Defrost Control
(See page 55 for more information)
Defrost Operation– Heat Pump
Units Only
This section applies to heat pump units only. Defrost operation
is required when compressor operation is in the Normal
Heating state when frost build up on the outdoor coil interferes
with the normal heat transfer from the outdoor coil to the
ambient air. When defrost operation is active, compressor
operation remains in the Normal Heating state but the device
control is overridden until the defrost operation is complete.
While defrost operation is active, the auxiliary supplemental
heat source in the unit is use to maintain comfort conditions.
Defrost Operation State
When outdoor coil defrosting is required defrost operation
transitions through the defrost states shown in the following
state diagram:
Figure 22: Defrost Operating States
Defrost Initiation (Heat Pump)
DPS heat pump units require a defrost cycle to remove frost
build-up that can accumulate on the outdoor coil during certain
heating operating conditions. The controller assumes there
is a potential for frost build-up on the coil when the Defrost
Temperature (DFT) is less than 14.0°F and the saturated
suction temperature is below 32°F.
When a compressor is operating under these potential frosting
conditions for at least the MinCmpOpTm= period (Default=10
minutes) and the accumulated compressor operation time
since the last defrost cycle exceeds the MinAccCmpTm=
value (Default=40 minutes), the controller can initiate a defrost
cycle base on temperature, time or on a manual request as
described in the following sections.
Temperature Defrost
A defrost cycle is initiated based on temperature when the
Defrost Temperature (DFT) drops below a defrost temperature
value (Tdef) and the saturated discharge temperature is below
109°F continuously for 3 minutes. The Tdef value is adjusted
automatically between (+) 14°F and (-) 13.0°F based on
outdoor air temperature. This value is not directly adjustable
but can be “biased” by setting the Tdef Adj= parameter
(Default=0°F) between -3.6°F and +3.6°F. Note: To improve
performance the Tdef value is also automatically adjusted after
each defrost cycle depending on the length of the cycle.
Timed Defrost
A defrost cycle is initiated based on time when the
compressors have accumulated enough time operating in
potential frosting conditions. The controller accumulates
minutes of operation in low frost potential (LoFrstAccTm=) and
high frost potential (HiFrstAccTm=) conditions. Whenever the
HiFrstAccTm= time plus ½ the LoFrstAccTm= time exceed the
MaxFrostTm= setting (Default=120minutes) a defrost cycle is
initiated.
Manual Defrost
A defrost cycle is manually initiated when the Manual DF=
parameter is set from No to Yes. Note: Once the defrost cycle
begins the Manual DF= parameter automatically reverts to No.
Defrost Initialization
This state prepares the system for switching the 4-way valve to
the cooling position
OM 1141-3 • MICROTECH UNIT CONTROLLER 120 www.DaikinApplied.com
heat puMp
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