Carrier 542E - Manual (Page 14)

deenergize indoor blower relay coil 2A and compressor con­
tactor coil 2D. All closed contacts return to their normally
open position and all three motors stop.
The heat pump has now returned to a “standby” condition
awaiting another “call” for heating by the room thermostat.
Except for the crankcase hater, all heat pump components
are now deenergized. If the room temperature continues to
rise, the heat pump will remain “off” and the cooling mode
will not start until the room thermostat SYSTEM switch is
moved to the COOL position (either COOL or AUTO when
using autochangeover thermostat P/N 34427DP115).
D. Defrost Cycle
The defrost control circuit consists of defrost timer 3M,
defrost thermostat switch 7M, and defrost relay 2P. The
defrost timer is factory-set for 90-minute intervals of
elapsed running time, which results in optimum heating
mode efficiency for most installations; however, the adjusta­
ble cam can be reset for a 30-minute interval when abnormal
climatic conditions dictate. These conditions include
excessive outdoor humidity at low outdoor temperatures or
any condition that results in frequent incomplete de­
frosting.
CAUTION: Kever adjust the factory-set 9()-minute interval
unless an observed defrosting problem exists. An occasional
deposit of frost on the outdoor coil, after a full 10-minute
time-terminated defrost cycle, does not indicate that a
problem exists. Frequent deposits of frost on the outdoor coil
that occur throughout a wide range of outdoor temperatures
indicate that a problem does exist.
With the heat pump operating in the heating mode, the
defrost cycle sequence of operation is as follows:
Defrost thermostat 7M switches to the closed position when
the outdoor coil refrigerant temperature drops to approx­
imately 30°F. When defrost timer 3M completes 90 minutes
of elapsed running time, the normally open timer contacts
between 3 and 4 close for approximately 10 seconds. If
defrost thermostat is not closed during the 10-second inter­
val, the defrost cycle does not begin and the defrost control
does not “look” for frost for another 90 minutes of elapsed
running time.
When normally open defrost timer contacts between 3 and 4
close for their 10-second interval, and defrost thermostat
switch 7M has switched close, the defrost control circuit is
completed and defrost relay coil 2P is energized.
Energized defrost relay coil 2P switches all of its contacts
and each of the following events occur simultaneously:
1. Normally open, high-voltage, defrost relay contacts be­
tween 7 and 9 close before defrost timer 3M contacts be­
tween 3 and 4 complete their 10-second interval. The
defrost control circuit is maintaned and defrost relay
coil 2P remains energized.
2. Normally closed, high-voltage, defrost relay contacts
between 7 and 8 open and the circuit through outdoor
fan motor 3D1 is broken. The motor stops running.
3. Normally open, low-voltage, defrost relay contacts be­
tween 1 and 3 close and complete the circuit to revers­
ing valve solenoid 5B. The reversing valve switches
from the normal heating mode position to the cooling
mode position and the hot discharge gas is directed to
the outdoor coil to melt the frost on the coil.
4. Normally open, low-voltage, defrost relay contacts be­
tween 4 and 6 close. Unit terminal E is now connected to
unit terminal W1 and the first bank of supplemental
electric heat is automatically energized. Terminals W1
and W2 of the room thermostat are internally con­
nected; therefore, any additional banks of supplemental
electric heat are also automatically energized.
The system is now heating the conditioned space with the
supplemental heaters while the outdoor coil is defrosting.
When the outdoor coil refrigerant temperature rises to ap­
proximately 67°F, defrost thermostat 7M switches to the
open position to terminate the defrost cycle; however, if the
defrost thermostat 7M has not terminated the defrost cycle
within a 10-minute interval, normally closed timer 3M con­
tacts between 3 and 5 automatically open and terminate the
defrost cycle.
When the defrost cycle is terminated by either the defrost
thermostat or the timer contacts, defrost relay 2P is
deenergized. Outdoor fan motor 3D1 restarts and reversing
valve solenoid 5B is deenergized. The reversing valve
switches to the nrmal heating mode position. The supple­
mental electric heaters are deenergized if the room ther­
mostat is not “calling” for second-stage heating.
The system has now returned to normal heating mode opera­
tion. After 90 minutes of elapsed running time, defrost timer
3M will automatically make another check to determine if
the outdoor coil requires defrosting.
E. Emergency Heat Operation
Thermostats P/N 34427DP115 and 34427DP118 have an
emergency heat switch (EM. HT.) which can be used if there
is a heat pump malfunction. When the switch is moved from
the NORM, position to the EM. HT. position, the compressor
and outdoor fan are deenergized, and the unit operates on
electrical resistance heat only.
F. Automatic Changeover Operation
When using autochangeover thermostat P/N 34427DP115,
switching to either heating or cooling is automatically con­
trolled by the room thermostat when the SYSTEM switch is
in the AUTO position.
To prevent unwanted cycling between heating and cooling
mode operation, the heating and cooling temperature selec­
tion levers provide for a 4°F minimum temperature setting
differential. For most installations, this differential will
probably be set closer to 10°F to comply with recommended
guidelines for energy conservation.
With both the SYSTEM and FAN switches in the AUTO
position, the heat pump will start operation in the heating
mode when the room temperature drops to a point that is
slightly below the heating control setting, or will start
operation in the cooling mode when the room temperature
rises to a point that is slightly above the cooling control set­
ting.
VHI. CARE AND MAINTENANCE
To ensure continuing high performance, and to minimize the
possibility of premature equipment failure, periodic mainte­
nance must be performed on this equipment. This packaged
heat pump should be inspected at least once each year by a
service person who is properly trained and equipped.
NOTE TO EQUIPMENT OWNER: Consult your local Dealer
about the availability of a maintenance contract.
WARNING: The ability to properly perform maintenance on
this equipment requires certain expertise, mechanical skills,
tools, and equipment. If you do not possess these, do not at­
tempt to perform any maintenance on this equipment other
than routine filter maintenance. A FAILURE TO HEED
THIS WARNING COULD RESULT IN SERIOUS PER­
SONAL INJURY AND POSSIBLE DAMAGE TO THIS
EQUIPMENT.
The minimum maintenance requirements for this equip­
ment are as follows:
1. Inspect air filter(s) each month. Clean or replace when
necessary.
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