Heating Systems 323
When the heating cycle occurs, the indoor coil, outdoor coil, and fans are
reversed. The outdoor coil is now responsible for extracting heat from the
outside air and passing it along the indoor coil, where it is released into
the duct network for distribution.
During the heating cycle, any refrigerant that is circulating in the
outside coil is changed into a low-temperature gas. It is purposely made
to be substantially colder than the outside air. Since heat energy always
moves from hot to cold, there is a transfer of heat from the outside air to
the cold refrigerant. In a sense, we can say that the heat of the cold outside
air is absorbed by the much colder refrigerant gas.
The compressor of the system is responsible for squeezing together
the heat-laden gas that has passed through the outside coil. This action is
designed to cause an increase in the pressure of the gas that is pumped to
the indoor coil. As air is blown over the indoor coil, the high-pressure gas
gives up its heat to the air. Warm air is then circulated through the duct
network to the respective rooms of the system.
When the refrigerant gas of the indoor coil gives up its heat, it cools
and condenses into a liquid. It is then pumped back to the outside coil by
compressor action. Once again it is changed into a cool gaseous state and
is applied to the outside coil to repeat the cycle. If the outside temperature
drops too low, the refrigerant may not be able to collect enough heat to
satisfy the system. When this occurs, electric-resistance heaters are ener-
gized to supplement the heating process. The place where electric heat is
supplied to the system is called the balance point.
Figure 12-9 shows an illustration of heat-pump operation during its
heating cycle. At (1), the heat is absorbed from the cold outside air by the
pressurized, low-temperature refrigerant circulating through the outside
coil. As (2), the refrigerant is applied to the compressor and compressed
into a high-temperature, high-pressure gas. At (3), the heated gas is trans-
ferred to the indoor coil and released as heat. At (4), warm air is circulated
through the duct network. Note that the supplemental resistance heat ele-
ment is placed in this part of the system. At (5), the refrigerant is returned
to the compressor and then to an expansion device, where the liquid re-
frigerant is condensed and then returned to the outdoor coil. The cycle re-
peats itself from this point.The Cooling Cycle of a Heat Pump
A heat pump is designed to respond as an air conditioning unit dur-
ing the summer months. For this to occur, the reversing valve must be