If you are fortunate enough to have hot water in your home in China, it may be produced in a variety of ways. For some, cold water is the only option. For others hot water is available in their housing unit or dormitory only during certain hours.
In the apartment complexes that we've had the chance to live in, we have always had 24 hour access to hot water, albeit by different methods. At Yan Bei Yuan there was a water heater on the wall which (as long as the batteries were working!) would ignite and heat the water directly when you turned the hot water on in the sink or shower. Last year in Tianzuo Guoji, it was just like back home in that there was no contraption visible. If you turned the hot water knob, hot water came out, any time, day or night.
This year at Xue Yuan Pai, yet again there was a contraption involved. A large tank hung from the bathroom wall. It needed to be switched on around 30 minutes before the water was needed so it could heat up. Given the varied length and heat needed for each of our showers (some of us take long, really hot showers, others quick and cool), timing was everything. As a result, whoever got up in the middle of the night would be charged with pushing the button so it would be ready for the first shower-taker. Then, depending on which family member went next, he or she would have to wait for the temperature indicator to reach the desired number.
This, of course, made for some really fun mornings. Especially the day when our electricity ran out...