Heating Mode - Carrier TRANSICOLD 69NT20-274 Operation And Service

b. Heating (See Figure 2-2.)
The unit will only heat when the controller is set
above ---10_C (+14_F) as relay TH is electronically
locked out to prevent heating when the controller is set
below ---10_C (+14_F).
If the air temperature decreases 0.5_C (0.9_F) below
controller set point, TH closes and the system enters the
heating mode which is designed to raise the container air
temperature. When TH closes, power flows through TH
contacts and the heat termination thermostat to energize
the heat relay (HR). This in turn energizes the heaters
and heat light. The evaporator fans continue to run to
circulate air throughout the container.
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TO CONTROL TRANSFORMER
= Energized Circuit
Figure 2-2. Heating Mode
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As the supply air decreases to the in-range tolerance
below set point, relay TI and the in-range light
de-energize (after 15 minutes time delay) and will remain
de-energized until the supply air increases to a tolerance
below set point. (Refer to section 1.13.4 Code 29)
When the temperature rises to 0.2_C (0.4_F) below
set point, TH opens (heating off) and the system again
enters the holding zone. The compressor and condenser
fan motor are not running as contactors C and CF remain
de-energized. The evaporator fans continue to run to
circulate air throughout the container.
A safety heater termination thermostat (HTT)
attached to an evaporator coil support, set to open at
54.5_C (130_F) will open the heating circuit if
overheating occurs.
= De-energized Circuit
2-4