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Old 01-03-2012, 07:10 PM   #16
how
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Sears GAs furnace issue with burner running too long


Sorry dmanmtl.

The R & W jumper approach was to eliminate the thermostat..BUT
I'd forgot that you mentioned on an earlier post that you watched your burners turn off when they reached a certain temp on your fan/limit control. If the burners turned off because the fan/limit control dial turned clockwise past the third temp nib, that indicates an overheating furnace or a fan/limit control that is faulty.
I take it you don't have an led light that is blinking a high limit code on a mother board in your furnace. Actually the only thing I know about your furnace is the older style fan/limit control you posted.
Age/ brand & model would help.

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Old 01-03-2012, 09:19 PM   #17
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Sears GAs furnace issue with burner running too long


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Originally Posted by how View Post
Sorry dmanmtl.

The R & W jumper approach was to eliminate the thermostat..BUT
I'd forgot that you mentioned on an earlier post that you watched your burners turn off when they reached a certain temp on your fan/limit control. If the burners turned off because the fan/limit control dial turned clockwise past the third temp nib, that indicates an overheating furnace or a fan/limit control that is faulty.
I take it you don't have an led light that is blinking a high limit code on a mother board in your furnace. Actually the only thing I know about your furnace is the older style fan/limit control you posted.
Age/ brand & model would help.
My machine is a Heil Ultimate 2 installed in about 1993/1994.

When I say that the burner is being turned off by the high limit switch, I may be mistaken, now that I am more cognoscente of this, I believe the tstat is actually click faintly right as the burner ****s off...I did not realize the tstat actually controlled the burner, I thought it just said "HEAT" and the furnace took over.

At this point I am really curious to understand how all this works. With regards to airflow, I am 99.9% sure I don;t have an issue as I had a new heatpump installed 2 years ago and they deal all the fancy tests then.
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Old 01-03-2012, 10:41 PM   #18
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Sears GAs furnace issue with burner running too long


If you watch the fan/limit control that you posted all the way through a heating cycle you will know what's going on and whether it's the thermostat or the fan/limit control that's shutting off the burners.
Turn the thermostat at least 5 degrees above the present room temp.
The fan limit dial will turn clockwise as the furnace heats up. The small indentation that lines up with the middle of the control gives you the supposed temp on the dial, of the house air stream in the heat exchanger. On your picture it's exactly where the first nib is.
The dial normally passes by this indentation with the first nib and when it gets to the second one, the fan will turn on. If the dial then continues to rotate up to the third nib, then the burners will shut off as the limit cuts off the electricity to the gas valve. You will probably hear the click of this control when it limits out. This cutting off of electricity to the gas valve circuit may be why your thermostat makes a clicking sound just as the burners turn off.
This will mean that the furnace has just overheated or that the fan/limit control is no longer properly calibrated to the real temp.

Poke a small hole in the furnace plenum, just beyond the line of sight of the heat exchanger, stick a thermometer in the hole and read it just as the furnace limits off, now subtract the room temp from that thermometer plenum reading and compare it to the recommended temp rise that is listed on your furnace plate.
Let us know what your measured temp rise is compared to what is recommended.

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Last edited by how; 01-03-2012 at 10:46 PM.
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