Unfortunately, a whole-house water heater is depreciated over 27.5 years (even though the real lifespan is much shorter than that).
It is considered part of the Plumbing System, which is depreciated over 27.5 years. If it needs to be replaced before 27.5 years, then you would get the rest of the deduction when it is disposed of.
However, as Carl said, if you qualify for the De Minimis Election and choose to make that election, you may be able to deduct the entire cost in one year as a supply or repair.
