Editing Autonomous building (section)

===Water heating===
{{Further|Solar hot water}}
[[Hot water heat recycling]] units recover heat from water drain lines. They increase a building's autonomy by decreasing the heat or fuel used to heat water. They are attractive because they have no lifestyle changes.

Current practical, comfortable domestic water-heating systems combine a solar preheating system with a [[Tankless water heater|thermostatic gas-powered flow-through heater]], so that the temperature of the water is consistent, and the amount is unlimited. This reduces life-style impacts at some cost in autonomy.

[[Solar water heater]]s can save large amounts of fuel. Also, small changes in lifestyle, such as doing laundry, dishes and bathing on sunny days, can greatly increase their efficiency. Pure solar heaters are especially useful for laundries, swimming pools and external baths, because these can be scheduled for use on sunny days.

The basic trick in a [[solar water heating]] system is to use a well-insulated holding tank. Some systems are [[vacuum]]- insulated, acting something like large [[thermos]] bottles. The tank is filled with hot water on sunny days, and made available at all times. Unlike a conventional tank water heater, the tank is filled only when there is sunlight. Good storage makes a smaller, higher-technology collector feasible. Such collectors can use relatively exotic technologies, such as vacuum insulation, and reflective concentration of sunlight.

[[Cogeneration]] systems produce hot water from [[waste heat]]. They usually get the heat from the exhaust of a generator or fuel cell.

Heat recycling, [[cogeneration]] and solar pre-heating can save 50–75% of the gas otherwise used. Also, some combinations provide redundant reliability by having several sources of heat.
Some authorities advocate replacing [[bottled gas]] or [[natural gas]] with [[biogas]]. However, this is usually impractical unless live-stock are on-site. The wastes of a single family are usually insufficient to produce enough [[methane]] for anything more than small amounts of cooking.