Although it is not recommended to apply a different voltage to a heater already designed for a set voltage, it can be acceptable to do so if the new voltage being applied is lower than the already-designed voltage. This will just result in a lower capacity and amp draw. It is not acceptable to apply a higher voltage that the already-designed voltage. Doing this will increase the capacity and amp draw of the heater, which can cause damage to the elements and components, and creates a risk of overheating and potential fires.