Trane LPC Install And Operation Instructions page 79

Mixed-air temperature control is used to
maintain the mixed-air temperature
above the mixed-air low-limit setpoint
(configurable). See Table O-SO-9. If the
air-handling unit does not have a mixing
box section, then mixed-air temperature
control is not required.
Heat only, cool only, preheat cool, or cool
reheat air-handling configurations with a
mixing box can be configured for mixed-
air temperature control. If cold outdoor air
conditions exist, depending on ventilation
requirements, the mixed-air temperature
can create freezing conditions. Mixed-air
temperature control reduces the outdoor
air damper below the minimum position
to maintain mixed-air temperature above
the mixedair low-limit setpoint.
Air-handling units with preheat can use
mixed-air preheat control to maintain
mixed-air temperature before reducing
ventilation. Cold entering air conditions
from the mixing box can be heated with
the preheat capacity to maintain the
mixed-air temperature above the mixed-
air low limit setpoint. Mixed-air preheat
control attempts to use preheat until it
has reached 100% capacity. At 100%
preheat capacity, if mixed-air tempera-
ture is below the low-limit temperature,
the mixed-air preheat control then lowers
the outdoor air damper below the
minimum position to maintain mixed air
above the mixed-air low-limit setpoint.
Table O-SO-9. Mixed-air temperature control
LPC-SVX01C-EN
Operation
If ventilation is not a concern, the Tracer
AH540/541 controller can be configured
for mixed-air temperature control when
preheat capacity is available. Mixed-air
preheat control is the best choice for
preheat air-handling units with ventilation
requirements.
Mixed-air temperature sensor
location
It is important to mount the mixed-air
temperature sensor in the proper
location according to the mixed-air
temperature control configuration.
Economizing
This sequence applies to both space
temperature control and discharge air
temperature control.
Economizing is a mode in which outdoor
air is used as a source of cooling capacity
before hydronic cooling. With a valid
outdoor air temperature (either hard-
wired or communicated) or a communi-
cated Enable command from the Tracer
Summit system, the Tracer AH540/541
controller uses the modulating econo-
mizer damper as the highest priority
source of free cooling.
sequence of
operation
Economizing is possible during the
occupied, occupied standby, unoccupied,
and occupied bypass modes. It requires a
mixed-air temperature sensor and an air-
handling unit equipped with a mixing box.
The mixed-air temperature sensor is
used as a low-temperature limit, to keep
mixed-air temperatures above freezing. It
also requires an outdoor air temperature
value to be present. If an outdoor
temperature is not available, a communi-
cated request can enable economizing.
The controller initiates the economizer
function if the outdoor air temperature is
cold enough to be used as free cooling
capacity. If the outdoor air temperature is
less than the economizer enable setpoint
(absolute dry bulb), the controller
modulates the outdoor air damper
(between the active minimum damper
position and 100%) to control the amount
of outdoor air cooling capacity. When the
outdoor air temperature rises 5°F above
the economizer enable point, the control-
ler disables economizing and moves the
outdoor air damper back to its predeter-
mined minimum position based on the
current occupancy mode or communi-
cated minimum damper position (see
Table O-SO-10).
79