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Service reference manual sequence of operations – Armstrong World Industries HWC PREMIER 183 User Manual

Page 29

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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.

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