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Gas furnace limit switch question


dimes's Avatar
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10-24-08, 05:29 PM   #1 (permalink)  
Gas furnace limit switch question

Hello
I have a 35 yr old gas furnace( forced air) that works perfectly and has never failed me. I routinely have it inspected every 2 to 3 yrs and I change the filters regularly. Today a service person came, at my request, to inspect my furnace and informed me that the gas lines are fine and there is no measurable carbon monoxide . He said however, that it is the "limit switch" that is turning off my burners, according to him the burners should remain on throughtout the whole cycle until the thermostat reaches the desired temperature. In my case what happens is the furnace burners come on then the fan starts maybe 30 secs later and heat flows into the rooms, however before the rooms are at the temp set on the thermostat the burner is shut down by the limit switch and the fan continues to run blowing warm air until the heat in the plenum reduces and then the burner comes on again and this goes on until the room temp is reached as set on the thermostat then the whole thing shuts down until the room temp drops and calls for heat again.He calims the heat exchanger must be all coated and the heat isnt getting out of the plenum fast enough and wanted to red tag my gas service until i bought a new furnace as he says my furnace is dangerous( he says that the limit switch is a safety device and if it fails that the burner would just keep going and eventually I'd have a metdown of the furnace and of course most likely a fire). I do remember another service person telling me a number of years ago that my furnace was actually too big for the house and that was why the temp in the plenum rose to the point that the limit switch was shutting down the burner.What do you all think ,do some furnaces operate the way mine does, in that the burner goes off and on throughtout the cycle ,determined by the limit switch until the temp set on the thermostat is reached?Is this dangerous or what ? I hope I've explained my situation clearly enough.
Thanks in advance for your imput.

 
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10-24-08, 06:16 PM   #2 (permalink)  
If that is actually what is going on then yes, it is potentially dangerous and thats one tough limit switch. A limit switch is a safety switch that should not be continuously operated because it indicates a problem. Have another company look at it for confirmation then ask for a quote on a new furnace and insist on seeing the heat loss calculation.

 
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10-24-08, 07:21 PM   #3 (permalink)  
It could be your last explantion -only - if the furnace did it from the day it was installed. But that is not the case. Right?

Since your problem has now begun, it is also conceivable the limit switch, over the years, has gotten weak. I would have the temp where the limit switch goes in, actually tested, to be sure. Then move on from there, depending on that reading.

IF you happen to have the old thermodisc limit switch, you can remove the switch and just test it by blowing a hot hair dryer on IT and a thermometer next to it at the same time, while you have ohm meter alligator clipped to the two terminals, and see when it trips out. I've changed out lots of mobile home limit thermodiscs (albeit many were operating stat) that went bad from 35 years ago furnaces. I was doing this 22 years ago, and it seems like yesterday.

This same scenario can happen to a circuit breaker in a house, where there is a lot of heat going thru it over the years. And over time it gets weaker and weaker and starts to trip out, even though you are not using any more electricity. And by putting in a new circuit breaker cures it.

 
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03-19-09, 03:16 PM   #4 (permalink)  
Replaced limit switches, now what?

My furnace is doing exactly the same thing. The flame stays on until the limit switch trips, then it starts cycling through again when it cools down. I replaced the high limit switch and the limit switch in the blower area. Still does it.
I removed the plenum box on top and looked at the heat exchanger. Extremely clean. I pulled the return duct and checked the intake area. Spotless, including the fan blades. I ran a cleaning brush up the tin chimney. Lovely. I listened carefully to the exhaust fan and the main blower fan. They sound like they are running at normal speeds. I even tried running it without the return air filter, sounds fine and plenty of air from vents.
The furnace was installed in 1993, the igniter replaced in 1998, 2000, 2002, 2004, 2006. The firebox was replaced n 2007, under warranty! The control board failed last September. Now the limit switch trouble.
Yes, I should replace this thing for its unreliability. I can't buy a new one and forget the idea of financing it. Not these days. I fix it or freeze.
Goodman GMP-075 3. Heated the house for fifteen years as long as I kept feeding it igniters, so it isn't likely to be an installation mistake.
What is the likelihood that the heat exchanger has failed? It was replaced by order of the Federal Government (safety hazard) in 2007.
Any thing I need to check before I give up?


