1, 4.27.2, 4.27.3, 4.27.4, and/or 4.27.5
TUs (Fan-Powered): 4.28.1 and/or 4.28.2
Air-Handling System (CHW/HW, Single Zone) / CAV: 4.10, 4.11.1, 4.12.1 or 4.15, plus 4.14
VAV: 4.10, 4.11.2, 4.12.2 or 4.15, plus 4.14
Air-to-Air Heat Recovery / Thermal Wheel: 8.10
Heat Pipe: 8.13
Run-Around: 8.15
CDW System / 3.15 or 3.16, plus 3.17
CHW System / 3.01 and 3.12, 3.13, or 3.14 [Edit for water- vs air-cooled chiller(s)]
HW Boilers / 3.12 and 3.23 or 3.24
HW Convection/Radiation / 3.27
HW Unit Heater / 3.26
Kitchen Exhaust / 7.10 and/or 7.12
Laboratory Exhaust / 7.13 and 7.14
Lighting / 9.10 and/or 9.13
Packaged/Split HVAC Unit (Single Zone, CAV or VAV) / 5.10 plus
5.10.1, 5.10.2, or 5.10.3
Packaged /Split HVAC Unit (Multiple Zone, VAV) / HVAC Unit: 5.20
TUs (Single Duct): 4.27.1, 4.27.2, 4.27.3, 4.27.4, and/or 4.27.5
TUs (Fan-Powered): 4.28.1 and/or 4.28.2
Packaged/Split DVA Unit / 5.30
Refrigeration Machinery Room / 3.18
Service Hot Water Heater / 6.10
Steam Boilers / 3.31 and 3.33
Steam Convection/Radiation / 3.37
Steam Humidifier / 4.09
Steam Unit Heater / 3.35
Steam-to-HW HX / 3.25
Index to Sequences
Section / ID / Description /1 / GENERAL DDC SYSTEM REQUIREMENTS
1.14 / Schedules of Operation
1.21 / Constant Speed Single-Phase or Polyphase Motor Control
1.22 / Two-Speed Polyphase Motor Control
1.23 / Variable Speed Single-Phase Motor Control
1.24 / Variable Speed Polyphase Motor Control
1.25 / Single-Phase and Polyphase Motor Status Monitoring
1.26.1 / Lead/Lag/Cascade Sequence
1.26.2 / Lead/Lag/Standby Sequence
1.27 / Exhaust Air Fan Start/Stop
1.30 / Outdoor Piping/Equipment Trace Heating Cable
1.33 / Hydronic Control Valve
1.37 / Steam Control Valve
1.39 / Electric Resistance Heating Control
2 / SITE CONDITIONS AND UTILITIES
2.01 / Outdoor Air Conditions
2.03 / Cooler or Freezer Temperature Alarm
2.04 / Sump Pump High Water Level
2.07 / Utilities Consumption
3 / CENTRAL PLANTS
3.1 Chilled Water Systems
3.11 / Chiller Controller Interface
3.12 / Chilled Water System, Single Chiller, Constant Flow
3.13 / Chilled Water System, Single Chiller, Variable Flow
3.14 / Chilled Water System, Multiple Chillers, Variable Flow
3.15 / Condenser Water System, Dedicated Cooling Tower
3.16 / Condenser Water System, Multiple Cooling Towers
3.17 / Condenser Water System, Cooling Tower Operating and Safety Controls
3.18 / Refrigeration Machinery Room Ventilation and Refrigerant Purge
3.2 Hot Water Systems
3.21 / Boiler Controller Interface
3.23 / Hot Water System, Multiple Conventional Boilers, Variable Flow
3.24 / Hot Water System, Multiple Condensing Boilers, Variable Flow
3.25 / Hot Water System, Steam-to-HW Heat Exchanger(s), Variable Flow
3.26 / Hot Water Unit Heater
3.27 / Hot Water Convection/Radiation
3.3 Steam Systems
3.31 / Boiler Controller Interface
3.33 / Steam System, Multiple Boilers
3.35 / Steam Unit Heater
3.38 / Steam Convection/Radiation
3.4 Closed Circuit Water Source Heat Pump Systems
3.41 / Heat Pump Water Loop, Variable flow
4 / AIR-HANDLING SYSTEMS
4.0 General
4.00 / Start/Stop Control
4.01 / Space Temperature/Humidity Setpoints and Monitoring
4.05 / Drain Pan Condensate Level Alarm
4.06 / Filter Differential Pressure Alarm
4.07 / Low Limit Thermostat Control and Alarm
4.08 / Preheat Control
4.09 / Duct Steam Humidifier
4.1 Single Zone Air-Handling Systems
4.10 / Single Zone Air-Handling Systems
4.11 / Space Temperature and Humidity Control
4.11.1 CAV System
4.11.2 VAV System
4.12 / Minimum Outdoor Air Control
4.12.1 CAV System
4.12.2 VAV System
4.14 / Demand-Based Ventilation Control
4.15 / Airside Economizer Control
4.2 Multiple Zone Single Duct Air-Handling Systems
4.20 / Multiple Zone Single Duct Air-Handling Systems
4.21 / Discharge Air Temperature Control
4.22 / Airflow Volume Control
4.23 / Minimum Outdoor Air Control
4.24 / Demand-Based Ventilation Control
4.25 / Airside Economizer Control
