Trane LPC Install And Operation Instructions page 76

Variable-air-volume operation always
maintains duct static pressure control in
all modes of operating with the supply fan
On. The air-handling unit duct static
pressure is maintained
by a duct static pressure control
sequence.
The supply fan variable frequency drive
in a variable-air-volume system is
controlled to maintain the duct static
pressure setpoint. When the fan is On, the
controller reads and compares the duct
static pressure input to the duct static
pressure setpoint and adjusts the supply
fan speed analog output signal (AO1) to
the variable frequency drive.
The duct static pressure signal can be
from a wired sensor or communicated
via a network variable. If the controller
does not have a valid duct static pressure
from a wired sensor or communicated,
the controller generates a Duct Static
Press Failure diagnostic and shuts down
the unit. The controller does not operate
duct static pressure control without a
valid duct static pressure input.
If the controller has both a hard-wired
and communicated duct static pressure
input, the communicated value is used for
duct static pressure control. The greater
of the two values, local (hard-wired) or
communicated, is used for duct static
pressure high limit shutdown.
The Tracer AH540/541 controller has a
configurable duct static pressure high
limit setpoint. If the duct static pressure
exceeds the duct static pressure high
limit setpoint, the controller shuts down
the unit and generates a Duct Static
Press High Limit diagnostic. This latching
diagnostic must be cleared from the
controller before the unit is allowed to
operate.
Valve Operation
This sequence applies to both space
temperature control and discharge air
temperature control.
The controller uses analog modulating
(0–10 Vdc or 2–10 Vdc) valves for heating
or cooling operation. The controller
supports one or two modulating valves
for hydronic heating, steam heat, and
76
Operation
hydronic cooling operation. The Tracer
AH540/541 controller supports both one-
and two-valve unit configurations.
Heating only, cooling only, and heating
and cooling controller configurations will
always use the heating analog output for
valve heating and cooling analog output
for valve cooling. For two-pipe
changeover configurations with only one
hydronic coil installed, use the cooling
analog output for heating/cooling valve
operation.
Normally, heating and cooling valves
remain closed any time the supply fan is
Off. Valves can open when the supply fan
is Off during:
• Entering water temperature sampling
• Freeze avoidance
• Certain diagnostic conditions
• Valve override open
The Tracer AH540/541 controller oper-
ates with either normally open or
normally closed valves. The normal state
of the valve is the position of the valve
when power is not applied. When power
is applied, the controller has full control of
the valve. For example, if the fan mode
switch on the zone sensor is in the Off
position, the controller closes the valve,
regardless if it is configured normally
open or normally closed.
Freeze-avoidance valve cycling
During low-temperature detect or freeze-
avoidance diagnostic conditions that
cause the unit to shutdown, the controller
opens all heating and cooling valves
100% to prevent the coil from freezing.
When hydronic or steam heat is present,
the controller cycles the 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 from 0 to 100%.
For example, if valve cycling, duty cycle is
configured for 25% (default), the control-
ler opens the valve for 75 seconds (25%
of 5 minutes) and closes it for 225
seconds.
sequence of
operation
T wo-pipe changeover
This sequence applies only space
temperature control. The controller
provides a two-pipe changeover function
when an air-handling unit has one
hydronic coil for heating and cooling
operation. Two-pipe changeover allows
the controller to provide heating or
cooling to the space depending on the
entering water temperature.
When a two-pipe changeover unit has
secondary electric heat the controller
uses the electric heat if the entering
water temperature is not appropriate for
heating. If entering water temperature is
appropriate for heating, the controller
uses hydronic heating and disables
electric heat operation.
Two-pipe changeover units without an
auxiliary source of heat, like electric heat,
determine their mode based on the
following sequence:
1. If the controlled space requires heating
or cooling, the controller changes from
heating to cooling and cooling to heating
based on the space temperature, active
setpoint, and integrated error between
the two. Once the controller changes
modes, it verifies the entering water
temperature.
2. If the controller does not have a valid
entering water temperature (hard-wired
or communicated), the controller as-
sumes hot water is present.
3. When the entering water temperature
is not appropriate for the desired capacity
(either heating or cooling), the controller
remains at 0% hydronic capacity.
Economizer cooling would remain
available if needed regardless of the
entering water conditions.
4. If the entering water temperature is
appropriate for heating or cooling, the
controller energizes the appropriate
output to control the space temperature
at the heating setpoint in heating mode or
cooling setpoint in cooling mode.
LPC-SVX01C-EN