18
88-4WCA4001-1A-EN
Sequence of Operation
G
Ge
en
ne
erra
all
Operation of the unit heating and cooling cycles is
automatic when the system is in the H
HE
EA
AT
T or C
CO
OO
OL
L
functions (the optional automatic changeover
thermostat, when in the A
AU
UT
TO
O position, automatically
changes to heat or cool with an appropriate room
temperature change). The fan can be set to O
ON
N,
causing continuous evaporator (indoor) fan operation
or set to A
AU
UT
TO
O causing fan operation to coincide with
heating or cooling run cycles. Continuous fan mode
during Cooling operation may not be appropriate in
humid climates. If the indoor air exceeds 60% relative
humidity or simply feels uncomfortably humid, it is
recommended that the fan only be used in the A
AU
UT
TO
O
mode.
C
Co
oo
olliin
ng
g M
Mo
od
de
e
N
No
otte
e:: The T
TS
SH
H and T
TS
SC
C are contacts that are internal
to the indoor comfort control.
With the disconnect switch in the O
ON
N position, current
is supplied to the control transformer. The cooling
cycle is enabled through the low voltage side of the
control transformer to the “R
R” terminal on the indoor
thermostat. With the comfort control set to A
AU
UT
TO
O and
T
TS
SC
C--1
1 contacts closed, power is supplied to the “O
O ”
terminal on the indoor thermostat to the switchover
valve coil ((S
SO
OV
V)). This energizes the switch-over valve
((S
SO
OV
V)) and places it in the cooling position (it is in the
heating position when de-energized).
When the indoor temperature rises 1-1/2 degrees, T
TS
SC
C--
2
2 contacts close, supplying power to the ""Y
Y"" terminal
on the indoor thermostat, and to the compressor
contactor ((C
CC
C)). This starts the outdoor fan motor and
compressor. The T
TS
SC
C--2
2 contacts also provide power to
the “G
G” terminal which provides power to the indoor
fan motor.
H
He
ea
attiin
ng
g M
Mo
od
de
e
With the comfort control set to O
ON
N, current is supplied
to the transformer. Starting at the “R
R” terminal on the
indoor comfort control, current goes through the
system switch (which is in “A
AU
UT
TO
O” position) to the
T
TS
SH
H--1
1 contacts. When closed, these contacts supply
power to terminal “Y
Y” on the indoor thermostat as well
as to the heating anticipator. The switch-over valve will
not energize because of the high resistance of the
heating anticipator in the thermostat. Power is
provided from “Y
Y” to the compressor contactor ((C
CC
C))
which starts the compressor and outdoor fan motor.
The indoor thermostat contact T
TS
SH
H--1
1 also provides
power to ""G
G"" terminal on the indoor thermostat
energizing the indoor fan motor.
S
Su
up
pp
plle
em
me
en
ntta
arry
y H
He
ea
att
The supplementary electric heat is brought on when
the indoor temperature drops 1-1/2 degrees below the
thermostat setting. T
TS
SH
H--2
2 contacts close providing
power to the “W
W” terminal on the indoor thermostat
and to the supplementary heater control circuit. An
outdoor thermostat may have been added to disallow
the second stage (if provided) of electric heat above a
selected outdoor temperature. If the outdoor
temperature falls below the setting on the outdoor
thermostat, this additional heater stage will come on.
When the outdoor air temperature rises, and the
outdoor T-stat setpoint is reached, the system will
revert back to first stage electric heating.
When the indoor ambient is satisfied, T
TS
SH
H--2
2 contacts
will open and the unit will revert back to the
compressor only heating mode and then off. For
e
em
me
errg
ge
en
nccy
y h
he
ea
att (use of supplementary electric heat
only), an emergency ((E
EM
ME
ER
RG
G)) heat switch is provided
within the comfort control. When placed in the
emergency heat position, it will disable the
compressor, bypass the outdoor thermostats, if
provided, and engage the supplementary electric
heaters and indoor fan.
D
De
em
ma
an
nd
d D
De
effrro
osstt O
Op
pe
erra
attiio
on
n
During the heating cycle, the outdoor coil may require
a defrost cycle which is determined by the demand
defrost control ((D
DF
FC
C)). This control continuously
measures the outdoor coil temperature ((C
CB
BS
S)) and the
outdoor ambient temperature ((O
OD
DS
S--B
B)) and calculates
the difference or delta-T measurement. When the
calculated delta-T is met, the demand defrost control
((D
DF
FC
C)) opens the circuit to the outdoor fan motor
((O
OD
DM
M)) and energizes the switch-over valve ((S
SO
OV
V)),
placing the unit in the cooling mode to defrost the
outdoor coil. The outdoor coil temperature sensor
((C
CB
BS
S)) terminates the defrost cycle, or times out after
fifteen minutes in defrost, the ((D
DF
FC
C)) energizes the
outdoor fan motor ((O
OD
DM
M)) and twelve seconds later de-
energizes the ((S
SO
OV
V)), which returns the unit to the
heating mode. Supplementary electric heat, if
provided, is brought on to control indoor temperature
during the defrost cycle.
D
De
effrro
osstt C
Co
on
nttrro
oll
The demand defrost control measures heat pump
outdoor ambient temperature with a sensor located
outside the outdoor coil. A second sensor located on
the outdoor coil is used to measure the coil
temperature. The difference between the ambient and
the colder coil temperature is the difference or delta-T
measurement. This delta-T measurement is
representative of the operating state and relative
capacity of the heat pump system. Measuring the
change in delta-T determines the need for defrost. The
coil sensor also senses outdoor coil temperature for
termination of the defrost cycle.
N
No
otte
e:: Refer to the SERVICE FACTS for fault detecting,
test sensor and checkout procedures.
