Figure 7-11.Heating cycle.
supply air, which is distributed throughout the
controlled space. This temperature change removes
moisture from the air and forms condensate, which
must be piped away. The compressor suction pressure
draws the cool vapor back into the compressor where
the temperature and pressure are greatly increased.
This completes the cooling refrigerant cycle. A
graphic presentation of the nine steps of the cycle is
shown in figure 7-12.
As the outside temperature drops, the heat pump
runs for longer periods until it eventually operates
continually to satisfy the thermostat. The system
"balance point" is when the heat pump capacity
exactly matches the heating loss. The balance point
varies between homes, depending on actual heat loss
and the heat pump capacity. However, the balance
point usually ranges between 15°F and 40°F. Either
electric heat or fossil fuels provide the auxiliary heat.
Heat pumps operating at temperatures below 45°F
accumulate frost or ice on the outdoor coil. The
relative humidity and ambient temperature affect the
degree of accumulation. This ice buildup restricts the
airflow through the outdoor coil, which consequently
affects the system operating pressures. The defrost
control detects this restriction and switches the unit
into a defrost mode to melt the ice.
The reversing valve is energized and the machine
temporarily goes into the cooling cycle where hot
refrigerant flows to the outdoor coil. The outdoor fan
stops at the same time, thus allowing the discharge
temperature to increase rapidly to shorten the length of
the defrost cycle.
If there is supplemental heat, a
defrost relay activates it to offset the cooling released
by the indoor coil.
Conventional heat pump applications use electric
heaters downstream from the indoor coil. This design
prevents damaging head pressures when the heat pump
and auxiliary heat run simultaneously. The indoor coil
can only be installed downstream from the auxiliary
heat if a "fuelmaster" control system is used. This
control package uses a two-stage heat thermostat with
the first stage controlling heat pump operation and the
second stage controlling furnace operation.
Water chillers (figs. 7-13 and 7-14) are used in air
conditioning for large tonnage capacities and for
central refrigeration plants serving a number of zones,
each with its individual air-cooling and air-circulating