Furnace repair work can be complicated and may require the assistance of a professional. However, some projects, such as replacing a furnace transformer, are fairly straightforward and can be accomplished with a bit of preparation – and for a low cost. But certain precautions must be taken to ensure your safety.

A service man working on a home furnace
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How to Replace a Furnace Transformer

Checking for Supply Voltage

You should use a voltage test meter to check the power running to your transformer. Typically, transformers use either 120 or 240 volts of power. Determining the voltage coming into the transformer is a key first step so that you can see if things are working as they should.

Transformers are comprised of two systems: One for supply voltage and one for secondary voltage. In many instances, especially with a 120-volt transformer, the supply wires are black and white, but hey may be different colors depending on your unit. The black wire is likely the live one, and the white is more likely the neutral wire. Check the manufacturer's instructions, if possible, to determine which wire is which before proceeding.

Use the black lead of the voltmeter to test the voltage of the black wire. Use the red lead of the voltmeter to test the white wire. The voltmeter should read 120 volts, provided that's the type of transformer you have. If it doesn't, move the red lead to a ground wire for testing. If that works properly, the white wire in your transformer is malfunctioning. If you still don't get a reading on the test meter, there's another problem.

Checking for Secondary Voltage

You'll follow the same process to check for secondary voltage. Wires present may be a variety of colors. In many cases, they are red or blue. Again, if possible, check the manufacturer's instructions to determine which wire is which before proceeding.

Use your voltage test meter and connect the red lead to the red wire. Place the black lead on the blue wire. Your voltage meter should display the number of volts that your transformer is rated for. If it does not, you'll need to replace the transformer.

Replacing the Transformer

If you need to replace a transformer, be sure to turn off power at the circuit breaker before proceeding. This is extremely important as live electricity can be hazardous. Inform others who are present of the project you'll be working on, and ask that they not turn the breaker back on.

Next, open the furnace cover so you have access to the transformer. If you've already checked your voltage as outlined above, this step should be taken care of. The transformer should have four wires connected to it. You'll need to use wire cutters to cut these so that you can disconnect the transformer. Remove the transformer using a screwdriver or socket wrench depending on how it is attached.

Before you wire the new transformer to the furnace, read the manufacturer's instructions very carefully. Be sure that you are clear as to which wires are the supply and which are the secondary wires. You will need to be sure they correspond to the furnace correctly.

Remove the old wires once you are through, using them as guides. Attach the new wires to the furnace using wire nuts. It may be easier to navigate the replacement of the transformer if you have already bolted or screwed the new part into the furnace.

When you are finished, carefully test the new transformer with a voltage tester as outlined above. To do this, you will need to turn the circuit breaker back on. If everything tests satisfactorily, replace the furnace door or other exterior coverings.