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My 8 yr. old Comfortmaker will turn on, burner will light, then will occasionally shut down 10 minutes later, even tho the temp is not up to the correct temp. Burner goes out, fan stays on, blows cold air. I can turn off the unit at power switch, then when I turn on the burner will light, sometimes staying on and other times going off after 15 minutes. Why?

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If you have the model number you should be able to get a troubleshooting guide from the manufacturer. In my RV furnace, this would probably be the control board, the igniter, or the sail switch. –  Jay Bazuzi Mar 13 '11 at 20:38
Sounds like it could be a failed/failing high limit switch. What might be happening is the limit switch is shutting off the burner, then causing the blower to stay on to cool down the "over heated" furnace. Does the blower turn off eventually, or does it continue to blow until you shut it off at the switch? –  Tester101 Mar 17 '11 at 16:08
I agree that it sounds like a limit switch issue, had a similar problem with my own furnace a couple of years back. Same symptoms, burn would turn on and then off but the blower fan would run for ages trying to cool the system down. –  Zach Aug 18 '11 at 2:56

1 Answer 1

Sound like this may be the same problem as described on Why does my furnace need frequent power resets?

You didn't mention if the furnace comes back on on its own or not. If it doesn't (or doesn't as often as it should), it sounds like the exact same problem I posted about in the above link: a dirty flame sensor. If this is the case, cleaning it will probably make the problem go away.

If the furnace is coming back on on its own, then:

  • Chimney is clogged causing exhaust to come back down the chimney (backflow)
  • Backflow sensor is clogged/broken
  • Flame sensor is dirty/broken
  • Control board is faulty

Checking the sensors is probably easiest.

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