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.

If the burners turn off while the fan is running could be caused by two things I can think of, one is that the limit is set too low, therefore the limit breaks before the house is warm. Another things is about the fan switch spinning when the burners turn off, Could some outside air be getting into the return air duct, if so this would make the fan cut off and 90 degrees F and then come back on when the burners get the temp back to the temp set on fan on setting. The 90 degree setting is a little low for the fan off setting, I would rather see something like 105 degrees.

Could it be possible that some outside air is getting into the return air duct, this would make the fan switch drop very fast making the fan cycle on and off at the one minute rate you mentioned.
The 90 degree fan off is a little low I would rather see something like 105 degrees.

This was occurring before the thermostat change.

The point is... the burners turn off exactly when the 'fan on' switches on, regardless of the limit set points. They turn off cleanly and abruptly, as if the limit had been tripped... but the limit is not tripping. Every once in a while, the burners will actually stay lit for a complete cycle.

I am now thinking the gas valve must be faulting. Any thoughts on that?

Put a voltmeter across the high limit switch, run furnace, and see if voltage changes when blower starts.

Good idea KC... why didn't I think of that. :)

Won't get to it until tomorrow... but I'll let you know what happens.

Believe it or not, (believe it), I have replaced many a bad part with... a bad part. New, but bad. Most common? Sequencers... and combination fan/limit controls.

I highly doubt that your valve is the cause of your problem.

Mark - I came to the same conclusion after I thought about it some more.

Anyway... I tested the limit side of the fan/limit control with a voltmeter to make sure it wasn't closing when the fan side closed. And this was confirmed... it's not closing. So, it is working as it should.

The only thing left that I can think to replace is the transformer/relay (White-Rodgers 90-112), which I should be able to do tonight if the part arrives as scheduled. I'll post an update.

I reviewed your whole thread, and a very unusual possibility comes to mind. Check the blower compartment for a current-sensing relay clamped to the heating speed blower motor lead. If one is present and the load is shorted, it could overload the transformer and deprive the gas valve of power. Unusual, but it explains why the burners go out as soon as the blower starts.

Thanks KC, I'll give that a look as well.
What would the current-sensing relay look like (size,shape,etc.)?

Typically, looks like a rectangular box, two wires leaving, and clamped to one of the blower leads. Usually found in the blower compartment, but could be anywhere along the length of the lead wire. This is a bit of a reach, but it does fit the "profile".

Thanks KC, I'll give that a look as well.
What would the current-sensing relay look like (size,shape,etc.)?


Here you go

Ok... so after some digging around and closer examination, I have discovered that my furnace has an auxiliary limit control (150 deg. Open). I didn't notice it before because I have little documentation for the unit, and it was tucked behind the blower in the blower compartment.

Anyway... can someone tell me what this aux. limit is doing? Does 150 deg. Open mean that it is turning the gas off if it senses >150 deg. And if so, how do I coordinate that with the fan/limit control (assuming the aux. limit is functional)?

*EDIT*
I forgot to mention that I didn't see anything resembling a current sensing relay. I also replaced the transformer/relay assembly. But the problem persists.