So you think you may have a bad transformer on your furnace but you aren’t sure how to be certain. Let us take a look at a typical, 24-volt furnace transformer
As you can see in this photo above, the transformer will generally be located near the furnace control board. This is because most furnace control boards need to be powered by a 24-volt source.
The transformer here is known as a “step-down” transformer, meaning that it takes an incoming power (input) voltage and “steps it down” to a lower voltage (output) necessary for various applications. The most popular are 24-volt transformers.
In order to begin testing a furnace transformer, you will need to have a digital voltmeter (shown in the photo above).
The first step is checking that the power to the furnace is in fact on. It would be awful hard to test voltages with the power supply off! So let us check the breaker and the furnace switch. The furnace should have a light switch either attached to the furnace itself or nearby (shown in the photo above)
The next step you will want to take is to remove the furnace doors and expose the area where the transformer is located. This next step is often overlooked by do-it-yourselfers. Furnaces have a safety door switch (shown above) that must be depressed in order to pass power to the transformer and furnace. You will need to temporarily tape this down with some electrical tape or you may use a zip-tie as in the example above.
The next step is to remove the 120-volt input wire connector to the transformer and insert the red meter lead into this connector and touch the black meter probe to any ground source (screw or bare metal frame) and check the meter reading as shown in the photos above. It should read 120-volts ac. If it does not, then your issue is incoming power-related and not the transformer itself. Recheck all of your connections, the power switch itself, and the breaker.
If it reads 120-volts, reconnect the input wire to the transformer and remove the labeled 24v connection and touch your red test lead from the meter to the 24v output post (shown in the photo above) and the black meter test lead to good ground. The meter should read 24-volts. If the meter does read 24-volts, your transformer is good. If this is the case, then check the fuse on your control board. If the fuse is blown, check all the low voltage (24-volt) wiring, there may be a short somewhere that caused the fuse to blow and when you replace the fuse, it will just blow again if not corrected beforehand.
If the meter does not read 24 volts ac, then you have a failed transformer and will need to replace it. Now before you go ahead and replace your transformer, you need to investigate why this happened. It is very rare that a transformer fails in a furnace on its own. Generally, it fails from some sort of electrical short in the wiring. So be sure to look over all the wiring and check for frays, missing insulation, and loose connections before you spend money and install another transformer.
As always, if you need any help diagnosing any issue on your furnace or ac unit, our experts at Fixmyfurnace.com are standing by with our virtual diagnostic tool.