Heat Pump
www .DaikinApplied .com 67
OM 1263 • MICROTECH UNIT CONTROLLER
Defrost Control
(
See page 29 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 required 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 16: 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
Execute Defrost
The 4-way valve is switched to the cooling position to defrost
the coil . This state manages the refrigerant in the system and
assures the 4-way valve is seated before defrosting the coil .
Execute Defrost continues for 10 minutes or until the Defrost
Temperature (DFT) exceeds 51 .8ºF or the discharge pressure
exceeds 426 .7 psi .
Terminate Defrost
After the defrost operation is complete, the 4-way valve is
switched to the heating position . Terminate Defrost manages
the system refrigerant and assures the 4-way valve is seated
before returning to normal heating compressor operation .
Defrost Inactive
When defrost operation is in the inactive state, action of the
compressor devices is a dictated by the applicable heating or
cooling compressor state .
Summary of Contents for MicroTech III DPS
Page 113: ......
