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Old 07-22-2009, 10:31 AM   #1
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Low Voltage Wire Shorting


The low voltage thermostat wire from a condensor unit outside to the thermostat control board above the air handler unit appears to be shorting and blowing the transformer. I installed an inline fuse to protect the transformer and ran a jumper wire from the control board to directly to the condensor unit. The Unit ran for 2 days with no problems, never blowing the fuse. This seemed to verify that there was a short somewhere in that existing wire (the house is 20 yrs old)....Instead of running all new wire, I decided to reconnect the existing wire at the condensor unit and look for evidence of bad wire in the attic (at least for the wire I could see). I went back as far as I could get to and spliced into the existing wire in the attic and ran new wire to the thermostat control board.... The unit ran fine for 1 night, in the morning the fuse was blown. I understand the basics of system components. Does the fact that unit ran for 2 days with no problems tell me that I need to replace the entire wire? Why did the unit run for 1 night with the short jumper? All the connections look good. I am hoping for any suggestions or insight on what to check next.

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Old 07-22-2009, 12:06 PM   #2
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Low Voltage Wire Shorting


What makes you believe its the cable (wire). Fuses blowing can be caused by many circumstances. If you think it is the cable, replace the whole run without any splices. Then if it happens again you will know it was not the cable. You should call an HVAC tech if it is not the cable.

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Old 07-22-2009, 12:13 PM   #3
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Low Voltage Wire Shorting


I believe its the cable (wire) because when I ran the new 200 ft wire from the attic down the stairs and out the window to the unit, it ran for 2 days without blowing the fuse. Considered leaving the wire for a few days to confirm, but the wife would not go for that more than a 2 days. Thats why I went back up in the attic and made the splice back as far as I could physically get to.
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Old 07-22-2009, 02:15 PM   #4
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Low Voltage Wire Shorting


Is there a basic test to verify that the thermostat is good? It is a 6 wire thermostat cable with heat pump system.
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