Daikin DCC090 Series, Service Instructions Manual

Page: 62 / 139

SYSTEM OPERATION
16
When the heat pump is on the heating cycle, the outdoor coil
is functioning as an evaporator. The temperature of the
refrigerant in the outdoor coil must be below the temperature
of the outdoor air in order to extract heat from the air. Thus,
the greater the difference in the outdoor temperature and the
outdoor coil temperature, the greater the heating capacity of
the heat pump. This phenomenon is a characteristic of a heat
pump. It is a good practice to provide supplementary heat for
all heat pump installations in areas where the temperature
drops below 45° F. It is also a good practice to provide
sufficient supplementary heat to handle the entire heating
requirement should there be a component failure of the heat
pump, such as a compressor, or refrigerant leak, etc.
Since the temperature of the refrigerant in the outdoor coil on
the heating cycle is generally below freezing point, frost forms
on the surfaces of the outdoor coil under certain weather
conditions of temperature and relative humidity. Therefore, it
is necessary to reverse the flow of the refrigerant to provide hot
gas in the outdoor coil to melt the frost accumulation. This is
accomplished by reversing the heat pump to the cooling
cycle. At the same time, the outdoor fan stops to hasten the
temperature rise of the outdoor coil and lessen the time
required for defrosting. The indoor blower continues to run and
the supplementary heaters are energized.
D
EFROST
C
ONTROL
NOTE: DCH models have one stage of mechanical heating.
The defrost accumulation period will start when either first or
second stage defrost thermostat closes. Defrost termination
occurs when both thermostats open or the 10 minute cycle
has completed.
During operation the power to the circuit board is controlled
by a temperature sensor, which is clamped to a feeder tube
entering the outdoor coil. Defrost timing periods of 30,60 and
90 minutes may be selected by connecting the circuit board
jumper to 30, 60 and 90 respectively. Accumulation of time
for the timing period selected starts when the sensor closes
(approximately 31° F), and when the wall thermostat calls for
heat. At the end of the timing period, the unit’s defrost cycle
will be initiated provided the sensor remains closed. When
the sensor opens (approximately 75° F), the defrost cycle is
terminated and the timing period is reset. If the defrost cycle
is not terminated due to the sensor temperature, a ten minute
override interrupts the unit’s defrost period.
R
EFRIGERATION
S
YSTEM
C
HECKS
Ensure the hold-down bolts on the compressor are secure
and have not vibrated loose during shipment. Check that
vibration grommets have been installed. Visually check all
piping and clamps. The entire refrigeration system has been
factory charged and tested, making it unnecessary to field
charge. Factory charges are shown on the unit nameplate.
F
INAL
S
YSTEM
C
HECKS
1. Check to see if all supply and return air grilles are
adjusted and the air distribution system is balanced for
the best compromise between heating and cooling.
2. Check for air leaks in the ductwork. See Sections on Air
Flow Adjustments.
3. Make sure the unit is free of “rattles”, and the tubing in the
unit is free from excessive vibration. Also make sure
tubes or lines are not rubbing against each other or sheet
metal surfaces or edges. If so, correct the trouble.
4. Set the thermostat at the appropriate setting for cooling
and heating or automatic changeover for normal use.
5. Be sure the Owner is instructed on the unit operation,
filter, servicing, correct thermostat operation, etc.