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My furnace model is Lennox Merit Series G40UH-69D-155-01. It was
installed Apr. 2001.

Here are my questions.
1) What is the possible problem?

The primary/secontary limit switch is open. Could be something else though.


2) What does "primary/secondary limit switch open" mean?

It is not working. Closed mean it is making contact or "on" while open
means "off".

3) Is it still under warranty? The Lennox website says "5-Year Limited
Warranty on covered components."

Yes, if it is a covered component. No if it is not a covered component.

Simplified, the thermostat senses the need for heat. It closes a swithch
that starts the furnace operation. The igniter glows, then the gas valve
opens to alow gas to flow to the burners. Once it reaches a pre-set
temperature, the blower kicks in and distgributes the heated air. When the
thermostat says "it is warm enough", the burner shuts down, but the blower
will run until a low limit switch says the heat exchanger is cooled off
enough. Then the blower stops. If the controls think the heat exchanger is
still hot due to a faulty switch/sensor, the blower will continue to run.

I'm not a technician so I can't give you more details than that. It is a
bit more comple than my simplified version and I'm not qualified to give
specifics. Call the heater guy.