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I think 220 volt gfci's work as well as the 110. Exception is that
neutral
not used - current in both power lines must be equal.

quoted from the Siemans link
"To protect against electrical shock, Siemens GFCIs continuously monitor the
difference in current between the hot and neutral conductors."

Since the heater does not have a neutral how will a GFCI provide any
increased safety?