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Old 01-19-2014, 02:56 PM   #1
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Furnace problem - open high limit device


I have an American Standard Freedom 80 furnace. I am receiving an "open high limit device" warning - four blinks. New filter, nothing blocking external returns. The blower is working, but it will not fire up. I have jumped the 3 limit switches I can find, one on the blower face, and two near the flame nozzles. Any other suggestions?

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Old 01-19-2014, 03:49 PM   #2
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Furnace problem - open high limit device


Any sensors on the plenum?? Any wiring disappearing to an area you can't see or get to?
Look at control board.. anything unhooked?
Shut off power, check sensor feeds with an ohm meter.
I may not have answered the question but that's the direction I would take to troubleshoot.
..Jon..

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Old 01-19-2014, 04:26 PM   #3
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Furnace problem - open high limit device


I believe that is the blower fan high limit. If that is thinking the heat exchanger is over heating, the fan will continue to run and the burners will not lite. It will most likely have to be replaced. Beenthere and Yuri will be along at some point to confirm this as they are better at this than I am.
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Old 01-19-2014, 04:27 PM   #4
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Furnace problem - open high limit device


Will probably get moved to the HVAC forum so stick with us.
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Old 01-20-2014, 11:15 AM   #5
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Furnace problem - open high limit device


Thanks for looking. Seems there was another limit switch that looked a lot different than the others. It is rectangular and is against the back wall behind the burners. Not sure what it is or does but there is a lollipop shaped part on the back side that sits against a piece of metal. That appears to be the culprit. Don't know if the piece is shot or if there is so much dust on the metal that it is interfering somehow. I haven't found a way to get to it without some major disassembling.
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Old 01-20-2014, 09:02 PM   #6
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Furnace problem - open high limit device


That is the high limit. the others are roll out switches, and an aux limit.

The limit goes bad from having opened and closed to often. So either you let your air filters get a little too dirty. or you have an air flow restriction.
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Old 01-25-2014, 02:22 PM   #7
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Furnace problem - open high limit device


I got a new high limit but the replacement isn't exact. Mine was a "L170-30F" and the replacement is "L170-40". The vendor told me it would not be a problem...but of course it is (I think). When I installed and ran the new limit, it blew the fuse on the control board. So do I need to replace the limit with the exact match or can I up the fuse?
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Old 01-25-2014, 03:13 PM   #8
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Furnace problem - open high limit device


Good chance a piece of the foil face of the insulation on the heat exchanger side of the panel the limit is mounted to is touching the limit. Remove the limit and fix the insulation. A bigger fuse won't help. it would only risk burning up the transformer.
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Old 01-25-2014, 03:53 PM   #9
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Furnace problem - open high limit device


-40 means it takes 10 deg F longer to reset and is perfectly safe. Opens at 170 and closes at 130 instead of 140, no problema. However the shape of it and length have to match. If it is hitting some foil then that may be a problem and you may have to get the Am Std one.

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