Mixed-Air Temperature Control - Trane Tracer AH540 Setup And Operation

Table 21. Mixed-air temperature control
Air-handling unit
configuration
No mixing box present
Heat only unit
Cool only unit
Cool reheat unit
Preheat cool unit
Preheat cool unit
CNT-SVX05B-EN
During occupied modes, the damper remains at a minimum damper posi-
tion, whether it is configured or communicated.

Mixed-air temperature control

The Tracer AH540/541 controller provides minimum ventilation require-
ments according to the effective occupancy mode. Ventilation require-
ments are maintained by mixed-air temperature control depending on
available heating and cooling sources, unit configuration, and mixed-air
temperature control type (configurable). Low mixed-air temperatures can
be a concern for units with hydronic heating and cooling.
Mixed-air temperature control is used to maintain the mixed-air temper-
ature above the mixed-air low-limit setpoint (configurable). See Table 21
on page 57. If the air-handling unit does not have a mixing box section,
then mixed-air temperature control is not required.
Mixed-air temperature
control type
(configurable)
None
Mixed-air temperature
control
Mixed-air preheat control
Heat only, cool only, preheat cool, or cool reheat air-handling configura-
tions with a mixing box can be configured for mixed-air temperature con-
trol. 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 mixed-
air low-limit setpoint.
Air-handling units with preheat can use mixed-air preheat control to
maintain mixed-air temperature before reducing ventilation. Cold enter-
ing 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 tem-
perature is below the low-limit temperature, the mixed-air preheat con-
trol then lowers the outdoor air damper below the minimum position to
maintain mixed air above the mixed-air low-limit setpoint.
Controller action
None
Reduce ventilation. Mixed-air temperature is maintained
above the mixed-air low-limit setpoint (48°F default, config-
urable) by reducing the outdoor air ventilation below mini-
mum position. The lower percent of outdoor air raises the
mixed-air temperature.
Preheat before reducing ventilation. Preheat capacity is
used to maintain the mixed-air temperature above the
mixed-air low-limit setpoint (48°F default, configurable). If
100% preheat capacity does not maintain the mixed-air tem-
perature above the mixed-air low-limit setpoint, outdoor air
ventilation is reduced below minimum position.
Outdoor air damper operation
57