FIGURE 4

FIGURE 4
In addition to a reversing valve, a heat pump is equipped
with an expansion device and check valve for the inside coil,
and similar equipment for the outside coil. It is also provided
with a defrost control system.
The expansion device performs the same function on the
heating cycle as on the cooling cycle. The check valves are
required due to the reverse flow of refrigerant when changing
from cooling to heating or vice versa.
When the heat pump is on the heating cycle, at which time
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 for the
refrigerant in the outdoor coil to extract heat from the air.
Thus, the greater the difference in outdoor temperature and
outdoor coil temperature, the greater the heating capacity of
the heat pump. Since this is characteristic of heat pumps, it