4.26 / “OFF” Period Space Temperature/Humidity Control
4.27 / Terminal Unit, Single Duct
4.27.1 CAV Terminal Reheat
4.27.2 VAV Cooling Only
4.27.3 VAV Terminal Reheat, Single Maximum Airflow Setpoint
4.27.4 VAV Terminal Reheat, Dual Maximum Airflow Setpoints
4.27.5 Supplemental Perimeter Heating
4.28 / Terminal Unit, Single Duct, Fan-Powered,
4.28.1 Series-Configured
4.28.2 Parallel-Configured
4.3 Multiple Zone Dual Fan Dual Duct Air-Handling Systems [FUTURE]
5 / PACKAGED DX COOLING SYSTEMS
5.10 / Packaged/Split HVAC Unit (Single Zone, CAV or VAV)
5.10.1 Space Temperature and Humidity Control (Separate Heating)
5.10.2 Space Temperature and Humidity Control (Air Source Heat Pump)
5.10.3 Space Temperature and Humidity Control (Water Source Heat Pump)
5.20 / Packaged HVAC Unit (Multiple Zone VAV)
5.30 / Packaged HVAC Unit (Dedicated Ventilation Air)
6 / SERVICE HOT WATER HEATING
6.10 / Service Hot Water Heater
7 / SPECIAL EXHAUST SYSTEMS
7.10 / Type I Kitchen Hood Interface [FUTURE]
7.12 / Type II Kitchen Hood [FUTURE]
7.13 / Laboratory: Room Controls
7.14 / Laboratory: Fume Hood Exhaust, Variable Air Volume, Multi-Fan Manifold
8 / AIR-TO-AIR HEAT RECOVERY
8.10 / Thermal Wheel Air-to-Air Heat Recovery [FUTURE]
8.13 / Heat Pipe Air-to-Air Heat Recovery [FUTURE]
8.15 / Run-Around Air-to-Air Heat Recovery
9 / LIGHTING
9.10 / Exterior Lighting
9.13 / Interior Lighting
1 - GENERAL DDC SYSTEM REQUIREMENTS
1.14. Schedules of Operation:
[Guideline: Two independent schedules for HVAC system operation and control are required:
"ON/OFF" schedule(s) shall determine HVAC systems and/or equipment start/stop times, define applicable operating setpoints, and define when specific sequences of operation are to be utilized.
“ON” hour is defined as the estimated latest hour at which the HVAC system/equipment may be commanded “ON” so that setpoint conditions are maintained within the space(s) served at the “OCCUPIED” start hour. This “ON” hour shall be adjusted by the “Optimized Start” application as specified in Section 230923.
“OFF” hour is defined as the hour at which the HVAC system/equipment shall be commanded “OFF.”
"OCCUPIED/UNOCCUPIED" schedule(s) shall determine ventilation modes.
Determine daily/weekly specific operating times and occupancy periods for each area of the facility and establish "ON/OFF" schedule for each HVAC system operation and, for air systems, the "OCCUPIED/ UNOCCUPIED" periods.
“System(s)” can be defined individually (e.g., AHU-3 or Chiller) or in groups (e.g., AHU-1 thru AHU-9 or Chillers 1 and 2).
“Hours” define the times for each day of the week for on/off and, as applicable, occupancy start/stop (note that times outside of the occupancy period, by default, define the unoccupied period).
Enter “weeks” of the year during which each weekly schedule applies (e.g., 1-52 for offices, etc. or 1-24/34-52 for schools).
Define multiple schedules as needed. For example, there may be a “normal” schedule, but also schedules for “holidays,” “vacation periods,”, and/or one or more seasonal schedules.]
System(s) / Period / Hours (1-24) Each Day of The Week / Weeks to Apply (1-52) /MON / TUE / WED / THR / FRI / SAT / SUN /
On Hour
Off Hour
Occupancy Start Hour
Occupancy End Hour
On Hour
Off Hour
Occupancy Start Hour
Occupancy End Hour
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1.21 Constant Speed Single-Phase or Polyphase Motor Control:
Keyed Notes:
1 NC Fire Alarm System Shutdown Relay Contactor
2 NC BCU/AHU Isolation Smoke Damper(s) End Switch(es), as Applicable
3 DDC System Start/Stop Relay
4 NC Mixed Air Low Limit Thermostat (Located Downstream of Preheat Coil)
Motor shall start/stop via DO point.
Monitor motor status as DI point in accordance with Sequence 1.25.
Emergency stop:
Hard-wired contact(s) shall stop motor in accordance with emergency elements defined in Section 230993.
Initiate alarm upon emergency stop.
