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I have a nordyne g7. It will start, the flames ignite, it…

Customer Question
I have a nordyne g7...
I have a nordyne g7. It will start, the flames ignite, it runs for 5-10 minutes then I get an open limit switch fault. The blower continues to run for 3 minutes, the green light stops flashing and red and green go solid and the process repeats.
Submitted: 2 years ago.Category: HVAC
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Answered in 6 minutes by:
2/12/2016
HVAC Technician: Brian HVAC Guy, HVAC Technician replied 2 years ago
Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
Satisfied Customers: 1,024
Experience: I have been an HVAC/R mechanic/technician for 30+ years.
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Hi Luke, I'm Brian, an HVAC expert here at Just Answer and I'm here to help!

The fact that the furnace runs for so long before shutting down on high limit usually indicates either a weak high limit switch, or diminished airflow because of a dirty air filter (the most common reason for this), dirty blower wheel or dirty air conditioning a-coil. I would check for the airflow conditions first and if all is clean then I would replace the limit switch with a new one with the same ratings. I would also check the temperature rise of the furnace and compare it to the data plate on the furnace. The plate should have a 'Max Temp Rise' or the like. That is the temperature difference from entering to leaving air on the furnace. If the furnace is running at or near the max, then there is some kind of diminished airflow problem.

Let me know if you have further questions on this subject. Otherwise, please remember to rate before leaving (3 stars or higher please!). Bonuses are always appreciated! Thanks for allowing me to help and thanks for using Just Answer!

Thank you,
Brian

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Customer reply replied 2 years ago
Air flow is not restricted. Fan blades are clean. The limit that is tripping is on the heat exchange. The ambient temperature yesterday was 10℉, today it is 25℉. The thermostat is set at 68℉. Today the problem is gone.
HVAC Technician: Brian HVAC Guy, HVAC Technician replied 2 years ago

Ok, it sounds like the high limit switch may be getting weak and tripping lower than it should. The way to double check that is to check the temperature rise by measuring temperature of the return air and the supply air and subtracting. Compare that to the "Max Temp Rise" on the furnace's data plate. If the temp rise is well under the max temp rise, then you most likely have a weak limit switch.

-Brian

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