CNT-SVX05A-EN
49
Sequence of operation
If ventilation is not a concern, the Tracer™ AH.540 controller can be configured
for
mixed air control
when preheat capacity is available.
Mixed air preheat
control
is the best choice for preheat air handling units with ventilation
requirements.
Table 28 — Mixed air temperature control
Economizer operation
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 hardwired or communicated) or a communicated Enable command from
Tracer Summit
™
, the Tracer AH.540 controller uses the modulating economizer
damper as the highest priority source of free cooling.
Economizing is possible during the occupied, occupied standby, unoccupied,
and occupied bypass modes.
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
controller disables economizing and moves the outdoor air damper back to its
predetermined minimum position based on the current occupancy mode or
communicated minimum damper position.
Air handling unit
configuration
Mixed air
control type
(configurable)
Controller action
No mixing box
present
None
None
Heat only unit
Cool only unit
Cool reheat unit
Mixed air control
Reduce ventilation. Mixed air temperature is maintained
above the mixed air low limit setpoint (50
°
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 cool unit
Mixed air preheat
control
Preheat before reducing ventilation. Preheat capacity is used
to maintain the mixed air temperature above the mixed air
temperature low limit setpoint (50
°
F default, configurable). If
100% preheat capacity does not maintain the mixed air tem-
perature above the mixed air temperature low limit, outdoor
air ventilation is reduced below minimum position.
