How to test a 24v furnace transformer

Written by anthony smith
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While complicated furnace repairs should be handled by a professional, a homeowner can often do basic troubleshooting involving parts like the furnace transformer. The transformer changes the standard 110-volt power available in homes to 24 volts, which is necessary for components like the thermostat. Anyone can test a furnace transformer and possibly avoid an expensive service call.

Skill level:
Moderately Easy

Things you need

  • Voltmeter or multimeter

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  1. 1

    Find the furnace transformer. It is a small rectangular box with the word "line" next to the wire connections on one side and "load" next to the wire connections on the other side.

  2. 2

    Turn on the multimeter and set it to display voltage. Touch the probes to the "line" side connections of the transformer and take note of the reading. It should read between 110 and 120 volts. If the reading is less, there is a problem with the incoming power to the transformer. This will have to be addressed before you can test the transformer again.

  3. 3

    Touch the probes to the "load" side connections of the transformer. A properly functioning 24-volt transformer should have a reading of 24 to 28 volts. When you have the proper 110 volts at the "line" side of the transformer but the "load" side voltage is not in the 24- to 28-volt range, the transformer is faulty and should be replaced.

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