System Operation Information, Heating Mode

to remove any dirt and lint that may have ac- cumulated in the compartment or on the blower and motor. Dirt and lint can create excessive loads on the motor resulting in higher than normal operating temperatures and shortened service life.

Heat Exchanger and Burner Maintenance The furnace should operate for many years without excessive soot buildup in the ﬂue pas- sageways, however, the ﬂue passageways, the vent system, and the burners should be inspected and cleaned (if required) by a quali- ﬁed serviceman annually to ensure continued safe operation. Particular attention must be given to identify deterioration from corrosion or other sources.

SYSTEM OPERATION

INFORMATION

Proper maintenance is most important to achieve the best performance from a furnace. Follow these instructions for years of safe, trouble free operation.

•Do not place combustible materials on or against the furnace cabinet or the vent pipe.

•Do not store gasoline or any other ﬂam- mable vapors and liquids in the vicinity of the furnace.

•Change or replace the air ﬁlters monthly during any period when the circulating blower is operating regularly.

•Always replace the doors on the furnace after servicing. Do not operate the furnace without all doors and covers in place.

•Avoid operating the furnace when win- dows and doors are open.

•Be sure that the thermostat is properly installed and is not being affected by drafts or heat from lamps or other appliances.

Sequence of Operation

Operating sequences for the heating, cooling, and fan modes are described below. Refer to the wiring diagrams (Figures 29) and the voltage ﬁeld wiring diagram (Figure 26).

Heating Mode:

1.On a call for heat the thermostat closes, applying 24 VAC to the W terminal on the control board.

2.The control board checks for continuity on the 24VAC limit control circuit (over-tempera- ture limit switch, ﬂame rollout switches and blocked vent switch in series).If an open limit is detected the control board will energize the inducer and the conditioned air blower. All other system functions will be inoperable until the limit circuit closes. While the limit is open, the red LED will pulse at a rate of

1blink per unit time.

3.The furnace control checks for continuity across the pressure switch (24 VAC). If the pressure switch is closed the heat mode sequence will not continue. If it remains closed for 10 seconds the red LED will blink

3times repetitively until the fault condition clears.

4.The inducer is energized.

5.The pressure switch will close. If the pres- sure switch does not close after 10 seconds the fault LED will blink 2 times repetitively and the inducer will continue to run until the switch is closed.

6.The inducer will pre-purge for 30 seconds and then the igniter will start its warm-up as follows:

Initial Power up: After 30 seconds of igniter warm-up the gas valve (24 VAC) will then open. The igniter circuit will stay energized for 3 seconds after the gas valve opens.

After Initial Power up: The control has a programmed adaptive ignition feature which varies the warm-up period as follows: If igni- tion is successful the warm-up is reduced by 3-seconds on each subsequent call for heat until ignition failure occurs. Upon igni- tion failure, the warm-up is increased by 3-seconds on the next try. If successful, the timing remains ﬁxed at this level. In general, whenever ignition failure occurs the warm-up interval is increased by 3-seconds on the next try. And if successful, it remains there. Minimum and maximum warm-up time limits are set at 6 and 54-seconds, respectively.

7.The furnace control must prove ﬂame via the ﬂame the ﬂame sensor 5 seconds after the gas valve opens. If ﬂame is sensed, all burners are on and the igniter cools off. If no ﬂame is sensed, the gas valve closes im- mediately and the inducer continues to run.