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ni2016 04-03-2013 05:24 PM

Bryant 394gaw000075 furnace issue
 
Hi,

My furnace (model # in subject line) was installed in 1990. Recently, it has had a tendency to shut itself off every couple days (i.e., the thermostat is still set at a certain temperature, but the actual temperature is far below and the heat obviously isn't coming on). When I go down, flip the electricity switch, thens lip back on, it proceeds to work again for a day or two, before stopping again.

I searched some online forums and heard the thermocouple might be the problem and went to buy a "universal" thermocouple, but it didn't fit - the "thermocouple" in this model is more of just a metal pipe. Does anyone know whether the issue might actually be the thermocouple or something else, and/or where I could find a replacement part if that's the issue?

Thank you in advance for the advice.

how 04-03-2013 06:05 PM

A photo of the pilot assy would help.

A thermocouple refers to a gas safety control for a pilot light. Do you have a continuously burning pilot light??? That would not usually be a 1990's furnace system.

Is the metal pipe you speak of, actually the gas supply line for the pilot?
I think some of those units might have had pilot warp switches. Do you have any wires going into the pilot assy??

JScotty 04-04-2013 12:22 PM

Quote:

Originally Posted by ni2016 (Post 1151728)
Hi,

My furnace (model # in subject line) was installed in 1990. Recently, it has had a tendency to shut itself off every couple days (i.e., the thermostat is still set at a certain temperature, but the actual temperature is far below and the heat obviously isn't coming on). When I go down, flip the electricity switch, thens lip back on, it proceeds to work again for a day or two, before stopping again.

I searched some online forums and heard the thermocouple might be the problem and went to buy a "universal" thermocouple, but it didn't fit - the "thermocouple" in this model is more of just a metal pipe. Does anyone know whether the issue might actually be the thermocouple or something else, and/or where I could find a replacement part if that's the issue?


Thank you in advance for the advice.

Does the "metal pipe" thermocouple you're describing look like this:
http://www.hamiltonhomeproducts.com/...le_qc_300.jpeg
If so that's a flame sensor not a thermocouple. It does basically the same thing, it senses the flame, but it works in a different way.

However if your furnace has a flame sensor it's definitely a possibility that it's the cause of your problem. The way those sensors work they pass AC voltage through the flame into the burners which are connected to the case ground. When the AC voltage passes through a flame it converts to DC voltage. The ignition board senses DC voltage off the case & it knows the burners lit.

What tends to happen is the sensor gets dirty & can only pass a low amount of voltage through the flame. Therefore the board can barely sense the flame. Sometimes it gets a strong enough signal, sometimes it doesn't. When it doesn't sense the flame it will lock out & the furnace will no longer try to ignite.

Remove the flame sensor clean it with some sandpaper or a wire brush. See if that fixes it.


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