What Is a Furnace Transformer?

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Any furnace that uses voltage other than standard commercial power needs a transformer. Whether for home or industrial furnaces, the transformer changes the voltage of the input power to a different voltage for use within the furnace.

How It Works

The input to the transformer connects to commercial power and goes through the primary winding--a coil of wires wrapped around a metal core. The primary induces voltage into a secondary coil, usually with fewer windings, to reduce the voltage.


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The Output

The reduced output voltage of the transformer goes to the control circuits of the furnace to perform the required electrical functions. That can involve temperature controls, timers and igniters.


Output Voltage

A universal NEMA type-D furnace transformer for domestic use has an output voltage of 26.5 volts for an input of 120 volts. However, specialty transformers for custom or elaborate furnaces could have different outputs.



A typical universal type-D transformer measures 2 3/32 inches high by 2 5/8 inches wide by 2 3/16 inches deep. It comes with 9-inch, color-coded wires attached to the primary and the secondary.


Industrial Transformers

Industrial high-performance transformers, used for induction heating processes, come with automatic cooling and several outlets for various loads. The transformers are available in single-phase, double-phase or three-phase input configurations for high-power operations.


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