Our furnace is pretty old and seems to have an intermittent problem. It will run fine for awhile and then it will kick on, and after the blower shuts off there is something you can still hear running, like a motor. It will just keep running until something is done. My boyfriend has taken to smacking the metal box and this usually works, but not all of the time, and it's also annoying and probably causing further damage to the unit. There is a black switch on the top of this box that will shut the motor off. Sometimes if you leave that switch off for awhile and turn it back on the furnace starts to function properly again. Sometimes turning the switch back on only makes the motor come back on and stay there again. It also has another intermittent problem, I am not sure if it is related or not. When the blower comes on, it will blow for a second or two and then shut off, then it will do it again 5 or 6 times. Sometimes it goes on and stays there until the cycle is completed like it is suppose to. Any advice? Thanks

What make, model is this? Are you trying to fix this yourself or just want to pare down the possibilities for a tech?

I don't think smacking the unit is doing any good.

It's a Duo-therm model 714080. I would like to try to repair it. Do you think the limit switch might be bad?

Thanks

The short cycling could simply be a temp sensor that needs annual cleaning. The draft inducer run-on could be a bad controller board. I'm not real familiar with that unit, so try posting at Mobile Home Repair (http://www.mobilehomerepair.com/). They are much more familiar with this unit.