Furnace is shutting down before reaching thermosat

[wreini]

I have a 13 yr old furnace and the furnace runs for anywhere from 10 - 40 minutes and then shuts down and will not start again. I can get it to restart by turning off a disconnect switch I have and on again. I can do this as soon as it shuts the gas valve (the fan is still running for cool down) and it will start right back up with the fan never turning off (other than for the 2 seconds that I turn off the power at the disconnect. I don't believe that it is the over temp or flue sensor as those have a mechanical lockout.

Any other ideas and how to test it?

Thanks

[Dean]

See the other posts about furnace lock out problems.

What type of ignition, standing pilot, intermittent pilot, spark ignition?
Electronic module?
What type of furnace, regular basement upflow - natural gas?
Do you have a meter to test with?

Pretty tough to guess what's happening, especially without more information.

[wreini]

I have an updraft natural gas furnace in the basement w/ a hot surface ignitor. It is not a high eff. I do have a multi-meter. The real strange thing is that last night it seemed to be working again. I live in Michigan and the temps have been for the last week in the single digits and last night was in the upper teens. Like I said, it is 13 years old and has not had a problem keeping temp in the house.

[LazyPup]

When you say it is turning off, i presume you mean the gas burner is shutting off, while the fan continues to run. That is a normal function. There is a heat anticipator adjustment in the thermostat. The purpose of the anticipator is to turn the burner off a bit before it reaches the temp set point, anticipating that the residual heat in the plenum will be sufficient to meet the calling temp without running over temp. If the burner were to stay on until the calling temp was satisfied the residual heat in the plenum and ductwork would push the room over the setpoint.

[wreini]

I don't think that it is the thermostat. The relay on it is still indicating that it is closed (Energized) and the temp can be 10 degrees colder than the set point and it still won't start. Also, the disconnect switch would not reset the thermostat, only the furnace.

[kactuskid]

When you turn the furnace switch off and then on, what you're doing is resetting the safety interlock. If you open up the furnace cabinet you'll see some small lights that are blinking, The number of blinks is equivalent to a problem that should be outlined in your furnace manual. The problem could be a blocked flue or a cracked ignitor or something else. If your furnace filter is clean and allowing air flow, then you need to call in a service tech to diagnose the problem. Too many variables to really diagnose over the net.