208v transformer in place of 120v

Furnace uses 120volt transformer stepping down to 24volt. Looking around van me and my coworker found a 208/240 to 24. No we didn't use it but we got curious about the problems that could create.

He thought it would fry the board but I felt like the line power is still 120 I think the transformer might just not properly convert 24volts on the other end which would maybe make the transformer over heat and fry out?