What many people (even news anchors on well-respected TV news channels!) call a
water heater.
You don't want to heat water that's already hot for
Pete sakes!! It's proper to call it a 'water heater' or you can even call it a 'hot water tank' -- but never a 'hot water heater'.
{Latoya}: Gary, I think the hot
water heater is busted; I can't get any hot water out of this faucet to
do the dishes!
{Gary}: Latoya, we don't have a 'hot water heater' -- we might have a hot water cooler, a
water heater, or a hot water tank, but no hot water heater! Which of those do you want me to check?