I have a Bryant gas sys. @ 20 yrs. Old. When the heat goes off the fan continues to run. If I cut it off on the wall and back on it will continue to run.

Most companies build overrides into their designs. For example, you would not want to turn the blower motor on until the heat exchanger was hot enough to release heat into the house, so they may hold off on starting the blower motor until a temperature level is met and then turn the blower motor on.

You would really have to read the furnace's schematic to see how it actually works. Usually the schematic is with the furnace or attached to the furnace. I'm not a HVAC guy but if a motor does not shut off it is because something did not tell it to.

Take a hard look at the motor circuits and see if you can see a shutoff timer. If you can find it on the schmatic, then trace backwards to see what conditions have to be made to turn off the motor.

A Volt/Ohm meter or a clamp on Am Meter would be real handy for you right now, or an experienced HVAC tech. Personally I'd vote for the tech.

One other quick check would be to make sure no one has changed the setting to have the fan run all the time. Usually this is a switch on the Thermostat. It will usually say, Fan ON --- Auto. Make sure it is set to Auto.

With a 20 year old unit you probably have a fan and limit control mounted to the furnace at the wall area behing the gas valve. Usually this is located to the right or left of the gas valve up near the top of the compartment.

Fan/limit controls do go out of calibration

See the sticky notes at the beginning of this board to locate the control.

Look at this page for the two types of fan controls.

https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313-2.html

Hvac,

How far off were my suggestions. I'm curious and I can always use the education.

My furnace, oil, was rejected by Noah when he was building the Ark. It was out of date by then. This heap of metal also supplies hot water to the rooms for heating the house.

Your suggestions were good. I usually try all the simple things first. Home owners usually do not have meters and many tools to work with but some of them do and some of them know how to use them.

Most of the time we are guessing at what is wrong because we are not there.

Bryant furnaces also has a few different setups for fan control. The one I mentioned is the easiest for a homeowner to find. If his system does not have that style then it could get more difficult.

The only way we learn in life is practise and that takes some time.
There are many things in the HVAC biz that I am not good at because my company never worked on the items.