Lennox G51MP Goes out after 1 hour


Old 12-11-10, 09:43 PM
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Lennox G51MP Goes out after 1 hour

My 7 year old Lennox G51 furnace has an intermittent failure where if it runs for more than 1 hour continuously the flames go out, it begins blowing cold air and the status LEDs indicate that it has entered a Watchguard state. This failure is repeatable and always happens after 1 hour.

When I turn the thermostat off and then on again, the furnace lights first time and continues without any problems... for an hour until it starts blowing cold air again.

If I do nothing and let the blower run continuously for another hour then the furnace will restart and work fine for another hour until it goes out again.

The watchguard mode indicates that either the
  1. burners failed to ignite (This isn't likely since I've never seen the furnace fail to light first time)
  2. limit open longer than 3 minutes
  3. flame sense lost >4 times

I've looked everywhere on the internet and I am absolutely baffled by this behavior! Does anyone have any idea what the problem might be? Or even how to describe the failure mode I'm experiencing?

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Old 12-11-10, 10:26 PM
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Have you cleaned the flame sensor?

Old 12-12-10, 03:48 AM
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If this only happens after the furnace has been burning continuously for an hour I would suspect the high temperature limit. The high temperature could be caused by a number of different things including a plugged return air filter, too high a gas pressure, too small of ductwork, (either supply or return) or too low a blower speed. If the furnace has been operating for a long time (months to years) without a proper return air filter then it could be a plugged heat exchanger.
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