Why doesn’t the gas furnace ignite

furnacegashvacrepair

I had a Lennox slp98dfv installed last year – it has a surelight ignition – it now just blows cold air – so I assume the pilot light is out – but not sure how to light it – as u cannot manually ignite the burners – any thoughts before I call in the repairman?

Best Answer

Diagnostic Display

According to the Installation Instructions, your model is equipped with a diagnostic display.

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To review the any error codes:

Press the diagnostic push button and hold it to cycle through a menu of options. Every five seconds a new item will be displayed. Release the button when the desired mode is displayed.

When a solid "P" is displayed, the furnace capacity/size is programmed.

When a solid "E" is displayed, the control enters the Error Code Recall mode.

Error Code Recall mode menu options:

  • No change (displaying error history) remains in Error Code Recall mode.
  • solid "b" exits Error Code Recall mode.
  • solid "c" clears the error history. Must press button while flashing "c" is displayed to clear error codes.

When the solid "-" is displayed, the control enters the Field Test mode.

Field Test mode menu options:

  • Solid "C" starts pressure switch calibration.
  • blinking "-" exits Field Test mode.

Firing Sequence

If you don't want to get into all that, you can simply use the firing sequence to try and troubleshoot the problem. Start by turning the breaker, and any serviceman switch on. Next turn the thermostat up, until it calls for heat.

First, you should hear a small click, ping, ding, or some other sound, to indicate the gas valve opened (may be too quiet to hear on some valves). Next you'll be able to hear a quiet hiss, as the gas starts to flow. The ignitor; either a spark gap or hot surface, will turn on. You'll either hear a rhythmic click-click-click, or see a small bit of metal turn red. At the same time, the draft inducer should turn on. This may be quite noisy, and could make diagnosing problems by sound difficult.

No Flame

If a flame is not sensed within a few seconds (may be variable depending on model), The gas valve will close, the ignitor will turn off, and the draft inducer will purge the system for 15 seconds (may be variable depending on model). The sequence should repeat again, for up to 5 tries (may be variable depending on model). If after the 5th try, there is still no flame. The system will purge for 20 seconds, then shut down and display and error code.

Flame

If a flame is sensed, the furnace will wait for the flame to stabilize. Once a good flame is detected, the indoor blower should start, and warm air will be blown throughout the house.

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