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V. CHECK HEATING CYCLE OPERATION
Place thermostat selector switch at HEAT and reset the
space set point above ambient temperature so that a call for
heating is ensured. Compressor will start within 5 minutes.
Observe system operation.
VI. CHECK COMPRESSOR OIL LEVEL
After adjusting the refrigerant charge, allow the system to
run fully loaded for 20 minutes. Running oil level should be
within view in the crankcase sight glass. Stop the compres-
sor at the field power supply disconnect and check the crank-
case oil level. Add oil only if necessary to bring the oil into
view in the sight glass. If oil is added, run the system for an
additional 10 minutes, then stop and check oil level. If the
level remains low, check the piping system for proper design
for oil return; also check the system for leaks.
If the initial check shows too much oil (too high in the sight
glass) remove oil to proper level. See Preliminary Oil
Charge, page 12, for proper procedure for adding and remov-
ing oil.
When the above checks are complete, repeat the procedure
with the unit operating at minimum load conditions. Unload
the compressor by disconnecting the field-control circuit lead
at TB2
.
Y2
Reconnect the field-control circuit lead when checks are
complete.
VII. FINAL CHECKS
Ensure all safety controls are operating, control panel covers
are on, and the service panels are in place.
VIII. SEQUENCE OF OPERATION
A. Heating
Place thermostat selector at HEAT and set temperature
selector above room ambient.
B. Cooling
Place thermostat selector at COOL and set temperature
selector below room ambient.
When thermostat calls for unit operation (either heating or
cooling), the indoor-fan motor starts immediately. The
outdoor-fan motors and compressor start within 3 seconds to
5 minutes depending on when unit was last shut off by ther-
mostat, because unit contains a Time Guard® circuit. When
first-stage cooling is required, thermostat (TC1) closes, caus-
ing the heat pump to start with an unloaded compressor.
When TC2 closes, demanding additional cooling, the com-
pressor loads to full load operation.
During heating, compressor is always fully loaded. When
TH1 demands first-stage heating, the heat pump starts
within 3 seconds to 5 minutes depending on when unit was
last shut off by thermostat, because unit contains a Time
Guard circuit. (The defrost board has speed terminals to
shorten this cycle.) When TH2 of the thermostat closes, aux-
iliary heat supply (electric strip heat) is energized in 1 or
2 stages depending on number of stages available and
whether outdoor thermostats are closed.
Defrost is achieved by reversal from heating to cooling cycle
and deenergization of outdoor-fan motors, allowing hot
refrigerant gas to defrost outdoor coil. Defrost is achieved
with a timer set to initiate defrost every 30, 50 or 90 minutes
(factory set at 30 minutes).
Defrost is initiated when refrigerant temperature leaving
the outdoor coil is measured below 27 F, (typically when the
outdoor ambient temperature is below 45 F as sensed by the
defrost thermostat [DFT]).
Defrost is terminated when: The refrigerant temperature
rises to 65 F (80 F for 541A180) at the DFT location on the
liquid line; or the refrigerant pressure rises to 280 psig at
the HPS2 location on the liquid line; or the defrost timer
completes the 10-minute cycle.
Malfunction
The high-pressure switch, loss-of-charge switch, oil pressure
safety switch, and compressor overtemperature safety are
located in a Signal-LOC™ circuit that prevents heat pump
operation if these safety devices are activated. A light at the
thermostat energizes when Signal-LOC circuit is affected.
The lockout system can be reset by adjusting the thermostat
to open the contacts (down for Heating mode, up for Cooling
mode), deenergizing the Signal-LOC circuitry. Compressor
overcurrent protection is achieved with a circuit breaker
which requires manual resetting at the outdoor unit control
box.
The unit is equipped with an oil pressure safety switch that
protects the compressor if oil pressure does not develop on
start-up or is lost during operation. The oil pressure switch
is of the manual reset type and therefore must be reset at
the outdoor unit. DO NOT RESET MORE THAN ONCE.
If oil pressure switch trips, determine cause and correct. DO
NOT JUMPER OIL PRESSURE SAFETY SWITCH.
To reset the oil pressure switch:
1. Disconnect power to the unit.
2. Press the RESET button on the oil pressure switch.
3. Reconnect power to the unit.
Unit is equipped with a no-dump reversing valve circuit.
When unit is in Cooling mode, reversing valve remains in
cooling position until a call for heating is requested by ther-
mostat. When unit is in Heating mode, reversing valve
remains in heating position until there is a call for cooling.
The crankcase heater is in a lockout circuit. If crankcase
heater is defective, compressor is locked off. Heat pump
remains off until corrective action is taken. The lockout cir-
cuit cannot be reactivated by adjusting the thermostat. To
reset the crankcase heater lockout, disconnect and then
reconnect power to unit.
C. Liquid Line Solenoid
To minimize refrigerant migration to the compressor during
the heat pump OFF cycle, the unit features a bi-flow liquid
line solenoid valve. The valve opens when the compressor is
energized, and closes when the compressor is deenergized.
This feature reduces compressor flooded starts, significantly
increasing compressor life.
D. Accumulator
The unit accumulator controls the rate of liquid refrigerant
to the compressor during heat pump operation. The accumu-
lator features a unique method for returning oil to the com-
pressor. The oil return mechanism is external to the
accumulator. The mixture of oil and refrigerant is metered to
the compressor by a brass orifice which is removable and
cleanable. The oil return mechanism also contains a solenoid
valve that opens when the compressor is ON and closes
when the compressor is OFF. This keeps the liquid refriger-
ant stored in the accumulator from draining to the compres-
sor during the heat pump OFF cycle, which further protects
the compressor against flooded starts.
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