DIY Appliance Repair: How to Test an Oil Furnace HV Transformer

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Most people do not think of furnaces when they think of home appliances, but in the strictest sense of the word, they are appliances, just as air conditioners, washing machines, clothes dryers, and electric ranges are appliances. Like any major appliance,

At the heart of every oil furnace ignition system is either a conventional high voltage transformer or an electric spark generator. Although the electronic spark generators are rapidly replacing the conventional high-voltage transformers in oil furnaces, new entry-level furnaces still use them as do older furnaces that still have many serviceable years left in them. In this article we will concentrate on testing those conventional transformers. They are simple devices compared to the electronic spark generators and easy to test, but testing them can be hazardous to your health if you do not follow the proper safety precautions when testing them.

Dangerously High-Voltage.

Back in the day when I was studying Radio and Television Repair, the instructor repeatedly warned us of two grave dangers—the dangerously high voltage on the Second Anode of the Cathode Ray Tube (CRT) and the danger of an imploding picture tube. There was 10,000 volts present on the Second Anode of the picture, in the case of Black and White sets and 25,000 volts on the Second Anode of the CRT in color sets, which were just becoming available. Ten thousand volts could kill you in a heartbeat, no pun intended. An imploding picture tube could shred every thing in the immediate area just as if it had been an exploding hand-grenade. Except for the imploding picture tube, the same dangers are preset when working on an oil furnace ignition transformer which has a secondary voltage between 10,000 and 14,000 volts.

A peek inside the transformer's “Can”.

The two white leads you see exiting from the top-right hand corner of the transformer are the transformer's 120-Volt AC primary winding leads. The two high-voltage terminals

are these two heavy “L” shaped terminals at the top front of the drawing.

Tools and test equipment required.

Preparing to test the Ignition Transformer.

Before beginning work on the oil furnace, turn off both the furnace branch circuit, circuit breaker on the service panel and the furnace's emergency disconnect switch. The furnace's emergency disconnect switch should be located within sight of the furnace and have a Red colored cove plate. The switch itself, other than for its Red cover plate may look like an ordinary light switch.

After shutting off the power to the furnace, remove the transformer for inspection and testing.

Inspecting and Testing the High-Voltage Transformer.

  • Wipe the porcelain insulator off with a clean, lint-free rag and check them for any indication of cracking or other damage. If the high-voltage insulators are damaged in any way, replace them before continuing with the test.

  • Attach the 120-Volt. 3-wire test cord to the transformer. Connect the black and the white insulated alligator clips to the transformer's primary leads and the green insulated or uninsulated grounding alligator clip to the transformer's can.

  • Connect the high-voltage test prove to the DMM and set the DMM to the AC Voltage function. Connect the grounding clip of the high-voltage probe to the transformer's can.

  • Plug in the 120-Volt test jumper and touch the tip of the high-voltage probe to each of the secondary terminals, as shown here.


Each terminal should read 5,000 volts. If the reading that you get is significantly lower than 5,000 volts, replace the transformer.

Alternative Test method

If you do not have a high-voltage probe for your DMM, you can test the secondary with nothing more than a well-insulated screwdriver as shown below.

A good transformer will produce a spark that easily spans a 3/4-inch air gap. If the spark appears weak, replace the transformer.


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