A good supply of hot water is only truly appreciated when it stops. And if it stops when you're in the shower, this realization can arrive quite abruptly. Fortunately, it doesn't happen that often. The systems that provide hot water are generally reliable and operate for years without a hitch. But when the hitch comes, some background can help. Even if you can't make the repairs yourself, you'll have a good idea what to tell the doctor when he arrives.