Last edited by rhinoguy; 03-19-09 at 04:11 PM. Reason: left something out
 
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03-19-09, 04:20 PM   #5 (permalink)  
Have you removed the limit switch to inspect it and inspect limit switch opening for signs of loose furnace insulation that may have fell on the limit switch? You could try to confirm that when the limit switch trips out, that the limit temp number, stamped on the limit switch, matches the temperature you get if you drill about a 1/8 inch hole above or below limit switch, in the same vertical line, and insert one of those 6 inch probe thermometers that go up to 220F, that you can buy in grocery stores or Harbor Freight, and see if the furnace limits at the temp it is supposed to, or if it is limiting too many degrees in advance of that high number listed on the limit switch. If your limit switch say says L200-30, that mean it would shut off at 200 and come back on at 170, for example.

When I get the chance, I have to get back to this rental that has limiting issues, and I have checked everything but remove the limit itself, as I suggested to you. A supply house heating pro told me they have been a service call witness to furnace insulation causing problems around the limit switch inside.

Have you pulled the blower out and looked up at the underside of the secondary heat exchanger? If someone neglected changing filters, the fins on the exchanger can plug with dust. The one I was working on, and need to do further work on, was partially plugged! And by me cleaning AND (for now, temporarily) putting the blower motor on high speed a/c speed, rather than med. high heat speed, I doubled the amount of time it takes now to high limit.

And if you pull out the blower, clean the squirrel cage vanes, as they can fill up with caked on dust, and cut down on the curve each vane is supposed to have, and reduce it's air moving capactity. I must have gotten a pound to come off the one I worked on!

 
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03-19-09, 04:55 PM   #6 (permalink)  
It's a bad practice to have your safety depend upon a safety switch shutting off the furnace. That should be corrected.

However, there are lots of things that could be inspected and tried before replacing the furnace.

It might be possible to increase the fan speed. It might be possible to reduce the gas pressure somewhat, thus reducing the BTU input to the furnace. It's possible the furnace might operate OK with a new filter. There may be warm air vents that are closed that could be open.

It might also be reasonable to replace a 35 year old furnace with one that is properly sized.

 
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03-19-09, 05:12 PM   #7 (permalink)  
Checked all of that

I took off the air inlet to the main blower and could see that the squirrel cage is very clean. I cleaned it by hand in 3-07 when I replaced the heat exchanger.
I tested both of the limit switches. The six inch long opened at 195, digital meat thermometer. The auxiliary limit opened at 137.
BOTH are NEW from D & L Appliance Parts.
I have a sinking feeling that the exchanger has got a leak. The eyelets failed on the original (CPSC recall replacement), I suspect the replacement has failed also. Is there an easy way to check this? When the original failed the flame flashed back into the manifold area and tripped the safety switches, it's not doing that this time.
GMP-075 3 Goodman, 1993

 
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03-19-09, 05:24 PM   #8 (permalink)  
Your furnace is 35 yrs old. Can a problem be fixed yes, but why not put money into somthing that is way more effecient and pay less for gas. Stop spending money on service and do yourself a favor get new furnace with 5 to 10 yr warranty and maybe it will last you another 35 yrs (maybe)

 
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03-19-09, 05:31 PM   #9 (permalink)  
Dimes...

if you have the silver covered Honeywell limit control you have 2 slide bar settings onit the "fan on" and the "hi limit"220F.this setting is if the motor fails to come on or the filter is completly blocked and the limit keeps heating up and shuts the main burner NOTE....shuts the burner off but doesn't lock the burner out it will cycle as you said but you are over heating the metal on the heat-X.35 years and you seem to have goos maint.going there,so now it is almost red tagged$$$$$$$$$$$.the thermostat calls the cycle in during heating but the burner should stay on during that stat call and not shut off till the space temp.is satisfied...make sure the fan is running LOW speed if you have A/C there FAN ON stat should be HI fan speed different then the heating LOW.chec those slide pin settings on that Honeywell controller if the HI is set anywhere near the "fan on limit" it will shut the burner off during a call for heat.sounds like scare tactics from here with the brain surgeon doing the service check.

