Trane TUC060B936A0 Fan Cycling in "Auto" mode.


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Old 12-25-12, 06:49 AM
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Trane TUC060B936A0 Fan Cycling in "Auto" mode.

The furnace runs fine in the fan Run mode. The problem is the Auto mode. In Auto, the furnace comes on fine. A little later the fan cycles off while the burners are still running. This process repeats over and over. The furnace gets quite hot as the blower cycles on and off. This repeats every minute or so. If I put the thermostat on the Run fan mode, the fan runs all the time and the burners cycle as needed, normally. This is an older furnace, about 1985 or so. I have it in the Run mode now. Why does it cycle in the Auto mode?

Thanks,

JimmyA
 

Last edited by JimmyA; 12-25-12 at 07:30 AM.
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Old 12-25-12, 08:19 AM
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If this is a 1985 furnace, you are probably cycling at the fan limit control.



I recommend having this 27 year old furnace professionally cleaned and checked.
 
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Old 12-25-12, 08:34 AM
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Limit Control

It looks like the whole dial has slipped 90 degrees. If I force it back, the fan works properly but I have to hold it there.
 
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Old 12-25-12, 08:48 AM
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Stripped or missing mounting screw?
If this is the case you can turn off power and probably add or replace a screw.
 
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Old 12-25-12, 08:53 AM
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Rust can build up on the burners of these furnaces and cause flames to roll out of the furnace sides. Rust can also cause a delayed ignition which can be a small explosion. How long has it been since this furnace was cleaned and checked?
 
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Old 12-25-12, 09:09 AM
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Furnace

It's been awhile. By the way, where can you buy a new limit control switch?

Thanks,

JimmyA
 
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Old 12-25-12, 09:19 AM
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Johnson supply, Johnstone, or Grainger.
They will want to know if the temperature probe is 6, 8 or 10 inches long.

There is usually a tag on the replacement reminding you to cut a jumper for low voltage limits.
 
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Old 12-25-12, 09:27 AM
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I just took it out

It's a Honeywell. Found numbers: B141195P01 and L4064A2881 6 9127 K

The dial is practically upside down. No wonder the switch isn't working. I don't understand how that could happen.
 
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Old 12-25-12, 09:32 AM
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Looks like the temperature probe is 5 inches long...
 
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Old 12-25-12, 09:43 AM
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https://customer.honeywell.com/resou...0s/69-0117.pdf

The first warning speaks of the jumper. The limit is usually low voltage...

"Failure to remove brass jumper, if limit switch is
in low voltage circuit, can cause electrical shock
hazard or damage low voltage"
 
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Old 12-25-12, 03:18 PM
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Looks Good

I hope I can get it wired correctly. Thanks for you kind help.

JimmyA
 
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Old 12-25-12, 04:47 PM
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I hope I can get it wired correctly. Thanks for you kind help
The most common question now on this forum is: "how can a keep an ancient unsafe machine running for as little money as possible?"

If you value your life, call for service - don't try to repair a 27 year old furnace which may not be safe any more. There are certain things that need to be checked once in a while, especially on something that's more than 15 years old - search this board for more info.
 
 

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