Application

Room Segment Functions
Device Control Functions HVAC
62
Siemens Industry, Inc.
Application Note
A6V11692055
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 electric
heating coil, two binary output.
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
Comments column for hints on parameters that affect interlock functionality.
Signal
Type
Description
Comment
AirFlSta
Boolean
Air flow status → Interlock signal, from a fan or a supply
damper, that keeps the coil locked unless that same fan or
supply damper provides airflow.
No related parameters in
electric coils. AirFlSta is hard-
coded.
AirFlHReq
Boolean
Air flow heating request → Initiates airflow (fan turns on or
VAV supply damper provides airflow) to satisfy airflow
support request for heating. Signal is sent to fan or VAV
supply damper depending on application.
See Configuration section for
parameters named
"...AirFlHReq".
AirFlHldH
Boolean
Air flow hold for heating → Interlock signal to fan or supply
damper requesting equipment protection airflow support for
a period of time, such as when the electric heating coil has
just turned off.
See Configuration section for
parameters named
"...AirFlHldH".
Available Status
When the heating coil is available for heating, the binary output signal for Heating
coil available for heating (HclAvlH) will be "Yes" (available). HclAvlH is used by
the system to regulate heating resources. For example, if there is a call for heating
but the heating coil is not available because it is already at maximum capacity
(because device mode is fully open), then HclAvlH will signal "No" and the next
available heating resource in the temperature control sequence will be activated.
For heating available status to be "Yes", device mode must equal "Control mode"
(modulation).