gas furnas cycles on/off due to high temp limit


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Old 11-30-11, 11:35 AM
J
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gas furnas cycles on/off due to high temp limit

My old Rheem furnace works fine (heats the house), but i have noticed that the burner will cycle off and back on when the thermostat is still calling for heat. I know that it is the high temp limit-Honeywell fan control. This limit opens the 24volt circuit and kills the automatic gas valve. The fan continues to run and in a few minutes the limit closes and the burner lights again. This continues untill the thermostat is satisfied.

The timing works like this...
-thermostat calls for heat
-burner lights
-fan comes on 1 min later
-in about 4 or 5 mins the burner shuts off
-fan continues to run
-4 or 5 mins later burner fires again.
this continues until the thermostat stops calling for heat

I have a clean filter and all supply registers are open. The intake for the furnace is unrestricted. The fan works fine, the squirrel cage spins freely. I have recently converted this furnace to electric ignition for the pilot which included replacing the gas valve. I know this problem was happening before i replaced the valve (was hoping this issue would go away with the new valve). I know some things about this unit because i just worked on it but i am no expert. I am not sure how these are chambered inside for the heat exchange. It doesn’t seem safe to have the 24v system switching on and off like this and the high limit probably should not be tripping. I have looked at the Honeywell control and the high limit is set to about 200 deg.

Short of replacing the furnace, is there something i should do? Maybe disassemble the unit and clean the insides?
Thanks
 
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Old 11-30-11, 08:55 PM
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Hey,
I just had the same problem with my furnace and watch closely when the repair man came over so I could maintain my own furnace. In the plentum, there is a high temp control limit. It looks like a long tube when you take it out. It is held in with two screws. I am not an expert, but it did help my situation.

Jim
 
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Old 11-30-11, 09:24 PM
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Sounds like where I would start; the high limit switch. Connect a multimeter to it and see if it is what is triping the burner on and off. Is the chamber really reaching 200 degrees or the limit switch is faulty and shutting off at 100?
 
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Old 12-01-11, 10:08 AM
J
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Thanks to both replies. It seems like you are both talking about the same thing. I agree that the High Temp Limit is the likely problem. I am going to pull it out and try to figure out a way to check calibration... maybe employing my wifes cooking equipment, possibly her toaster oven. I will have to do it while she's not home . Toaster oven with a multimeter will likely give me a better idea if this thing is working properly. Thanks again
 
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Old 12-01-11, 10:29 AM
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Guessing that the limit switch is the problem doesn't cut it. Use a voltmeter to find out if the burners are being shut off when and if the limit switch is opening.

Some limit switches have a dial about 3" in diameter which display the temperature being measured and allow you to determine if the limit switch is shutting off the furnace by looking at it.

Limit switches that shut off the burner are not the problem. The question is WHY it is shutting off the burner. Much the most likely cause of that is something that is obstructing the airflow across the heat exchanger.


But first you need to check and see if the limit switch is shutting off the burner.
 
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Old 12-01-11, 01:33 PM
J
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Will do

Thanks SP... will check the high temp limit with a multi meter tonight and will post what I find.
JG
 
 

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