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I have a furnace (Model: NUGK050CF01) that has recently been experiencing some weird behavior. The furnace will come on (I can see the flames) and it will run for a little over a minute. Then the flames will go out for about 7 minutes, then the cycle repeats itself. The blower is on continuously while all of this happens.

I've done some research and this kind of behavior can be consistent with a dirty flame sensor or if the furnace isn't getting enough airflow. I have checked my air filter and it looks fine. I was unable to located the flame sensor and it looks like my unit does not have one. I've included some pictures.

Furnace

Furnace model number

  • Hello, and welcome to Stack Exchange. That's a pretty old unit; this and this may be the service manual. – Daniel Griscom Dec 7 '16 at 11:23
  • Are you certain there isn't an internal thermostat which shuts off the burner head when the local temperature gets too high? Any way to check on the output air temperature? – Carl Witthoft Dec 7 '16 at 14:30
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    I agree with Carl, this sounds like a typical case of tripping the high limit (in other words, the furnace is overheating). When you say the blower is on, are you talking about the fan which sends air throughout the house or the combustion air blower (large brown thing) directly below the burners? – Connor Bredin Dec 7 '16 at 14:34
  • Does the unit display any error codes? – Tester101 Dec 7 '16 at 14:52
  • Do you have a manometer and a multimeter? If not, go get them. – Tester101 Dec 7 '16 at 15:57
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The high limit was indeed being reached. This was confirmed by watching the unit trip the high limit after the burners were turned on.

I had thought that the unit was getting enough air since my filter was new. It turned out that the secondary heat exchanger was packed full of matted dog hair and dust. I'll explain how I cleaned it.

  1. Remove service panels, disconnect wires (orange stars), disconnect spark plug (pink star). Then remove the two screws marked with the blue stars to removed electrical box, gently set aside. Furnace with service panels removed
  2. Disconnect the 3 wires attacked to the blower, be sure to note how they were connected, then remove the two screws that keep the blower from sliding out on the metal track (visible in the upper left). Then slide out blower. Furnace blower
  3. Things should then look like this. The secondary heat exchanger is in the back and above the visible foil. Furnace with blower removed
  4. This is what the heat exchanger looked like, and what it looked like after a bit of cleaning. Filthy heat exchanger Cleaner heat exchanger

I then vacuumed the rest of the heat exchanger as well as everything else to try and reduce future buildup. Everything is then put back in reverse order.

Thank you everyone for your help!

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