Daikin DBC Series, Installation Instructions Manual

16

HEAT PUMP OPERATION

Cooling Cycle

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”.

Heating Cycle

The heat pump operates in the heating cycle by redirecting 

refrigerant flow 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 figures 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 flowrator distributors and 

perform the same function on the heating cycle as on the 

cooling cycle. The flowrator distributors also act as check 

valves to allow for the reverse of refrigerant flow.

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.

Defrost Control

Note:

 DBH 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.