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Old 02-19-2019, 12:24 PM   #1
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Problem with no apparent problem


Furnace trips multiple times / day starting this winter on Limit 13 Lockout, furnace is 5 years old but the house is cold each morning / afternoon.

An inspector checked out the furnace and gave it a perfect pass.

A furnace repair guy came over and couldn't find any issue except the furnace actually is getting hot, so the sensor is probably good. He said the blower was good and the exchanger is not cracked.

Intake and exhaust are clear

Vents and returns are all clear and wide open

Used a boroscope to look inside ducting and found some lego pieces but nothing substantial

all vents feel like they get the same air flow.

Not sure what else to check since it is heating the house but still tripping. Is there a proper way to test the limit switch, like an actual temperature it has to reach?
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Old 02-19-2019, 02:59 PM   #2
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Re: Problem with no apparent problem


You may be able to find the trip temperature stamped on the limit switch. The tricky part is getting a thermocouple on the right spot to get a quick measurement of the peak temperature when it trips.
You would also want airflow and heat rise measurements. Either your flow is being restricted somewhere or the burners are over fired.
Surprised the repair guy didn't check any of these.
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Old 02-19-2019, 07:05 PM   #3
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Re: Problem with no apparent problem


This happened to me once and they called it a "runaway heater." The gas company will come out, free of charge, if you're smelling gas. If they don't smell it, they won't charge you and you'll get a professional to look into it for free.
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Old 02-20-2019, 10:16 AM   #4
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Re: Problem with no apparent problem


There's no gas smell. 2 professional furnace people have looked at it already. First said there was "no problem" even as the furnace tripped in front of him. The second said the furnace is "hotter than usual" but couldn't find any reason it would be like that.
Quite an annoying problem as the house is quite chilly for the last few weeks but ... there is "no problem".
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Old 02-20-2019, 12:48 PM   #5
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Re: Problem with no apparent problem


It has to be one of these things:
-Sensor is worn out and tripping too low
-Airflow is restricted somehow
-Gas pressure is too high

You could try replacing the temperature sensor. That would be easy and cheap.
If you had a digital oven thermometer you could try to get a temperature reading.
Try running with and without the air filter. Time how long it takes to cut off with and without the filter.
I would not recommend you try to adjust gas pressure. That should be done by someone with gas appliance experience.
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Old 02-20-2019, 02:11 PM   #6
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I'm leaning towards bad sensor, even though it was deemed fine. How do I read this? Pic attached

Quote:
Originally Posted by chiraldude View Post
It has to be one of these things:
-Sensor is worn out and tripping too low
-Airflow is restricted somehow
-Gas pressure is too high

You could try replacing the temperature sensor. That would be easy and cheap.
If you had a digital oven thermometer you could try to get a temperature reading.
Try running with and without the air filter. Time how long it takes to cut off with and without the filter.
I would not recommend you try to adjust gas pressure. That should be done by someone with gas appliance experience.
Attached Thumbnails
Problem with no apparent problem-img_20190220_130455_1550693403909.jpg  
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Old 02-21-2019, 09:36 AM   #7
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Re: Problem with no apparent problem


Have you done a temp rise test on this unit? Easy to do. Get a meat thermometer that can register down to room temp. Place the thermometer into a heat vent and record the temp when the furnace is running. Do this on 3 or 4 vents around the house. Next, go to the furnace and check the temp on the return air near where it enters the furnace. You may have to drill a small hole into the return ducting to do this. Record this temp when the furnace is running. compare the return to the supply and see what the difference is. In the manual or on a label in the furnace there should be a temp rise listed for the furnace. You should be withing about 10% of this number. My suspicion is it is too high and the furnace is shutting down on overtemp. It sounds like neither tech checked this. This it Tech 101 stuff.

