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How to Short-jump a Furnace

Short-jumping a furnace is a technique for starting the unit by bypassing the in-line thermostat, which is a different component from the wall thermostat typically used to adjust indoor temperature. Essentially a diagnostic technique, short-jumping allows you to determine whether the in-line thermostat is defective. This is not a permanent solution to a malfunctioning thermostat because the furnace blower motor will run continuously. Do not leave the furnace hot-wired this way without repairing any problem.

Things You'll Need

  • Screwdriver set
  • 18-gauge wire
  • Wire strippers
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    • 1

      Cut off the furnace at the circuit breaker or pull the fuse in a fuse box.

    • 2

      Take out the screws holding the cover on the side of the furnace to locate the in-line thermostat inside. The thermostat has a series of wires connected to the screw contacts.

    • 3

      Loosen the screws labeled L1 and L2 on the in-line thermostat.

    • 4

      Strip 1/2-inch of insulation from both ends of the 18-gauge wire using the wire strippers.

    • 5

      Wrap one end of the wire around the L1 screw and the other end around the L2 screw. Tighten the screws.

    • 6

      Remove your hands and all tools from the open furnace cabinet before switching on the circuit breaker or replacing the fuse. If the furnace blower motor activates immediately, you have a defective in-line thermostat that must be replaced for the furnace to operate efficiently.