Magic chef furnace main burner whiting down after fan comes on


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Old 11-09-10, 05:51 AM
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Magic chef furnace main burner shuting down after fan comes on

I have a magic chef 80% furnace. The pilot lights. The main burner comes on,heats up,fan comes on. Then the main burner shuts down. The fan limit does not reach high limit. But burner is shuting down. Tried a new control module same problem. Replaced flame sensor. Could it be my fan limit switch and how do I check it?
 
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Old 11-09-10, 06:10 AM
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You need to identify what is shutting off the burners. Odds are that's the thermostat or pressure switch opening.

Since you apparently have an intermittent pilot ignition system, a dirty pilot burner could also be the problem.

You need an AC voltmeter to check to see if the thermostat or the pressure switch is opening.
 
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Old 11-09-10, 07:03 AM
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There is no pressure switch. It has an old t87 tstat on it. The fan limit seems to be jumping not running smooth. Is there any way to check it? I will try new stat. Thank you for the help.
 
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Old 11-09-10, 07:07 AM
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Changing out parts on a sheer guess is a waste of time and money. You need an AC voltmeter if you are going to diagnose this problem. If you don't have one and don't want to get one, hire a pro to do the repairs for you.
 
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Old 11-09-10, 07:14 AM
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I have a volt meter and will try what you suggested. I can jump to stat out. Right? It needs updated any how. But I just didn't know how or if you could check the limit switch.
 
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Old 11-09-10, 09:16 AM
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Use the furnace chassis as ground for one connection to your voltmeter. Use the other test lead to connect to the connection you want to check for voltage.

Test the "R" furnace connection, which should have 24 VAC. The "W" furnace connection will have 24 VAC to it when the thermostat is calling for heat.

I wouldn't recommend changing out the thermostat until the furnace is repaired, since that would add additional uncertainty to the repair problem. Deal with one issue at a time.

The limit switch shuts the burners off if the furnace overheats, and allows the furnace fan to continue to circulate air to cool off the heat exchanger. If the limit switch is operating normally you will have 24 VAC on both sides of the limit switch. If it's open and shutting off the burners you will have 24 VAC going into it and 0 VAC coming out of the limit switch.
 
 

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