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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

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:

  • dirty air filter
  • 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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Your furnace is shutting down on limit. There can be a number of causes for this.

Start by making sure you have a clean filter in the furnace. A dirty or restrictive filter can cause this issue. Alternatively you could also have a dirty or plugged up A/C evaporator coil which is located above your furnace. This can also cause a furnace to cycle off on high limit, however is uncommon unless you have run your furnace without a filter for prolonged periods of time. Also if you have a high efficiency furnace, there is more of a chance to have the secondary heat exchanger plug up rather than the A/C coil.

You can also have a blower motor issue. The motor can be faulty or seizing up causing it to not spin fast enough to move enough air through the furnace. Another cause may be that you have a faulty limit that is opening prematurely.

The major issue that can cause this problem is a heat exchanger issue. If your furnace is cycling off on high limit, and everything checks out, you may have an internal blockage of the heat exchanger. If this is the case either the heat exchanger or the furnace will need to be replaced. A combustion analysis of the furnace is required to prove this is the issue.

Additionally, with a high efficiency furnace you could have a bad ventor motor gasket which is allowing air to be drawn in behind the motor. Big cracks in the condensate collector pan can also cause this but is very uncommon.

If your unit operates normally and will only sometimes trip on limit, then you may have a faulty limit that is opening prematurely. The furnace will automatically reset after the limit re-makes or after 4 minutes (whichever is longer). If your unit continues to not reset well after the 4 minute mark, to may have a faulty limit that is not closing after the unit cools down.

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