Air conditioners, heat pumps and furnaces all have components such as blowers and compressors that run on 120- or 240-volt power, but the controls for these components contain sensitive electronic circuitry that would be damaged by the high voltage. It's the job of the transformer to step down the high voltage to one that is more manageable — usually 24 volts. The thermostat, being part of the control system, also operates at 24 volts and gets its power from the transformer.
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A transformer is a simple device with no moving parts, and it's rare for one to fail. When one does, it's usually because of a loose internal contact or a burnt-out wire. It's fairly easy to test a transformer to see if it's causing your air conditioning system problems, but first you have to know where it is and what it looks like.
The transformer on an air conditioning unit is either inside the air handler or in the compressor housing, which is outdoors. It's usually attached to an inside wall and is recognizable by the wires attached to it.
AC Transformer Characteristics
The transformer for a central air conditioning unit is not much bigger than the transformer in the plug you use to charge a laptop. It's a square or rectangular box seldom wider than 4 inches and not much longer than that. The shape as well as the number of brackets it has for attaching to the air handler or compressor housing varies with brand, but you can always identify it by its wires.
On an air conditioning unit that uses 240-volt power, which is most of them, you'll see four large-diameter primary wires connected to one side and two smaller wires coming from the other side. These are the 24-volt secondary wires that feed the control system. The primary wires are white, black, red and sometimes orange, while the secondary wire colors are variable — often blue, green or red.
Most Common Transformer Location
Most central air conditioners are split systems, meaning the compressor and compressor fan are outdoors and the evaporator coils are indoors inside the air handler. The air handler usually contains a blower at the bottom and ductwork at the top. If the system includes a furnace, it will also be in the air handler housing.
The vast majority of the time, the transformer for the control panel is located inside the air handler. It's often near the bottom bolted to a back wall or, if the system includes a furnace, to a side wall. The transformer may also be mounted to an outside wall of the air handler. In cases where the transformer is outside, it's usually in the compressor housing and easy to reach when you remove the access panel.
How to Test a Transformer
You can verify that a transformer is working by testing the output voltage. Since you have to do this with the power on, you can potentially get a shock, so it's best to leave this to a licensed heating, ventilation and air conditioning pro.
If you do want to do it yourself, you'll need a multimeter set to measure AC volts. First, check the input voltage by putting one probe on the white wire and the other probe on each of the others in turn. You should get readings between 120 and 240 volts each time. Now touch the probes to the output terminals, and if the meter doesn't read close to 24 volts, the transformer is bad and needs to be replaced.