Gas and Oil Home Heating Furnaces - fan stays on

I have a fairly old forced air gas Janitrol, at least 30 years old. The fan will not stop running when the heating cycle completes. The furnace has done this twice before this winter, and has gone back to normal after 24 hours of continous fan running. This time it has been running for almost three days. Is there anything I can do before call for service? I have tried some cleaning around the fan area, wiping dust off different areas, but to no avail. I have run the furnace by turning the power switch on and off during the day, and letting it run at night.

the fan limit sticking. in your model i believe it is located on the firewall of the furnac above the gas valve, possibly a rectangular cover. in side is a round dial with 3 settings. one brings the fan on at proper bonnet temp, off after heat cycle, and a high limit that opens the circuit to the gas valve if an over heating codition occurs. sounds like the switch is sticking. it is attached to a round spiral bimetal probe that sticks into the heat exchanger area. try this, believe it or not, when heaqt cycles off and fan stays on, hit the side of the furnace with your hand, MANY will cycle off as this proves the limit switch is sticking. also, because this device contains a safety, (the high limit) and it's relatively inexpensive price, i would replace it.

Thank you very much! Your suggestion worked. I hit the furnace several times without anything happening. Then I took the back of a screwdriver and hit the inside of the furnace 2-3 times, and sure enough the fan stopped. I waited for the next heating cycle to make sure the fan went on and off and normal, and it did. Considering the age of the unit, I think it would be wise to look for something new this summer. Thanks again, I appreciate your swift response, which helped me solve this problem.

stick again, a new furnace is a great idea due to the age of yours.