CNT-SVX05A-EN
47
Sequence of operation
Steam valve cycling
During low temp detect or freeze avoidance diagnostic conditions, which causes
a unit shutdown, the controller opens all heating and cooling valves 100% to
prevent the coil from freezing. When steam heat is present, the controller cycles
the steam heat valve output on, then off over a period of five minutes
(configurable) to prevent excessive unit cabinet temperatures. The heat valve
output Open position is configurable 0 to 100%.
For example:
If steam heat cycling is configured for 25% (default), the
controller opens the steam valve for 75 seconds (25% of 5 minutes) and
closes it for 225 seconds.
Caution!
An air handling unit with greater than 15 psig of steam could create
excessive cabinet temperatures with the valve 100% open.
Face and bypass damper operation
The face and bypass damper modulates a percentage of air to the face of the heat
coil and around the coil (bypass) to maintain the supply air temperature setpoint.
The air passing through the hot water coil is mixed with the air bypassing the
coil to produce a desired discharge air temperature.
The Tracer
™
AH.540 controller supports face and bypass operation for low
outdoor temperature heating modes of operation only. During low outdoor
temperatures, when the outdoor air temperature is lower than the face and
bypass heat modulation setpoint, the heating valve is fully opened and the
face and bypass damper is used for heating to prevent the coil from freezing.
During economizer cooling operation, when outdoor temperature is less than
the face and bypass heat modulation setpoint, the face and bypass damper
is full bypass.
The face and bypass heat modulation setpoint is the outdoor air temperature
(40
°
F default) at which the controller changes over to face and bypass heating
operation. The face and bypass heat modulation setpoint can be changed using
the Trane Rover™ service tool.
When the outdoor air temperature is greater than 3
°
F above the face and
bypass heating modulation setpoint, the hydronic heating valve is modulated
to maintain discharge air temperature. The face and bypass damper is
positioned for full face air flow. When the outdoor air temperature is less than
the face and bypass heating modulation setpoint, the controller fully opens the
heating valve and uses the heat face and bypass damper to modulate heating
capacity to maintain the desired discharge air temperature.
Table 26 — Face and bypass damper operation based upon outdoor air
temperature
Note 1:
The outdoor air temperature must rise 3
°
F above the face and bypass heating enable point before face and
bypass heating operation is disabled.
During diagnostic and fan off conditions, when the controller shuts down unit
operation, the face and bypass damper is in the full bypass position. During
freeze avoidance operation or a
Low Temp Detect
diagnostic, the face and bypass
damper is driven to full face.
Outdoor air temperature
Hydronic heating valve
Face and bypass damper
Outdoor air temperature is
greater than face and
bypassheat modulation
setpoint (Note 1)
Modulated to maintain
desired setpoint
Full coil face
(operation disabled)
Outdoor air temperature is less
than face and bypass heat modu-
lation setpoint
100% open
Modulated to maintain desired
setpoint
(operation enabled)
