When the thermostat calls for heat, the fan kicks on immediately, heats home until thermostat says it is warm enough. Heat gas kicks off and fan dies. 30 seconds later, the fan limit control kicks fan on again for about 1 minute and then stops.

I don't think this is a fan limit control problem. What could be happening?
Scott

Weird. I don't see how a simple failure of either the relay or the limit switch could cause that. I would hate to tell you to spend hundreds replacing the control board when that wasn't the problem. You other choice may be to call a technician and pay a $100 in labor plus the circuit board that you could have replaced easily.

If you have a digital thermostat, it could be having a problem too. You could isolate that by removing the thermostat, connecting the red and white wires, and letting the furnace run for a while and then breaking the connection. If it runs normally, the problem is in the thermostat.

I guess you could clean up the limit switch and check all the connections, but that is grasping at straws.

This is a circa 1984 whirlpool furnace. It has no circuit board whatsoever that I can find. What I do see and perhaps suspect is the "combustion air relay"
There is small button on this to manual activate a heating cycle. If I push in the button hard, gas, ignition and fan all come on at the same time. If I push the button in half way, gas comes on, ignition, heat is generated until limit control switch turns on fan, which is what I think is the normal way this should operate. Does this indicate a bad combustion air relay, or is this somehow being jumpered so that when heat is called for, the fan always runs?
That just wouldn't make much sense to me.
Thanks for the comments,
Scott

This is a new problem and nothing else was worked on recently? Do you have a fan that provides combustion air? It almost sounds like the combustion air relay is turning the air circulation fan on too. I am not seeing how such a problem could have developed itself. Trouble shooting and repairs are easier with individual components rather than a board. Can you find a schematic for the furnace?

To answer your questions, yes it has a combustion air fan. Yes I see a schematic of inside the cover of the furnace.

I replaced the thermostat this summer as the old one got smashed. However, I have tested the furnace with the thermostat wires at the furnace disconnected with same problem. I found this "problem" when the furnace would run for a few minutes (november in minnesota) and then stop with the fan kicking back on a few seconds later to finish the cooling.

After figuring out how the limit control is supposed to work, I discovered that basically the limit control is not even being used. The furnace kicks on gas, ignites and fan all at the same time, and kills them all at the same time.
However, because there is still quite a bit of heat in the chamber, the limit control then heats up and cycles the fan back on. So I'm positive the limit control is working properly... I think.

The manual fan control at the thermostat works as it should, as does the airconditioning (call for cold, fan on) I kind of am thinking that this could still be a fan relay also. This is a whirlpool from 1984. Any ideas on parts?
Scott

Turns out that the combustion relay is providing power to the ignitor, gas valve, the combustion air fan, AND the low-speed blower fan and the schematics show that it should be that way.

That means that this furnace is NOT waiting for the chamber to heat up and then the limit switch turns on the blower fan. Since the limit control high setting was at 140 or so, it NEVER turns on the fan. Thus when the thermostat stops asking for heat, the combustion relay shuts off and removes power to the low-speed blower fan. Heat remaining in the chamber is causing the limit switch to finally kick back on to cool down the chamber (thus the furnace running, shutting off, and the fan kicking back on 30 seconds later for several minutes)

Here is what I have done and I hope it makes sense:
I LOWERED the high limit switch setting. I turned on the heat and let it run for 3 minutes to get a true HIGH limit reading on the dial, then turned the high limit switch down to just a little below that. Low setting is now at about 90 and high limit is at about 125 or so (who knows how accurate the dial is?? )

So here is what happens now,
Thermostat asks for heat. Combustion relay turns on gas, ignitor, combustion air blower and low-speed blower fan.
Chamber heats up to more than limit switch (~125 degrees) and turns on low-speed blower fan (already running)
Thermostat quits asking for heat, drops gas, ignitor, compustion air blower and low-speed blower fan. However because the limit switch has now turned on, the fan continues to run until chamber cools to 90.

Does this make sense?
I guess I've never seen a furnace that just turns on the blower from the start, I thought they were always supposed to heat up first.
Scott

I think you about have it. I would disconnect the wire from the combustion relay to the low speed fan relay and let the limit switch turn on the fan after it heats up.

A lot of what I know, I have worked out like you adjusting the limit switch. Posting back like you did helps and encourages me plus helps others that may read your answer. Yours is one of many questions I couldn't answer, but was able to help somebody figure out.

Can you give me a basic idea of what the high and low limit control settings are typically?

My feeling is that the limit switch was replaced (I found the old one next to the blower fan underneath the control panel) but the installing tech did not realize how this was wired or he would have set the limits as I did or changed the wiring as you are suggesting.
At this point, I have no idea what the "NORMAL" settings are.
I'd like to pull the wire as you state. I think it is better to not blow cold air throughout the house but wait until the air is hot.

Thanks again for your help and constructive comments.
Scott

Somebody new showed up at https://www.askmehelpdesk.com/showthread.php?t=14562

It is an oil furnace, but his figures may be better than my guess. Sure wouldn't hurt to have some good help here. I have helped with many problems, but don't have very wide experience. We wiil see how he does.