Please bear with me as I know little if anything about heaters but am patient when it comes to sorting through a home repair problem, handy with tools and can follow directions.

I have a 25 year old central forced air gas furnace. According to the label inside, it is a Carrier model number 396GAW000075. I first noticed that occasionally after the fan would shut down, following its cool down cycle, about 30 seconds later it would come back on for about a minute and then turn off. Then I discovered, over the last two nights, right after the set-back time on the thermostat, if the heater happened to be on, the fan would continue to run long after the burners shut down. I checked the thermostat and confirmed that the fan was set to "automatic". I feared that I would have to unplug the furnace as the only means to shut off the fan. As a last resort, I over-rode the temperature set on the thermostat and got the burners to light again. This time the fan shut off following the cool down cycle. As yet, I have not noticed the continuous running happen at any other time.

In searching this and other sites for a diagnosis of my situation, it seems that I may have a failing fan limit control switch. If that is the case, none of the answers I looked at described where it would be located on the furnace or what it looks like.

Any assistance would be greatly appreciated.

Gerry

That system uses a control board for the blower(no fan limit control) A sticking relay or tripped limit will keep the blower running. Are the filters clean?

Yes they are both clean and in properly. I have pictures of the unit and the schematic if they will be of any help.

I appreciate the assistance.

I did manage to remove the schematic from the furnace upper cover and scan it. Hopefully it will be legible enough to help sort out what I need to do. I also included photos of both the upper and lower portions of my furnace to aid in identify model and in identifying where things are so I know what to do.

Again, thanks for the help.

I have the same furnace. When the blower stays on for long periods, i.e. gas burn is over, the relay [2A on the schematic] is stuck. Mine would stick intermittently. If you take the control board out, you may be able to remove the clear plastic cover on the relay and use a fine file to clean the points that make the contact for 110VAC powering the blower. [The white nylon base of the relay has 2 protrusions that fit into retangular holes in the clear cover.] I think an extra pair of hands makes getting the cover off easier. I actually unsoldered the 4 points that attach the relay to the CB, removed the relay, then opened it and cleaned the points.

This solved the main problem, i.e. blower not going off. The blower still seems to intermittently come on and off a couple of times at the beginning and end of each heat cycle. Maybe the time delay part of the CB is at fault?

You can find what is listed as the replacement circuit bd for this model on the internet for less than $100 [eBay the cheapest offerings], however all the photos show 2 relays on the CB, not just 1 as the original. Maybe they redesigned for independent relays for heat and cool cycles, but I don't have AC. So I thought I would stop at 'servicing' the relay until I get a new furnace.

This old furnace is probably operating at less than 60% efficiency. A new one would be in the mid 90%s. Gas savings should be substantial.

While I appreciate the response, I repaired this problem just about 2 years ago by replacing the circuit board with a new one, bought on eBay for less that $50 if I recall correctly. I service it annually and it still runs just fine (knock on wood).

Again, thanks for taking the time.