Transformers difference

[e3belectrical]

what is the difference between furnace Transformer and Auto Transformer?

 

[BradtheRad]

An auto-transformer (also known as an autoformer) has one winding, combining the function of two windings.
One terminal usually serves as a common. Thus it does not isolate the incoming and outgoing sides.

'Furnace' transformer probably refers to a common step-down type, with the primary winding being isolated from the secondary. It is essential to prevent high voltage from contacting gas or liquid fuel.