I have an older Sears "Fifteen" horizontal gas furnace (#867767840) located in the crawl space. Recently it has been short cycling, meaning it starts a normal cycle, runs for approximately 1 minute, stops for several minutes, then starts another 1 minute cycle. This repeats until the set temperature is achieved. Also, the pilot light has gone out twice in the past two weeks.

Filters are new, all vents are open, returns are unobstructed.

So, I first replaced the thermocouple. And the pilot hasn't gone off again in several weeks, but the short cycling continues. I then replaced the thermostat. No change. What other things should I check? I am thinking it must be either a leak in the heat exchanger, or a faulty limit switch. Any help would be appreciated.

On the old systems there is a fan limit control. It cuts the furnace off if it overheats and also controls the blower so when the temperature is right the blower turns on and then off. Check the fan and limit.

Also

It could also be the blower motor since if it will not run or turn on the unit will cut off on high temperature.

Check the rating plate for the specified temperature rise (difference between the return and supply air temperatures)-usually a range is given. If you find that the rise is higher than the specified range, the furnace is overheating. This usually results from inadequate airflow or excessive gas input. Overheating will generally cause short-cycling via high limit control.

Thanks for your responses.

KC13 - where should I take the temperature readings to determine the rise? Also, what would cause excessive gas input?

I had a chance to observe the cycling of the fan limit control, and I noticed the following:

Limit set points:
1.Fan off ~90 deg.
2.Fan on ~130 deg.
3.Limit off ~175 deg.

Starting temp. in house is 61 deg.; Set thermostat to 68 deg.

The furance receives the call for heat, burners turn on, limit begins to spin.
The fan turns on normally as soon as the 'Fan On' limit is tripped.
But as soon as the fan starts blowing air the limit quickly spins down, and within 45-60 seconds it trips the 'Fan Off'. Adjusting the 'Fan Off' set point down to about 70 deg. Seems to keep it from tripping prematurely, but this also causes the fan to run for quite some time after the gas shuts off.

Is this indicative of a weak/faulty limit? Or is there something else causing the air to cool down too fast after the fan turns on?

It is a good possibility that the bimetal used in the control has lost its tension. Time for a new one.

It is also a possibility that the furnace is not firing at a full rate which could cause the same situation. Check the gas pressure leaving the gas valve or at the gas train tap to see if it matches the rate plate.

Thanks again hvac1000.

Can you clarify...
The rate plate you mentioned. Is that on the gas valve or the furnace?

On the furnace. It will be in inches of water column

I just noticed something.

When the limit control triggers the fan to run, the burners are turning off. Thus, the air cools down quite quickly, and the short cycle ensues. I assume that the burners should stay lit until the thermostat has been satisfied.. And in fact, I have seen the burners stay lit after the fan comes on, but this seems to happen sporadically and infrequently. What would cause this?

Does the limit trip before the fan starts?

At what indicated temperature does this occur (on the limit dial face)? The fan control portion of the fan/limit control isn't working right, or the switch trippers are not set correctly. As someone said, may be time for a new one.

Yeah it's weird... the limit control "appears" to be functioning properly.

I just watched it go through a few more cycles. The burners fire up, the limit control spins until it clicks the 'fan on' set point, then the burners go off and the fan comes on, the limit spins down until the 'fan off' set point clicks, the burners come back on... (repeat). Except that on the third or fourth cycle... the burners finally stayed on when the 'fan on' set point clicked.

Anyway... I think I'll be replacing the limit control as the next measure, unless anyone has a better idea. Thanks again for the input everyone.

Try turning the "fan on" down just a little, see what that does.

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Replace fan and limit control. Its old and worn out.

Well, I replaced the fan/limit control, but the problem persists.

I'm not sure what else to do.
My only other guess as to the problem would be a short in the wiring (or incorrect wiring) between the thermostat and furnace, or between the furnace and gas valve. (I've never changed the wiring, other than replacing the thermostat.) Seems that it has to be something electrical in nature since the burners are turning off in synch. With fan turning on.

Any more ideas?

Wait a minute, I don't think you mentioned replacing the thermostat before... did this problem develop afterward?

When the blower comes on does the flames 'roll out' or get squirrely? If so you may very well have a cracked heat exchanger.