13
another day when conditions are more suitable but-
DO NOT FAIL TO TEST. If the outdoor ambient is low
and the unit operates properly on the heating cycle,
you may check the pressure cutout operation by
blocking off the indoor return air until the unit trips.
13. Once the heating has been confirmed, raise the
temperature setting until the second stage heating
makes contact. Supplemental resistance heat, if installed
should now come on. Make sure it operates properly.
14. For thermostats with emergency heat switch, set
thermostat to Emergency Heat mode. The heat pump
will stop, the blower will continue to run, all heaters
will come on and the thermostat emergency heat light
will come on. Confirm heaters operate normally.
Final System Checks
15.
Check to see if all supply and return air grilles are
adjusted and the air distribution system is balanced for
the best compromise between heating and cooling.
16. Check for air leaks in the ductwork. See Sections on Air
Flow Adjustments.
17. Make sure the unit is free of “rattles”, and the tubing
in the unit is free from excessive vibration. Also make
sure tubes or lines are not rubbing against each other or
sheet metal surfaces or edges. If so, correct the trouble.
18.
Set the thermostat at the appropriate setting for cooling
and heating or automatic changeover for normal use.
19. Be sure the Owner is instructed on the unit operation,
filter, servicing, correct thermostat operation, etc.
Refrigeration Performance Check
Check that compressor RLA corresponds to values shown in
Appendix B. RLA draw can be much lower than values listed at
low load conditions and low ambient condensing temperatures.
Values in Appendix B can slightly exceed at high load
conditions and high ambient condensing temperatures.
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 the “condenser” becomes the
“evaporator”, “evaporator” becomes “condenser”, “cooling”
becomes “heating”.
Heating Cycle
The heat pump switches from cooling cycle by redirecting
refrigerant flow through the refrigerant circuit external to
the compressor. This is accomplished by 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.
The following figures show a schematic of a heat pump on
the cooling cycle and the heating cycle. The heat pump is
equipped with thermal expansion valves for the indoor and
outdoor coils. It is also provided with a defrost control system.
Heating
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
During operation the Defrost signal to the circuit board
is controlled by a temperature sensor, which is clamped
to a feeder tube entering the outdoor coil. Defrost timing
periods of 30,60 and 90 minutes may be selected
