Furance trips main breaker when fan switch reaches low limit


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Old 10-28-12, 05:28 PM
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Furance trips main breaker when fan switch reaches low limit

I have a1988 Rheem Imperial 90 Plus. When I turn on the heat the furnace exhaust fan comes on, then spark and flame. The fan limit switch rotates and as soon as it reaches low limit and signals the blower fan motor to turn on it trips the main breaker in the fuse box. I removed the fan switch with bracket but was unable to remove the switch from the bracket or the wires from the switch. It looks like the only way to remove the switch is to cut the two red wires since they go to a plug. The two black wires will disconnect; one goes to a snap disk and the other to the pressure switch. I don’t think I will find the direct replacement switch so cutting the wires is not a problem assuming I have diagnosed the problem correctly. Three months ago I replaced the blower fan motor and capacitor. The blower fan motor works with the AC and when switched on manually at the thermostat once the furnace cools down below low limit so I don’t think it is the blower fan motor. How can I test the switch? What else might the problem be? Thank You for any help.
 
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Old 10-28-12, 05:52 PM
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Welcome to the forums! What size service do you have? What is the overcurrent protection device amperage rating on the air handler? Just difficult to believe it will trip out a 100 or 200 amp main breaker before it tripped out a 30+ amp breaker for the unit. Can you post pictures of your panel, opened up and the control unit on the air handler. It will help if we can see what you see. http://www.doityourself.com/forum/el...your-post.html
 
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Old 10-28-12, 09:20 PM
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I'm guessing that just the breaker for the furnace is tripping and not the whole house main breaker. If the main for the house is tripping

If that replacement blower motor is a multispeed multi tap type motor maybe you wired the speeds incorrectly. I've seen an issue where the ac worked fine and the FAN ON position worked fine but when the heat stat in the furnace told the fan to run....bang.
 
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Old 10-28-12, 09:53 PM
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Just the furnace breaker is triping. The motor has 3 speeds but the other two wires for the two remaining speeds are capped. If I want a different speed I connect to a different wire. I had a newer unit in a different house where all the motor speed wires went to the contol box. Not on this 1988 unit. Sorry for the confusion.
 
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Old 10-29-12, 04:45 PM
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I replaced the fan limit switch today but the problem remains. There are six wires that come from the motor. One brown and one brown/white these go to the capacitor.
Yellow Blue and Red the three different speeds for the motor. Blue and Red are capped and yellow goes to the fan limit switch. The black wire goes to a terminal block or maybe it is a relay. There is also what looks like two transformers. My guess is one is for the thermostat not sure why there are two. Since the motor and capacitor is only three months old and the fan runs when the thermostat is switched to manual I am assuming the motor and capacitor are not the problem. Not sure if that is a correct assumption. Any ideas where to look next? Thank You
 
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Old 10-29-12, 05:05 PM
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How about you post a picture of the wiring schematic for your furnace (typically located behind the blower door).
 
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Old 10-30-12, 09:36 PM
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Update

Update. I have a gas fireplace which I am using to keep the house warm but the room where it is located is hot while other rooms cold. At 4:30 I turned on the furnace fan at the thermostat to distribute the heat. The furnace fan was still running at 5:30 when I left but the breaker was tripped when I returned at 8:30. I reset the breaker and the furnace fan ran for one hour before I turned it off.
Next I turned the switch off at the furnace and turned the furnace fan and heat on at the thermostat. I wanted to see if both running at the same time would trip the breaker. I turned on the furnace at the furnace and both the exhaust fan and blower fan started and ran fine. The furnace went through normal startup and the new fan limit switch approached the lower limit to turn on the house fan. Because the blower fan was already running the fan limit switch did not reach low temperature to turn on, it might have eventually. I turned fan limit switch dial by hand; it did not trip the breaker. Next I turned the blower fan to auto at the thermostat, the blower fan turned off because I was below limit on fan switch. Within a few minutes the fan limit switch reached lower limit and signaled the blower fan to turn on which tripped the breaker.

I noticed the furnace is not the only thing on the breaker. But the only other draw is an outlet with a clock and cell phone charger. I will address this but I do not think it the problem since it has been like that for ten plus years already.
When I replace the motor three months ago it was replaced with a ½ horse. The old motor was 1/3 horse.
I am going to try to post a schematic requested but it is over 50KB limit so I will have to see what I can do.
When I replace the motor three months ago I replaced the capacitor also; a ten dollar part. The neighbor’s blower fan stopped working also and the repair person replaced the capacitor and charged $350.
 
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Old 10-30-12, 11:12 PM
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Update 2

Since my last update the testing has resulted in whatever part is causing the problem to permanently fail. With the thermostat heat off and fan on auto and the switch at the furnace off when I turn on the switch at the furnace the breaker immediately trips. Permanent short! I am thinking the heat/cool relay. Took the whole control box out and will take to parts supplier Johnstone Supply to see if they can determine problem.
Will keep posting on findings.
 
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Old 10-31-12, 05:31 PM
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Problem Solved

I assumed the wire color from the old motor was the same for the new motor.
Motor was fine but wired wrong. The yellow wire was connected to the heat side but it was a line wire not a speed wire. The wire connected to the cool side was a speed wire. That is way it worked on cool but not on heat. It cost me $100 for a part I did not need but I learned a lot and probably at least that if I would have called someone. Now if I can remember until the next time.
When I had the control box on the counter at the parts store a contractor saw it and said “an old Reem” I was impressed.
Thanks for the help it got me to the finish line
 
 

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