Daikin DBC Series - Heat Pump Operation

16


When the heat pump is in the cooling cycle, it operates
exactly as a Summer Air Conditioner unit. In this mode,
all the charts and data for service that apply to summer
air conditioning apply to the heat pump. Most apply on
the heating cycle except that “condenser” becomes
“evaporator”, “evaporator” becomes “condenser”, “cooling”
becomes “heating”.

The heat pump operates in the heating cycle by redirecting
refrigerant ow through the refrigerant circuit external to
the compressor. This is accomplished with through the
reversing valve. Hot discharge vapor from the compressor
is directed to the indoor coil (evaporator on the cooling
cycle) where the heat is removed, and the vapor
condenses to liquid. It then goes through the expansion
device to the outdoor coil (condenser on the cooling cycle)
where the liquid is evaporated, and the vapor goes to the
compressor.
When the solenoid valve coil is operated either from
heating to cooling or vice versa, the piston in the reversing
valve to the low pressure (high pressure) reverse positions
in the reversing valve.
The following gures show a schematic of a heat pump
on the cooling cycle and the heating cycle. In addition
to a reversing valve, a heat pump is equipped with an
expansion device and check valve for the indoor coil, and
similar equipment for the outdoor coil. It is also provided
with a defrost control system.
The expansion devices are owrator distributors and
perform the same function on the heating cycle as on the
cooling cycle. The owrator distributors also act as check
valves to allow for the reverse of refrigerant ow.
When the heat pump is on the heang cycle, the outdoor
coil is funconing 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 dierence in the
outdoor temperature and the outdoor coil temperature,
the greater the heang capacity of the heat pump. This
phenomenon is a characterisc of a heat pump. It is
a good pracce to provide supplementary heat for all
heat pump installaons in areas where the temperature
drops below 45° F. It is also a good pracce to provide
sucient supplementary heat to handle the enre heang
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 heang cycle is generally below freezing point,
frost forms on the surfaces of the outdoor coil under
certain weather condions of temperature and relave
humidity. Therefore, it is necessary to reverse the ow
of the refrigerant to provide hot gas in the outdoor coil
to melt the frost accumulaon. This is accomplished by
reversing the heat pump to the cooling cycle. At the same
me, the outdoor fan stops to hasten the temperature
rise of the outdoor coil and lessen the me required for
defrosng. The indoor blower connues to run and the
supplementary heaters are energized.

Note: DBH models have one stage of mechanical heang.
The defrost accumulaon period will start when either
rst or second stage defrost thermostat closes. Defrost
terminaon occurs when both thermostats open or the 10
minute cycle has completed.