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4.6.5
Heat Mode
When Heat Mode is required the system operates with evaporator fans energized. Operation of heaters is deter-
mined in accordance with the calculated Controlling Temperature (CT) and the difference between Setpoint (SP) in
accordance with the following table:
HTCON1 may also be activated when the unit is in Continuous Operation Cooling to assist in balancing unit capacity
with the load. In all other operating states and modes HTCON1 and HTCON2 will not be activated.
4.6.6
Null Mode
For descriptions of operation in the Null Mode, refer to
4.7
DEFROST
Defrost is an independent cycle that overrides cooling and heating functions in order to de-ice the evaporator.
When the unit enters defrost the MessageCenter will display DEFROST CYCLE STARTED for the first 10 seconds
and then display the default message for the remainder of the cycle. DEFROST is displayed in the Operator Mes-
sage Panel, along with the setpoint while the unit is in the cycle.
Defrost is allowed when the DTT is less than 40.0°F (4.4°C).
OR
If DTT sensor alarm is active, the RAT is less than 45.0°F (7.2°C).
OR
If the alarms for both DTT and RAT alarms are active, then defrost can be initiated when the SAT is less than
45.0°F (7.2°C).
4.7.1
Defrost Initiation
Defrost can be initiated by pressing the MANUAL DEFROST key or automatically by the control system. Control
system initiation is based on coil condition or expiration of the defrost timer.
a. Defrost based on coil condition
Defrost based on coil condition will be initiated when blockage is sufficient to cause an air pressure differential
across the coil great enough to close the contacts of the defrost air switch (DAS).
OR
The control system has calculated there is a loss of heat transfer due to ice on the coil. This method of defrost initi-
ation is called a saturation based demand defrost. This method is allowed when the unit has been in low speed or
electric mode cool for five minutes, the evaporator fans have been in operation for seven minutes, and the evapo-
rator expansion valve is operating at less than 90%.
OR
When the RAT - 1DTT temperature is greater than 36°F (20°C) for 15 continuous minutes. This method of defrost
initiation will only be used for 3 consecutive defrosts. After three consecutive defrost initiations by this method the
counter will be reset to allow this defrost initiation again only after a defrost cycle is initiated by another method
(timer, air switch or manual).
Control Temperature with Fuel Saver OFF
HTCON1
Switch On
when CT-SP is equal to or less than -0.54°F (-
0.3°C)
Switch Off
when CT-SP is greater than -0.36°F (-0.2°C)
HTCON2
Switch On
when CT-SP is less than -3.6°F (-2.0°C)
Switch Off
when CT-SP is greater than -3.24°F (-1.8°C)
Control Temperature with Fuel Saver ON
HTCON1
Switch On
when CT-SP is less than -1.44°F (-0.8°C)
Switch Off
when CT-SP is greater than -1.26°F (-0.7°C)
HTCON2
Switch On
when CT-SP is less than -3.6°F (-2.0°C)
Switch Off
when CT-SP is greater than -3.24°F (-1.8°C)
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