Armstrong World Industries 122, 243, 302, 242, 123, 203, 182, 183 manual Limit Opens

SRM-HW/HWC 2/99

Service Reference Manual

9.When W de-energizes, the gas valve immediately shuts down and the induced draft blower also stops after several seconds.

10.The blower control board starts a countdown to blower “off”. Approximately 90 seconds later, the circulating air blower shuts down.

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 circuit between terminals C and NC on the limit switch opens. This closes the circuit between terminals C and NO on the limit switch, interrupting the 24-volt signal to the pres- sure switch and the rollout switch and causing terminal P.SW on the ignition control to de- energize. The gas valve immediately closes, as there is no longer a 24-volt signal at terminal V1 on the ignition control. With 24 volts still going to the blower control board and terminal TH on the ignition control, the circulating air blower and induced draft blower continue to run.

2.The circulating air blower and the induced draft blower continue to run as long as there is a call for heat from the thermostat or the limit switch remains open.

3.When the limit switch senses that the temperature in the unit is low enough, the circuit between C and NO opens and the circuit between C and NC closes.

4.With the limit switch closed, the unit again makes a trial for ignition and returns to normal operation.

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 coming from terminal NC on the limit switch. Terminal P.SW on the ignition control de-energizes, as does terminal V1. With loss of the 24-volt signal to the gas valve, the valve closes immediately.

2.Terminals TH on the ignition control and W on the blower control board remain energized. The induced draft blower and the circulating air blower continue to run.

3.When the pressure switch senses that there is enough negative pressure within the flue, it will again close the circuit between the limit switch and terminal P.SW on the ignition control.

4.The unit makes a trial for ignition and returns to normal operation.

SEQUENCE OF OPERATIONS

2-25

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. This interrupts the 24-volt signal going to terminal P.SW on the ignition control, causing terminal V1 on the ignition control to de-energize. This interrupts the 24-volt signal to the gas valve and immediately closes it. Since terminal TH on the ignition control and W on the blower control board remain energized, the induced draft blower and the circulating air blower continue to run.

2.The 24-volt signal to the ignition control can only be restored if the rollout switch is manually reset.

3.Once the cause of the rollout switch being tripped has been determined, the switch can be reset.

4.With the rollout switch reset, terminal P.SW on the ignition control again energizes. The ignition control makes a trial for ignition and operation returns to normal.