Step-down transformers use wire coils wrapped around an iron core to convert high-voltage electricity to low-voltage electricity. In a furnace, the low-voltage electricity flows from the transformer to a thermostat. When the thermostat senses a demand, a switch inside the thermostat closes. This sends the low-voltage electricity back to the furnace's low-voltage controls. After the appropriate control makes contact, the low-voltage electricity travels back to the transformer, where the circuit completes. Before replacing a gas furnace's transformer, a technician will find and repair the electrical short, which usually occurs in a different part of the low-voltage circuit.
Turn the circuit breaker, in the circuit breaker box that controls the gas furnace, off. The correct circuit breaker has a "Furnace" label. Some furnaces have an electrical service panel near the unit. Follow the wires out of the unit and if they lead to a box, open the box's lid and turn the switch off.
Open the furnace's front cover with the correct-size nut driver. Some furnaces need the bottom panel removed to access the top panel's mounting screws. If the furnace has a handle on the bottom panel, lift the panel up while pulling the handle out. Furnaces with a handle usually only use mounting screws to hold the top panel in place.
Follow the wires from the terminal block, the metal block connected to the the wires that penetrate the furnace's cabinet, to the transformer. The transformer, a square component with a metal band, hase four wires in use. Two of the wires lead to the terminal block and the other two connect to the low-voltage controls. Any extra wires, used in multi-voltage transformers, have wire nut caps.
Cut the transformer's wires with wire cutters. Leave any unused wires capped and uncut. Cutting the wires will help identify the correct wiring sequence while wiring the new transformer.
Unscrew the transformer's mounting screws, found on the transformer's base, with a nut driver. Almost all mounting screws use either 1/4- or 5/16-inch nut drivers.
Remove the disconnected transformer.
Observe the old transformer's wiring diagram. Determine which color wire goes to what, paying special attention to the high-voltage side. Often the high-voltage side uses the white wire for neutral and the black wire for power, but in the cases where a transformer can handle different high-voltages the colors may change and each voltage will have its own color wire. The low-voltage, often--but not always--uses yellow and red or yellow and blue wires and will have a "24V" identification.
Analyze the new transformer's wiring diagram. Find the two low-voltage wires. Find the two high-voltage wires that match the old transformer's high-voltage wires, use the wiring diagram not wire colors. Twist a wire nuts on each extra high-voltage wire. Do not twist any wires together.
Mount the new transformer in the furnace, using the removed mounting screws.
Connect the new transformer's low-voltage wires, using the old transformer's cut low-voltage wires as a guide. The low-voltage wires from a transformer connect to low-voltage control wires with wire nuts. Untwist the wire nut, remove the cut wire from the old transformer, twist the correct wire from the new transformer to the control wire and secure it with a new wire nut. Repeat this for the second low-voltage wire.
Connect the new transformer's high-voltage wires using the old transformer's wires as a guide. In some cases the transformer's wires connect directly to the terminal block. If so, unscrew the old wires from the terminal block and replace them with the correct wires from the new transformer. Sometimes wire nuts hold the high-voltage wires to jumper wires. In that case, remove the old wire nuts and cut high-voltage wires, twist the new high-voltage wires to their respective jumper-wire and secure them with new wire nuts.
Replace the furnace's covers and turn the electricity to the furnace on.