Trane Tracer Uc page 50

Calibration, Operation Modes, and Control
Safety and Maintenance!
Trane does not recommend enabling electric reheat in pressure dependent mode. There are
potential safety and maintenance issues that can occur if the electric reheat is allowed to
energize when the airflow passing over the element is too low or is below the Airflow Setpoint
Minimum Local Heat. Refer to the equipment catalog for Trane F-Style VAV boxes for minimum
airflow settings based on box inlet size and heating element kW.
Electric Heat
Electric Heat Pulse-width Modulation (PWM)
Single-duct units can have from one to three stages of electric heat pulse-width modulation. Fan
powered units can have up to three stages of electric heat pulse-width modulation. Energizing for
a portion of a three-minute time period modulates the electric heater. This allows exact load
matching for energy efficient operation, and optimum zone temperature control. The heating
minimum airflow setpoint is enforced during reheat. The amount of reheat supplied is dependent
on both the degrees that the space temperature is below the active heating setpoint and the time
that the space temperature has been below the active heating setpoint. If not already OFF , reheat
de-energizes when the zone temperature rises more than 0.5°F (0.28°C) above the heating setpoint.
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Electric Heat SCR
SCR Heat is a heat controller that controls a single stage electric heater proportional to an analog
signal. Typically with SCR heat, the heater is turned on and off on a very short cycle time to give
the proportional heat. This allows the exact load matching for energy efficient operation and
optimum zone temperature control. The heating minimum airflow setpoint is enforced during
reheat. The amount of reheat supplied is dependent on both the degrees that the space
temperature is below the active heating setpoint and the time that the space temperature has been
below the active heating setpoint. If not already off, reheat de-energizes when the zone temperature
rises more that 0.5°F (0.28°C) above the heating setpoint. The analog output signal is proportional
to the amount of reheat required. When reheat is de-energized, the cooling minimum airflow
setpoint is activated. The Trane VAV unit's SCR electric heater module takes a 0-to-10Vdc control
signal from AO2. Less than 0.2Vdc, heat if off. Above this to 10Vdc, the heater on-level is modulated
up to 100% on.
Important: Discharge air temperatures should not exceed between 100°F and 110°F , with 95°F
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– For electric reheat, the unit is in Pressure Independent mode or the Pressure Dependent
Mode Reheat Enable point is enabled.
Stage 1; the ON time is proportional to the amount of reheat required. For example, when 50%
of stage 1 capacity is required, reheat is ON for 90 seconds and OFF for 90 seconds. When 75%
of stage 1 capacity is required, reheat is ON for 135 seconds and OFF for 45 seconds. When 100%
of stage 1 capacity is required, reheat is ON continuously.
Stages 2 and 3; the same ON time logic as stage 1, except stage 1 is always energized. For
example, when 75% of unit capacity is required, stage 1 is energized continuously, and stage
2 is ON for 90 seconds and OFF for 90 seconds. When reheat is de-energized, the cooling
minimum airflow setpoint is activated.
being optimal for zone temperature and comfort control. At elevated discharge air
temperatures, room stratification may result in uneven air distribution and zone
temperature complaints. Refer to
CAUTION
Table 6 and Table 7, p. 51 for fan powered reheat.
VAV-SVX07B-EN