Service reference manual sequence of operations – Armstrong World Industries HWC PREMIER 183 User Manual
Page 29
Service Reference Manual
SEQUENCE OF OPERATIONS
2-19
SRM-HW/HWC 2/99
LIMIT OPENS
If the
limit switch
opens for any reason during a call for
heat, the following happens:
1.
If the
limit switch
senses that the temperature in
the unit is too high, the contacts between terminals
C
and
NC
on the
limit switch
open and the con-
tacts between
C
and
NO
close. This interrupts the
24-volt signal to the
pressure switch
and also de-
energizes terminal
P.SW
on the
ignition control
.
The
gas valve
closes immediately, as the 24-volt
signal to terminal
P.SW
is no longer present. The
induced draft blower
and the
circulating air
blower
continue to run. Power to
blower control
board
terminal
W
is maintained. The blowers
continue to run until the
limit switch
closes or the
heat call at the
thermostat
is satisfied.
2.
When the
limit switch
senses that temperatures in
the unit are normal, the contacts between terminals
C
and
NO
open, the contacts between terminals
C
and
NC
close and operation of the unit returns to
normal.
PRESSURE SWITCH OPENS (BLOCKED FLUE)
1.
If blockage of the flue occurs, the negative pressure
in the
induced draft blower
is reduced. At the set
point of the
pressure switch
, the contacts open.
This interrupts the 24-volt signal to terminal
P.SW
on the
ignition control
. The
gas valve
closes
immediately, as the relay in the
ignition control
opens and interrupts the signal to terminal
V2
of the
ignition control
.
2.
Terminal
W
on the
blower control board
stays
energized and the
circulating air blower
contin-
ues to run. If the negative pressure is restored, the
pressure switch
closes and sends a 24-volt signal
to the
P.SW
terminal on the
ignition control
. The
ignition control
makes a trial for ignition again and
operation returns to normal.
Note: If the
P.SW
and
TH
terminals energize at the
same time, the
ignition control
will not respond.
A
pressure switch
being stuck closed is an
example of what would cause this to happen.
ROLLOUT SWITCH ACTIVATED
1.
If the conditions in the burner compartment cause
the
rollout switch
to trip, the contacts on the switch
open, interrupting the 24-volt signal going to termi-
nal
P.SW
on the
ignition control
. This causes the
ignition control
to de-energize terminal
V1
,
interrupting the 24-volt signal to the
gas valve
and
immediately closing it.
2.
The reason the
rollout switc
h was tripped must be
determined before corrective action is taken.
3.
The signal to terminal
P.SW
can only be restored if
the
rollout switch
is reset, which can only be done
manually. The
W
terminal on the
blower control
board
remains energized and the
circulating air
blower
continues to operate. Once the
rollout
switch
is reset, the operation of the unit returns to
normal.
CALL FOR COOLING
Line Voltage
On a call for cooling, the thermostat closes the circuit
between
Y
,
G
and
R
.
1.
With
Y
energized, the
contactor
closes. This
sends line voltage to
compressor
terminal
C
,
causing the
compressor
to start. The
outdoor fan
also starts, drawing air through the
outdoor coil
.
2. Terminal
G
on the
blower control board
also
energizes at the same time as
Y
. This starts the
timer on the
blower control board
. Approximately
15 seconds later (or immediately - see note on page
2-20), the terminal marked
COOL
energizes. This
sends line voltage to the
circulating air blower
,
starting the blower.
3.
The unit continues cooling as long as both
Y
and
G
are energized. When the cooling call is satisfied,
the circuit between
Y
,
G
and
R
is interrupted. The
contactor
immediately opens, interrupting power to
the
compressor
and the
outdoor cooling fan
.
4.
With the low voltage signal to terminal
G
on the
blower control board
interrupted, the board starts
a countdown to blower “off”. After approximately 90
seconds (see note on page 2-20), the cooling speed
relay opens and the
circulating air blower
shuts
down.
Low Voltage
1.
On a call for cooling, the
thermostat
closes the
circuit between
R
and terminals
G
and
Y
.
2.
The 24-volt signal energizes terminal
G
on the
blower control board
, starting the fan “on” timer.
Approximately 15 seconds later (or immediately -
see note on page 2-20), the relay closes. This
sends line voltage to the
COOLING
speed terminal
on the
blower control board
, which starts the
circulating air blower
.
3.
The signal from
Y
energizes the
contactor
, starting
the
compressor
and the
outdoor cooling fan
.
4.
When the cooling call is satisfied, both
G
and
Y
are
de-energized. The
contactor
opens immediately,
causing the
compressor
and the
outdoor cooling
fan
to stop.