Application

Room Segment Functions
Device Control Functions HVAC
54
Siemens Industry, Inc.
Application Note
A6V11692053
Smart Infrastructure – Building Products
2019-04-01
Interlocks
Interlocks are (typically) binary signals that ensure equipment protection.
Additional signals help coordinate the interaction sequences between HVAC
devices.
A signal's direction In / Out (and function, to some degree) depend on the HVAC
device. For example, an airflow status (AirFlSta) interlock signal can be output from
a supply damper or a fan, and sent as input to a coil or an OA damper. In the table
that follows, descriptions of a signal's functionality reflect its usage by the
specified HVAC device.
NOTE:
Signals in table are internal signals, not BACnet objects. For this reason they are
not visible in the tool, but the parameters associated with them are. See comment
column for hints on parameters that affect interlock functionality.
Signal
Type
Description
Comment
AirFlCReq
Boolean
Airflow cooling request → Initiates airflow (damper opens) to
satisfy airflow support request from cooling coil.
Only if cooling coil is present
in terminal unit
AirFlHReq
Boolean
Airflow heating request → Initiates airflow (damper opens) to
satisfy airflow support request from heating coil.
Only if heating coil is present
in terminal unit
AirFlHldH
Boolean
Airflow hold for heating → Interlock signal from electric
heating coil indicating the coil needs equipment protection
airflow support. The damper is prevented from closing for a
period of time.
Uses equipment protection
(Priority 5)
AirFlSta
Boolean
Airflow status → Interlock signal that keeps the coil locked at
equipment protection (Priority 5) unless the supply damper
provides airflow.
See C onfiguration section for
parameters named AirFlSta
AirFlVavReq
%
Airflow VAV request → In a fan-powered box (FPB)
application, the VAV damper sends a request signal
(AirFlVavReq) requesting airflow, so that the box can meet an
airflow support or ventilation demand.
Fan-powered box (FPB)
application
Setpoint is percent of nominal
airflow configuration
FanSta
Boolean
Fan state → Interlock signal used in series fan-powered box
applications: Fan state (FanSta) must be True (fan is
running) before damper is allowed to open. This ensures that
airflow from the damper does not rotate the series fan while
at rest.
In series fan-powered box
applications, parameter
EnMonFanSta must = Yes.
Configuration
BACnet Objects
Description
Object
Object Type
Pre-loaded
Value
Supply air VAV duct area 1
Supply air VAV duct area 2
VavSuDuctArea1
VavSuDuctArea2
ACnfVal
0.349 ft
2
(0.0xx
m
2
)
Supply air VAV duct shape 1
Supply air VAV duct shape 2
VavSuDuctShape1
VavSuDuctShape2
MCnfVal
Round