#BertLife 4 – Why Does the Fuse Blow? Magic?
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Bert’s customer had called another company previously and said that the thermostat needed to be replaced, and the low-voltage fuses kept blowing. Bert goes out to investigate the source of the fuse problem with isolation diagnosis to see if the thermostat really was the culprit.
When Bert isolates the thermostat by pulling it off (and removing 24v from any of the other wires), securing the wires underneath a wire nut, and seeing if the short keeps happening. If the fuse keeps blowing, then the problem is NOT in the thermostat. Problems may include shorts elsewhere caused by nicked wires or rubouts at the condenser or air handler.
Sure enough, the fuse keeps blowing without the thermostat in the circuit. Bert disconnects the wires and isolates the outdoor unit from the indoor unit. He does the spark test at the air handler, where he connects a 24v hot wire to the thermostat wires to see which one sparks; a large spark indicates a short. You may occasionally get small sparks on the contactor coil or reversing valve, but Bert’s reversing valve (orange) wire has a spark that is WAY too large.
Bert disconnects the condenser and isolates the orange wire in the chase. Despite the condenser being disconnected, the spark is still much larger than it should be, indicating to Bert that the short is somewhere in the chase. He brings out his meter and checks the resistance, confirming his suspicions that the orange wire is shorting out in the chase.
So, Bert routes some new wire, taking us into the attic with him and alongside the house to strap the wire. While strapping the wire, Bert hears the equipment come back on, indicating that his job was a success. He caps off the episode with a celebration dance.