Service Reference Manual
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
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
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
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
SEQUENCE OF OPERATIONS
2.The reason the rollout switch 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
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
Low Voltage
1.On a call for cooling, the thermostat closes the circuit between R and terminals G and Y.
2.The
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
