Payne 100U42-19A wont shut off


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Old 02-10-21, 11:11 AM
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Payne 100U42-19A wont shut off

I have an old Payne/Carrier 100U42-19A gas furnace/ac unit. When I run the heat the unit has a problem shutting off. Sometimes it will shut off after 10-20 minutes. Sometimes it won't at all. If I go out to the garage and hit the side of it with my hand, it shuts off then. The problem doesn't occur when I'm using the A/C, only the heater. Could this be a bad thermocouple?
 
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Old 02-10-21, 12:21 PM
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I would start with checking for a loose connection.
 
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Old 02-10-21, 01:41 PM
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Hi, is this controlled by a thermostat, it doesn’t shut off even when the Stat is satisfied?
Geo 🇺🇸
 
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Old 02-10-21, 05:19 PM
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Welcome to the forums.

Old.... yes it is. Probably in the 35 year old category.
Please make sure to have this furnace inspected for heat exchanger rust thru issues.

It's not going to be a loose connection.
It's not going to be a thermocouple problem.
It could possibly be a thermostat problem or sticking gas valve.
If the problem happens again...... pull one of the wires off the gas valve. If the burner still operates....
call for repair ! A sticking gas valve can be an explosive problem.


 
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Old 02-14-21, 07:02 AM
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Yes controlled by a thermostat.
 
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Old 02-14-21, 07:26 AM
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I would second getting the unit checked out and verified safe. Cracked and rotten heat exchangers are common in older units. And present a risk of fire and carbon monoxide poisoning.
 
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Old 02-14-21, 07:55 AM
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It doesn't have a problem turning the burner off. After the burner turns off, after a period of time (letting the burners cool off a bit I assume) the fan keeps running. If the fan is still running when the thermostat clicks on and sends the signal to the valve to turn the gas on, there's an issue. Sometimes a dangerous ignition. What tells the fan to turn off once the burners have cooled down?
 
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Old 02-14-21, 09:06 AM
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What do you mean by dangerous ignition?
Checking the heat exchanger is MANDATORY. And should be done before any more diagnosis is started. If the heat exchanger is cracked, your furnace is dead.
 
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Old 02-14-21, 10:07 AM
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You didn't mention anything about the blower in your first post.
It sounded more like a burner issue.
 
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Old 02-14-21, 01:38 PM
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Sorry about that. I should have said blower not unit.
 
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Old 02-15-21, 04:26 PM
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What tells the fan to turn off once the burners have cooled down?
 
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Old 02-15-21, 04:28 PM
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I believe your unit has a fan/limit switch that services that purpose.
 
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