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1.22 Two-Speed Polyphase Motor Control:
Keyed Notes:
1 HOA Switch with Auxiliary Disconnect Mounted in Starter Cover
2 Motor Starter Contactor or Relay Holding Coil, Low Speed
3 Motor Starter Contactor or Relay Holding Coil, High Speed
4 High Speed Auxiliary Contactor, NC
5 Low Speed Auxiliary Contactor, NC
6 Manual Speed Selector Switch Mounted in Starter Cover
7 DDC System Start/Stop Relay, Low Speed
8 DDC System Start/Stop Relay, High Speed
9 NC Fire Alarm System Shutdown Relay Contactor
10 NC BCU/AHU Isolation Smoke Damper(s) End Switch(es), as Applicable
Motor shall start at low speed, stop, and/or switch from one speed to the other via two (2) DO points.
Monitor motor status as DI point in accordance with Sequence 1.25.
Emergency stop:
Hard-wired contact(s) shall stop motor in accordance with emergency elements defined in Section 230993.
Initiate alarm upon emergency stop.
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1.23 Variable Speed Single-Phase Motor Control:
Provide interface between DDC system and manufacturer-provided electronically commutated motor (ECM) integral speed controller.
Motor shall start/stop via DO point.
Monitor motor status as DI point in accordance with Sequence 1.25.
Emergency stop:
Hard-wired contact(s) shall stop motor in accordance with emergency elements defined in Section 230993.
Initiate alarm upon emergency stop.
Modulate motor speed from minimum speed to 100% speed in direct response to AO point.
Adjust minimum speed setpoint to value, as approved by the designer, using the following procedure: Manually set ECM controller minimum speed output to 6 Hz (10% of maximum) unless (1) dictated otherwise by the equipment manufacturer or (2) defined otherwise by sequence(s) of operation. Observe driven equipment to ensure that it is visibly rotating. If not, increase controller minimum speed output in increments of 2 Hz until the unit driven equipment is visibly rotating. Finally, ensure that the current sensing relay required by Sequence 1.25 confirms operation at the minimum speed setting.
Initiate alarm via DI point if ECM controller status/fault contact closes.
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1.24 Variable Speed Polyphase Motor Control:
Provide interface between DDC system and manufacturer-provided variable frequency drive (VFD) for motor speed control.
Motor shall start/stop via DO point.
Monitor motor status in accordance with Sequence 1.25.
Emergency stop:
Hard-wired contact(s) shall stop motor in accordance with emergency elements defined in Section 230993.
Initiate alarm upon emergency stop.
Modulate motor speed from minimum speed to 100% speed in direct response to AO control point.
Adjust minimum speed setpoint to value, as approved by the designer, using the following procedure: Manually set VFD minimum speed output to 6 Hz (10% of maximum) unless (1) dictated otherwise by the equipment manufacturer or (2) defined otherwise by sequence(s) of operation. Observe driven equipment to ensure that it is visibly rotating. If not, increase VFD minimum speed output in increments of 2 Hz until the unit driven equipment is visibly rotating. Finally, ensure that the current sensing relay required by Sequence 1.25 confirms operation at the minimum speed setting.
Monitor motor speed feedback from VFD via AI point.
Initiate alarm via DI point if VFD status/fault contact closes.
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1.25 Single-Phase and Polyphase Motor Status Monitoring:
Motor "ON" or "OFF" status shall be monitored via DI point contact closure at current monitoring relay motor amperage ”trip” setpoint.
Adjust current monitoring relay “trip” setpoint to 5% above the anticipated minimum operating motor amperage. For variable speed motors, coordinate “trip” setpoint with minimum motor speed.
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1.26.1 Lead/Lag/Cascading Sequence (Parallel Operation):
DDC system shall incorporate “Lead/Lag” sequence as part of any sequence of operation requiring sequential “ON/OFF staging” of multiple HVAC components designed to operate in parallel, as follows.
Designate HVAC component as “Lead” in the reverse order of its number of operational hours.
Designate HVAC component(s) as “Lag” in the direct order of its number of operational hours.
Command lead HVAC component “ON” via DO point.
Confirm lead HVAC component operation on basis of (1) motor status in accordance with Sequence 1.25 or (2) OEM component controller safety/status as DI point.
In the event of lead HVAC component failure:
Initiate alarm.
Command lead HVAC component “OFF” via DO point.
Command next lag HVAC component “ON” via DO point.
Confirm standby HVAC component operation on basis of (1) motor status in accordance with Sequence 1.25 or (2) OEM component controller safety/status as DI point.
In the event of lag HVAC component failure:
Initiate alarm.
Disable HVAC system of which the failed HVAC component(s) is a part in its entirety.
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1.26.2 Lead/Standby (Alternate Operation): DDC system shall incorporate “Lead/Standby” sequence as part of any sequence of operation requiring the “ON/OFF alternating” use of two or more HVAC components, as follows:
Command lead HVAC component “ON” via DO point.
Confirm lead HVAC component operation on basis of (1) motor status in accordance with Sequence 1.25 or (2) OEM component controller safety/status as DI point.