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Discussion Starter · #1 ·
Day and night - N9MSB0802120A1. Roughly 6 years old.

So my limit switch keeps tripping. I used a multimeter to verify that it reads 24v when it trips. I also tested continuity and it was good. I removed the limit switch and used a jumper to keep the furnace running while using a meat thermometer to check temperature in the port. Temperature never came close to the 200F specified on the limit switch.

So I replaced the limit switch and it's still tripping. Air filter brand new and still trips if removed. Blower is 1 year old, furnace 6 years old. All registers are open and should be clean. Unfinished basement so only registers on upper levels. No kids or pets that might stuff something in the registers. There are no dampers either. AC unit was replaced last summer, so the coil should be relatively clean.

What else could it be? Evey time a tech comes out, they suggest something different with a huge expense. Kind of like going to a jiffy lube and being up sold a cabin air filter replacement for 75 bucks.

Moreover, it never locks out on me. And I only seem to have the issue when bringing the house to temperature in the morning. Say from 65 to 70. It runs fine during the day and night to keep things at temperature.
 

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Have you checked the temperature rise? Supply minus return temp after 10+ minutes.

Low airflow or over-fired.

When did the problem start? Are all your vents clear and open?
 

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Discussion Starter · #3 ·
Have you checked the temperature rise? Supply minus return temp after 10+ minutes.

Low airflow or over-fired.

When did the problem start? Are all your vents clear and open?
I have not checked temperature rise. I am away for the holidays but will do this when I return.

All vents in home are open and free from obstructions. Problem started this year as soon as I switched from AC to heating the house.

I have a new filter, checked exhaust and supply pipes. I also cleaned out the condensate trap and hoses running to the pressure switch.
 

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Could be dirty coil, blower motor not slower than normal (capacitor or motor failing?), gas pressure drifted and it's now over-fired.

If everything is within normal limits, could be the limit is just tripping early. How long did you do the test for?
 

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You got the schematics to tell us exactly what that limit switch is supposed to be sensing ? (i.e temperature, exhaust back pressure .......???)
 

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Discussion Starter · #8 ·
Could be dirty coil, blower motor not slower than normal (capacitor or motor failing?), gas pressure drifted and it's now over-fired.

If everything is within normal limits, could be the limit is just tripping early. How long did you do the test for?
I replaced the limit switch, so I don't think the new one would be tripping early. It's rated 200F 20F. So it should cut off the gas/burners at 200F and then the blower runs until its 180F or less. At that point the furnace kicks on again.

Anyhow, I did the test with the meat thermometer foe about 15 - 20 minutes. It never reached 150F. Maybe a meat thermometer is not very accurate. I wonder if an infrared would provide better results.
 

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Infrared thermometers aren’t very accurate.
I suspect your meat thermometer isn’t very accurate either. And would explain why your seeing a 200° limit trip at 150°.
 

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I replaced the limit switch, so I don't think the new one would be tripping early. It's rated 200F 20F. So it should cut off the gas/burners at 200F and then the blower runs until its 180F or less. At that point the furnace kicks on again.

Anyhow, I did the test with the meat thermometer foe about 15 - 20 minutes. It never reached 150F. Maybe a meat thermometer is not very accurate. I wonder if an infrared would provide better results.
Did you remove the limit and run the furnace with the meat thermometer stuck in the same place as the limit was? If not, then you aren't getting a good enough reading. Anyways, is this a multi-stage furnace? From your description, I am thinking that the thing is kicking into high stage while it is trying to recover from setback, and that's when it overheats and trips. Your furnace is likely oversized, and it's probably hooked up to undersized ductwork. This, sadly, is way more common of a problem than it should be. If it is a 2 stage furnace, I would try disconnecting 2nd stage (usually "W2") and see how that goes. You might find that the furnace is still able to keep up just fine with only the low stage working.
 

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Discussion Starter · #11 ·
Did you remove the limit and run the furnace with the meat thermometer stuck in the same place as the limit was? If not, then you aren't getting a good enough reading. Anyways, is this a multi-stage furnace? From your description, I am thinking that the thing is kicking into high stage while it is trying to recover from setback, and that's when it overheats and trips. Your furnace is likely oversized, and it's probably hooked up to undersized ductwork. This, sadly, is way more common of a problem than it should be. If it is a 2 stage furnace, I would try disconnecting 2nd stage (usually "W2") and see how that goes. You might find that the furnace is still able to keep up just fine with only the low stage working.
It's a single stage furnace. I removed the limit switch and used a jumper to keep the furnace running while using the meat thermometer in the port the switch was in.

How am I supposed to tell if it's ovwrsized? I believe it's 80,000 BTU and finished sq footage is 2300 with 1700 unfinished in basement. Duct work not yet run in basement.
 

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Equipment isn’t sized via square footage. Yours could very well be oversized.
Good place to start would be to clock the gas meter and take the temperature rise across the furnace.
 

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Discussion Starter · #13 ·
Equipment isn’t sized via square footage. Yours could very well be oversized.
Good place to start would be to clock the gas meter and take the temperature rise across the furnace.
Per the furnace panel, temperature rise should be between 40F and 70F. I took readings at return of 68-70F and supply heated up to around 120F. At this point the limit switch tripped again.
 

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Discussion Starter · #15 ·
Where did you take the temperature readings? Should have been directly at the furnace.
Yes, readings were at the furnace. Return was taken about a foot from where the filter is installed.. Where the return comes in. Supply taken above heat exchanger, but not in direct sight. Just after the duct does a 90 degree.
 

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I’d say your furnace is either over fired or you have an airflow issue.
Is the blower wheel clean? Evaporator clean?
You can also clock the meter and check gas pressure.
 

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Discussion Starter · #17 ·
I’d say your furnace is either over fired or you have an airflow issue.
Is the blower wheel clean? Evaporator clean?
You can also clock the meter and check gas pressure.
Blower and wheel are clean. There's no debris inside the unit. Again, it's only six years old. I don't think it's getting hot enough to trip the limit. I would think a poor temp rise test would show bad airflow. But temp rise well within specs. I've again used a highly accurate digital thermometer to measure the port the limit sits in and it's only hitting 135F while running at least 15 minutes. The limit trips about 5-10 minutes into operation.
 

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Did you have your thermometer in the port for the high limit, sensing the same area the limit does, or hold it at the opening.
 

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Discussion Starter · #20 ·
Hi, how did it work before the “A” coil was installed?
I’d say your furnace is either over fired or you have an airflow issue.
Is the blower wheel clean? Evaporator clean?
You can also clock the meter and check gas pressure.
I pulled the access panel for the evap coil. It's clean as a whistle. Not even a spec of rust. Everything is clean. I don't have a manometer to check gas pressure. I still think it's not an overheating issue but more a problem with the limit switch..or the wiring to the switch. But switch was replaced with identical and system will run fine jumpered at that connection.
 
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