Lennox International Inc SL280UH090V48B Heating Operation With Two−Stage Thermostat

Applications Using A Single−Stage Thermostat See figure 38 for ignition control sequence

B − Heating Sequence −− Integrated Control Thermostat Selection DIP Switch 1 ON 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 (7 or 12 min- utes).

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.

2 − Once the control receives a signal that the low pres- sure switch has closed, the combustion air inducer be- gins a 15−second pre−purge in low speed.

NOTE − If the low fire pressure switch does not close the combustion air inducer will switch to high fire. After a 15 second pre−purge the high fire pressure switch will close and the unit will begin operation on high fire. After 10 to 20 seconds of high fire operation the unit will switch to low fire.

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 30−second ON−delay. When the delay ends, the indoor blower motor is ener- gized on the low fire heating speed and the HUM con- tacts are energized. The integrated control also initi- ates a second−stage on delay (factory−set at 7 minutes; adjustable to 12 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. 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 low speed post−purge. The field−selected indoor blower off delay begins. The indoor blower operates at the low−fire heating speed.

7 − When the combustion air post−purge period is com- plete, the inducer, the HUM contacts as well as the 120V ACC terminals are de−energized. The indoor blower is de−energized at the end of the off delay.

HEATING OPERATION WITH TWO−STAGE THERMOSTAT

1	15	35	39	30* second 80	5 SEC	blower
ON	Pre−Purge	Ignitor Warm−up	Trial For	blower	Post	delay
OFF			Ignition		Purge
1 stg heat demand
low speed CAI
ignitor
low fire gas valve
indoor blower low heat

2 stg heat demand	30 seconds RECOGNITION PERIOD
high speed CAI
high fire gas valve
indoor blower high heat

* Conventional thermostat 30 seconds, icomfort Touch® thermostat set for 30 adjustable 15 to 45 seconds.

FIGURE 37

HEATING OPERATION WITH SINGLE STAGE THERMOSTAT

		1		15		35	39	30* second80
	ON		Pre−Purge		Ignitor Warm−up		Trial For
								blower
							Ignition
	OFF
heat demand
low speed CAI
ignitor
low fire gas valve
indoor blower low heat

5 SEC

Post Purge

blower

delay

7 or 12 minutes after heating demand begins, furnace switches to high fire (depends on setting)

high speed CAI

high fire gas valve indoor blower high heat

* Conventional thermostat 30 seconds, icomfort Touch® thermostat set for 30 adjustable 15 to 45 seconds.

FIGURE 38

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