I have a old Duo Therm furnice. I have had to replace the thermial fan switch, and the man. Limit switch. Now it is warming up and the man. Limit switch shuts it all down before it reaches the temp. and then that has to be reset. My fan is blowing every few min and only for 30 sec. even if the furnice is cooled down. I'm lost can someone please help me... :confused:

Soulds like your fan and limit control is defective or out of range.

OR

There could be a problem with the blower motor or motor circuit.