Open limit circuit


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Old 12-30-09, 02:53 AM
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Open limit circuit

I woke up this morning to a cold house. Went down to the furnace, and the blower was the only thing running. Shut off the furnace for 1 minute, turned it back on same thing. I would hear the combustion blower kick on and then the main blower kick on, but no burners. When I opened the circuit board, there was 1 flash and it was a open limit circuit. As far as I know nothing is obstructed. Its an older miller gas furnace. Right now the only thing that kicks on is the fan blower on the furnace as soon as the furnaces power is turned on.
 
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Old 12-30-09, 05:19 AM
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clean filter?

Usually an open limit is due to a airflow problem which causes the heater to heat up and trip a limit, do you have a clean air filter??
or it could be worse and the heater may have rolled out due to a bad heat exchanger, in either case, have a licensed repairman come out to look at the heater.
 
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Old 12-30-09, 06:55 AM
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The filter is clean and the furnace hasn't been used more than maybe 5 years of usage due to us burning wood.
 
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Old 12-30-09, 07:40 AM
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I pulled some switches and tapped lightly on them. Checked everything, turned the switch on to the furnace and the light stayed on. When it ignited and was heating I checked all the burners and they all are even with nice blue flames in the heat exchanger. When the blower kicked on nothing changed as far as the flame patterns. Maybe it was a fluke or a stuck switch. It ran for 20 minutes then without any problems.
 
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Old 12-30-09, 10:18 AM
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If you have a mobile home furnace and ac check the coil at the bottom of the furnace, it is probably plugged.
 
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Old 12-30-09, 10:32 AM
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Its not a mobile home. We have a 2400 sqft home, with no ac. The furnace is probably 25 years old.
 
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Old 12-30-09, 12:06 PM
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When limit switches fail, they usually fail by failing to stay closed completely, shutting off the burners.

That could cause the symptoms you describe, including the furnace starting to work after jogging various parts.

This is probably more likely to happen after the furnace hasn't been used for an extended period of time, as well.


If this condition recurs, do you have a multimeter to use to check to see if the limit switch is blocking the 24 VAC power being applied to it?


Limit switches are usually relatively inexpensive. You could consider replacing it if you wanted to avoid having to wait around for it to fail again.
 
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Old 12-30-09, 01:50 PM
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I don't have a multimeter. Need to buy one though. As far as the temperature limit switch, its not built into the temperature sensor is it? Thats just 2 long pieces of steel with a dime sized temperature sensor on the end. The limit would be the one further down the line mounted to the combustion blower right? It had a small red button in the middle. I would assume thats what was sticking. I find out that things do go wrong when the furnace isn't used too often. But even then wood beats pro-pain any day.
After looking at the parts, I guess that temperature control does have a limit built in. I'm just not sure which one to buy? I was thinking it was a 200 degree cutoff but I'm not exactly sure.
 
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Old 12-30-09, 03:46 PM
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The 2 metal arm/dime sized disc one is an automatic resetting high limit switch. Mounts to furnace above combustion area. On the high limit fiber plate it should give numbers like 200L-40, meaning it trips out("opens") at 200(instantly shutting down the burners) and will "close" at 160.

The other switch with the red pin, is another limit switch that trips out if the exhaust gas temp is too high. That one you have to manually reset it by pushing in that red pin button.
 
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Old 12-30-09, 05:25 PM
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Ok, thanks for the bit of info. I will probably just order a replacement for it.
 
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Old 12-31-09, 07:00 PM
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It would really pay for you to buy even a cheap voltmeter-ohmeter, to test with, rather than take guesses, and buy parts without knowing for sure. Often, with furnace malfunctions, the reason a part trips out is because the part is actually doing it's intended job. That there is actually a problem with the furnace, and then the switch trips out the way it is designed to, to protect you.

I just bought 2 more test meters for myself at a nationally known tool store chain for $2.99 ea., on sale. They seem to have these meters on sale more often than not, so it seems to me, anyway.
 
 

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