62-12259 4–12
4.8 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).
4.8.1 Defrost Initiation And Start
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.
4.8.1.1 Defrost Based on Coil Condition
Defrost based on coil condition will be initiated when blockage is sufficient to cause an air pressure differ-
ential 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 initiation 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 evaporator 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 three 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 initi-
ated by another method (timer, air switch or manual).
TIP
Ice is not the only thing that will cause the air differential to increase across the evaporator coil. Shrink
wrap, paper, plastic bags, and other such items when caught in the return air stream and pulled up
against the evaporator coil or the return air grille can also cause the DAS contacts to close.
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