Dan
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Old 02-21-2019, 03:22 PM   #8
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Thanks Dan, that is something I can easily do when I get home, good suggestion. A vendor tech is supposed to come over today or tomorrow and check it out. Luckily the weather has been warmer. Just nice to have properly working equipment


Quote:
Originally Posted by danpik View Post
Have you done a temp rise test on this unit? Easy to do. Get a meat thermometer that can register down to room temp. Place the thermometer into a heat vent and record the temp when the furnace is running. Do this on 3 or 4 vents around the house. Next, go to the furnace and check the temp on the return air near where it enters the furnace. You may have to drill a small hole into the return ducting to do this. Record this temp when the furnace is running. compare the return to the supply and see what the difference is. In the manual or on a label in the furnace there should be a temp rise listed for the furnace. You should be withing about 10% of this number. My suspicion is it is too high and the furnace is shutting down on overtemp. It sounds like neither tech checked this. This it Tech 101 stuff.

Dan
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Old 02-21-2019, 05:55 PM   #9
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Re: Problem with no apparent problem


Quote:
Originally Posted by frustrated25 View Post
There's no gas smell. 2 professional furnace people have looked at it already. First said there was "no problem" even as the furnace tripped in front of him. The second said the furnace is "hotter than usual" but couldn't find any reason it would be like that.

Quite an annoying problem as the house is quite chilly for the last few weeks but ... there is "no problem".
I normally don't like to bash other "professionals", but this case is just sad. I would have fired the guy who saw it trip and then just shrugs his shoulders at it. We wouldn't have charged for his time.

Like others have said, this is pretty basic. The temp rise will let us know what's going on.

BTW, that limit trips at roughly 180*f and resets at roughly 160*f.

What is the model number of the furnace?

Cheers!
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Old 02-27-2019, 06:33 PM   #10
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Re: Problem with no apparent problem


Problem fixed:
The vendor for the furnace came out and found the furnace was starting fine, but then staying on high fire instead of going to low fire for regular heating. The furnace was indeed getting too hot.
There were no exterior signs of a fault (like burnt circuit board or loose wires), but something wrong in the logic. He replaced the motherboard and the furnace is working as it should now.
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Old 02-27-2019, 07:03 PM   #11
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Re: Problem with no apparent problem


Quote:
Originally Posted by frustrated25 View Post
Problem fixed:
The vendor for the furnace came out and found the furnace was starting fine, but then staying on high fire instead of going to low fire for regular heating. The furnace was indeed getting too hot.
There were no exterior signs of a fault (like burnt circuit board or loose wires), but something wrong in the logic. He replaced the motherboard and the furnace is working as it should now.
Very interesting. So it was firing in high stage, but the fan was running for low stage? Odd, and interesting.

Thanks for the update. It'll help others.

Cheers!
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Old 02-27-2019, 09:46 PM   #12
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Quote:
Originally Posted by supers05 View Post
Quote:
Originally Posted by frustrated25 View Post
Problem fixed:
The vendor for the furnace came out and found the furnace was starting fine, but then staying on high fire instead of going to low fire for regular heating. The furnace was indeed getting too hot.
There were no exterior signs of a fault (like burnt circuit board or loose wires), but something wrong in the logic. He replaced the motherboard and the furnace is working as it should now.
Very interesting. So it was firing in high stage, but the fan was running for low stage? Odd, and interesting.

Thanks for the update. It'll help others.

Cheers!
Correct. The air flow would reduce, as it should, to a low stage flow but the gas pressure would remain set for firing on high stage.
Honestly I haven't looked into it further, maybe it was something as simple as a broken resistor?? They replaced the whole board and hopefully will be more robust than the old one.
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Old 03-01-2019, 07:24 AM   #13
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Re: Problem with no apparent problem


99 times out of 100 it is failed or failing electrolytic capacitors. It wouldn't likely be resistors. Electrolytic caps often bulge when they go bad. I have repaired a number of computer motherboards by replacing caps. And replacing all electrolytic caps is the first thing that the radio restorers do when they restore old time radios.
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Old 03-01-2019, 07:45 AM   #14
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Re: Problem with no apparent problem


Quote:
Originally Posted by tmittelstaedt View Post
99 times out of 100 it is failed or failing electrolytic capacitors. It wouldn't likely be resistors. Electrolytic caps often bulge when they go bad. I have repaired a number of computer motherboards by replacing caps. And replacing all electrolytic caps is the first thing that the radio restorers do when they restore old time radios.
That may be true, but these boards are classified as safety devices and therefore it's not recommended to perform any component level repair on them, even if you're more then capable. The liability is just too high. (flame safeguard and limit circuits.)

Cheers!
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