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Unread 10-27-2009, 06:02 PM   #1
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HVAC Guys, what does an aux limit switch do?

We had some really strong winds in my neck of the woods this morning ahead of a winter storm that is moving in. The wife called me at work and told me that the furnace was no longer blowing hot air. I took the LED code (31) and did some research on our model of furnace and came up with PRESSURE, DRAFT SAFEGUARD,AUX LIMIT SWITCH issue.

I came home and found the little switch near the very bottom of the flue and was able to reset it. Now the furnace works fine.

Is it safe to assume that if the switch trips again, I have an obstruction somewhere? Is it possible that the high winds tripped the switch to begin with?

Thanks,
Robert
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Unread 10-27-2009, 06:52 PM   #2
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Has it continued to work fine since? Could be many things. First and foremost, what brand/model is the furnace?
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Unread 10-27-2009, 07:13 PM   #3
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Quote:
Originally Posted by 5.0_CJ View Post
Has it continued to work fine since? Could be many things. First and foremost, what brand/model is the furnace?
It is a Bryant 373LAV. It seems that the Carriers have the same codes.

And yes, it seems to be working fine now.
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Unread 10-27-2009, 07:48 PM   #4
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the wind may have cancelled the draft

Check your vent cap on the roof, it's supposed to prevent the wind from blowing down the vent. Most likely it was wind related. The blower is supposed to provide a positive pressure out the vent, the wind could have blown down the vent cancelling the blower pressure. If the aux switch is around the burner, the wind made the flame come out where it's not supposed to around the burner and the aux switch detected it and tripped the unit off.
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Unread 10-27-2009, 07:54 PM   #5
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HVAC Guys, what does an aus limit switch do?

Also it could have had air in the line, this sometimes happens. I also agree with Jyro.
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Unread 10-28-2009, 10:24 AM   #6
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Quote:
Originally Posted by jyro View Post
Check your vent cap on the roof, it's supposed to prevent the wind from blowing down the vent. Most likely it was wind related. The blower is supposed to provide a positive pressure out the vent, the wind could have blown down the vent cancelling the blower pressure. If the aux switch is around the burner, the wind made the flame come out where it's not supposed to around the burner and the aux switch detected it and tripped the unit off.
That is good advice... unfortunately, I have 6" of snow on my roof today so I will not be checking my vent cap.

Maybe I am not talking about the aux switch. Maybe it is some type of pressure switch. I found some diagrams of my machine and the aux switch is behind the blower. The switch I am talking about is where the flue ends.

Sometime during the night, the switch flipped again. I released it and the furnace started ok again. I got up in the attic to see if I could do anything from in there but I don't think I can. I checked the flue to make sure that it was evenly warm all the way up. I don't smell roasted chicken so I am pretty sure that there is not a bird stuck in there.

Thanks a lot guys. I knew I could count on dfwstangs to know this type of stuff.
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Unread 10-28-2009, 10:14 PM   #7
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make sure the vent is above the snow

I have never worked on a unit in that climate. Could snow block the vent?

I also found this--- http://www.hvacmechanic.com/forums/r...ages/11793.htm

Last edited by jyro; 10-28-2009 at 10:29 PM.
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