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.