 
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03-19-09, 05:37 PM   #10 (permalink)  
I'd be surprised if a Goodman GMP is 35 years old. Twenty would be about right.

What's the serial number of the furnace, which can be obtained from the rating plate in the burner compartment?

 
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03-21-09, 08:53 PM   #11 (permalink)  
Two different furnaces

There are two different furnaces being discussed here. One fellow has a thirty five year old beast, I have a Goodman made in 1992, installed in 1993. The Goodman is the one that eats igniters, limit switches and now looks as though it has got a hole in the heat exchanger.
The Goodman is spotlessly clean because I cleaned it with brushes, "Cyclone" cleaner and too much time. It runs until the limit switch turns it off and sets the lamp to flashing four times.
The older machine is doing the same thing, that's why I jumped on this thread instead of starting a new one.
BTW, if I take a long time between posts it is because my DSL service is running down sixty year old wires and the phone company is not planning to fix the problem. Cable high speed is three times as expensive.

 
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03-21-09, 08:54 PM   #12 (permalink)  
Do you have AC on this system? If so is the coil clean?

Have you checked the heat rise on the furnace? (the temp into the return air at the furnace and the temp leaving the furnace.. the rated heat rise of your unit should be on the data plate of the furnace.)

If your furnace is over heating due to lack of air flow it will show up in the heat rise of the unit.

 
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03-24-09, 02:33 PM   #13 (permalink)  
Posted By: cyberdead Do you have AC on this system?

Have you checked the heat rise on the furnace?

If your furnace is over heating due to lack of air flow it will show up in the heat rise of the unit.
I do not have A/C.
I have not checked the heat rise. Can I just drill a small hole and insert a thermometer, then close the hole with a sheet metal screw?
I see no indication of poor air flow. I have strong suction at the return and plenty of air from the floor registers.
I bought 44 filters at a bankruptcy auction for less than fifty cents each. I ALWAYS have a clean one.

 
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03-24-09, 05:31 PM   #14 (permalink)  
I have not had the chance to review the whole thread and am in a hurry.

Yes, you can drill the hole and insert skinny 1/8 inch x 6 inch probe thermometer.

Have you pulled your limit switch to inspect if anything is obstrudting it inside? Also, have you cleaned the underside of the secondary heat exchanger by removing the blower assembly and laying down inside the furnace, on your back, and inspecting the underside fins of the secondary?

Have you removed the blower and cleaned each and every cupped vane on the squirrel cage to maximize airflow?

Do you know if the correct blower speed wire was put on heat. Even on furnaces with no a/c, you might have a 3-speed motor where you can select speeds.

On last furnace I did, they had heat at med. hi, and a/c on hi. ( I temporary swapped the 2 until I can get back there and resolve similar high limit problem.

 
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01-20-10, 09:09 PM   #15 (permalink)  
Limit switch

I have just experienced a limit switch failure on a 9 - 16 year old Janitrol furnace. It went out during the night and I woke up with the fan running and a cool house. I removed the switch and checked with an ohmmeter = infinite ohms. I then tapped it on a table and remeasured = 0 ohms. I replaced it in the furnace and it worked several days, normally, then went out again. It again measured infinite ohms. I replaced with a like unit = OK. Indicates a limit switch can go bad without any other influencing circumstances.

 
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01-20-10, 11:06 PM   #16 (permalink)  
Hey rhern,


I can't recall ever encountering a properly wired limit switch that failed to open when it was supposed to.

I replaced many limit switches that remained open when they were supposed to be closed, as you describe. That's the common way that limit switches fail, in my experience.

 
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