Bad limit switch or something else?


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Old 01-12-11, 12:59 PM
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Bad limit switch or something else?

I have a goodman gas furnace that is at least 10 yrs old, I don't know the model #. The Controller flashed a diagnosic code that the pressure switch wasn't closing. After inspection I realized the inducer motor wasn't running and replaced it. Now it goes thru the ignition cycle fine and everything is running fine for about 2 minutes. Then the gas turns off for about a minute, and then it restarts the ignition sequence. And it runs fine again for another few minutes and then the gas turns off again. It repeats like this. The filter was very dirty so I replaced it but the condition persisted. I suspected the limit switch was opening and bypassed it and it heats normally until the thermostat is satisfied. Besides it being a bad limit switch B1370008, is there anything else that may cause this symptom?
 
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Old 01-12-11, 03:45 PM
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those furnaces were known for a cracked heat exchanger...I would get carbon monoxide detectors...think about it......if the limit switch is opening,it must be getting too hot.you need to have someone inspect it as soon as possible...
 
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Old 01-12-11, 05:48 PM
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It's very unlikely the limit switch is bad. It's very likely that the furnace is overheating and causing the furnace to shut off.

Do NOT bypass the limit switch except for testing!
 
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Old 01-17-11, 09:57 AM
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Not the limit switch

I replaced the limit switch and the problem was not resolved.

If the heat exchanger is cracked, Is it economically feasible to replace or just buy a new furnance?

Also, I noticed the flames from 3 gas jets are not contained in the heat exchanger tubes, a small amount is hitting outside the opening of the tubes. Could this be causing the limit switch to trip?

I am going clean the evaporator coils to see I can improved the air flow.

thanks for your input.
 
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Old 01-17-11, 10:43 AM
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Yes, you need to improve the air flow so the heat produced by the burners is stripped away by the airflow, preventing the furnace from overheating.

The flame rollout condition you describe could be a variety of things.

I wouldn't borrow trouble by supposing the heat exchanger is cracked. You need an accurate diagnosis of the problem by a competent repairman.


The usual rule of thumb is that IF a heat exchanger has failed, it's worthwhile replacing them if you get the heat exchangers for free under warrentee and paqy for their installation.
 
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Old 01-17-11, 03:40 PM
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flames

on this furnace,if the flames are not contained ,and as you say hitting the sides,I would be very concerned that you have a cracked heat exchanger,please have carbon monoxide detectors on each level of your home,and get it checked soon
 
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Old 01-17-11, 04:02 PM
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Air flow not the problem

I vacuumed out the evaporator coil which did have alot of crap on it, and tested it with the access panel off so the heated air could flow unobstructed and still experienced the same short cycling.

The model is a GMP075-03 made by Janitrol. If the heat exchanger is cracked would it cause this condition? Any idea how to get a warrenty exchange? How do I remove it? Is it located behind the inducer? The unit is installed in a closet, so I only have access from the front.

Thanks for your input
 
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Old 01-17-11, 04:21 PM
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I believe I've found the answers to where the heat exchanger is located and how it remove it. But still looking for answers the the other questions.
Thanks
 
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Old 01-17-11, 04:36 PM
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heat exchanger is most likely in warranty meaning a part only. Supply house will not/ should not sell to you. Average labor on that furnace is 3 - 5 hours to replace book time. This is not a DIY project.!!!!!!!!!!!!!!!!!!!!!
 
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Old 01-19-11, 10:23 AM
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Working again

I took the advice of several of you and got a carbon monoxide monitor. And thankfully it did not sense any CO. It appears the furnace was running hot, and the limit switch was doing its job. By decreasing the gas supply it now runs within its nominal range and doesn't short cycle. Thank you all for your suggestions.
 
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Old 01-19-11, 02:43 PM
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just because the CO alarm has not gone off does not mean that there is not an unknown issue with the heat exchanger. It should still be looked at by a pro. A CO alarm is just that an alarm it is not a sensitive $1000 tester. The alarm will not have an alarm event unless there is 40 - 70 PPM CO for an hours time span. You could still have a low level of CO present and never know it. GET THE FURNACE SERVICED ANNUALLY BY A PROFESSIONAL!

A Guide to Prevent Carbon Monoxide Poisoning
 
 

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