Lennox International Inc Gas Units, G61MP Series Units Heating Sequence of Operation

Heating Sequence of Operation

NOTE − The thermostat selection DIP switch on the control board is factory−set in the −STAGE" position.

Applications Using a Two−Stage Thermostat

A − Heating Sequence −− Control Board Thermostat Selection DIP switch in T wo−Stage" Position (Factory Setting)

1 − On a call for heat, thermostat first−stage contacts close sending a signal to the integrated control. The inte- grated control runs a self-diagnostic program and checks high temperature limit switches for normally closed contacts and pressure switches for normally open contacts.The combustion air inducer is ener- gized at low speed and the humidifier terminal is ener- gized.

2 − Once the control receives a signal that the low−fire pressure switch has closed, the combustion air induc- er begins a 15−second pre−purge in low speed.

3 − After the pre−purge is complete, a 20−second initial ig- nitor warm−up period begins. The combustion air in- ducer continues to operate at low speed.

4 − After the 20−second warm−up period has ended, the gas valve is energized on low fire (first stage) and ignition oc- curs. At the same time, the control module sends a sig- nal to begin an indoor blower 45−second ON−delay. When the delay ends, the indoor blower motor is ener- gized on the low fire heating speed. The furnace will continue this operation as long as the thermostat has a first−stage heating demand.

5 − If second−stage heat is required, the thermostat sec- ond−stage heat contacts close and send a signal to the integrated control. The integrated control initiates a 30−second second−stage recognition delay.

6 − At the end of the recognition delay, the integrated con- trol energizes the combustion air inducer at high speed. The control also checks the high−fire (second stage) pressure switch to make sure it is closed. Once the the control receives a signal that the high−fire pres- sure switch is close, the high−fire (second stage) gas valve is energized and the indoor blower motor is en- ergized for operation at the high−fire heating speed.

7 − When the demand for high−fire (second stage) heat is satisfied, the combustion air inducer is switched to the low−fire heating speed and the high−fire (second stage) gas valve is de−energized. The low−fire (first stage) gas valve continues operation. The indoor blower motor is switched to the low−fire heating speed.

8 − When the thermostat demand for low−fire (first stage) heat is satisfied, the gas valve is de−energized and the field−selected indoor blower off delay begins. The combustion air inducer begins a 5−second post−purge period.

9 − When the combustion air post−purge period is com- plete, the inducer and humidifier terminal are de−ener- gized. The indoor blower is de−energized at the end of the off delay.

Applications Using A Single−Stage Thermostat

B − Heating Sequence −− Control Board Thermostat Selection DIP switch in tage" Position

NOTE − In these applications, two−stage heat will be initi- ated by the integrated control if heating demand has not been satisfied after the field adjustable period (10 or 15 minutes).

1 − On a call for heat, thermostat first−stage contacts close sending a signal to the integrated control. The inte- grated control runs a self-diagnostic program and checks high temperature limit switches for normally closed contacts and pressure switches for normally open contacts.The combustion air inducer is ener- gized at low speed and the humidifier terminal is ener- gized.

2 − Once the control receives a signal that the low−fire pressure switch has closed, the combustion air induc- er begins a 15−second pre−purge in low speed.

3 − After the pre−purge is complete, a 20−second initial ig- nitor warm−up period begins. The combustion air in- ducer continues to operate at low speed.

4 − After the 20−second warm−up period has ended, the gas valve is energized on low fire (first stage) and ignition oc- curs. At the same time, the control module sends a sig- nal to begin an indoor blower 45−second ON−delay. When the delay ends, the indoor blower motor is ener- gized on the low−fire heating speed. The integrated control also initiates a second−stage on delay (factory− set at 10 minutes; adjustable to 15 minutes).

5 − If the heating demand continues beyond the second− stage on delay, the integrated control energizes the combustion air inducer at high speed. The control also checks the high−fire (second stage) pressure switch to make sure it is closed. Once the control receives a sig- nal the high−fire pressure switch is closed, the high−fire (second stage) gas valve is energized and the indoor blower motor is energized for operation at the high−fire heating speed.

6 − When the thermostat heating demand is satisfied, the- combustion air inducer begins a 5−second post−purge. The field−selected indoor blower off delay begins.

7 − When the combustion air post−purge period is com- plete, the inducer and humidifier terminal are de−ener- gized. The indoor blower is de−energized at the end of the off delay.

Page 52