Water heat recycling
Water heat recycling (also known as drain water heat recovery, greywater heat recovery, or sometimes shower water heat recovery) is the use of a heat exchanger to recover energy and reuse heat from drain water from various activities such as dish-washing, clothes washing and especially showers. The technology is used to reduce primary energy consumption for water heating. Standard units save up to 60% of the heat energy that is otherwise lost down the drain when using the shower.
The technology is fully recognized in Canada and USA by LEED for homes, Energy Star for New Homes Canada. Within the United Kingdom heat recovery is recognized and utilized within HVAC, and Greywater.
Typical retail price for a domestic drain water heat recovery unit ranges from around $400 to $1,000 Canadian. For a regular household, water heating is usually the second highest source of energy demand. The energy savings results in an average payback time for the initial investment of 2–10 years according to Natural Resources Canada, Canadian Center for Housing Technology and US DOE.
Heating water accounts for 18% of the average household utility bill. Installing a water heat recycler reduces energy consumption and thus greenhouse gas emissions and the overall energy dependency of the household. The cold water that is put into a water heating device can be preheated using the reclaimed thermal energy from a shower so that the input water doesn't need as much energy to be heated before being used in a shower, dishwasher, or sink. The water entering a storage tank is usually close to 11°C but by recovering the energy in the hot water from a bath or dishwasher, the temperature of the water entering the holding tank can be elevated to 25°C, saving energy required to increase the temperature of a given amount of water by 14°C. This water is then heated up a little further to 37°C before leaving the tank and going to the average shower.
Explanation of Input Water Heat http://www.ecohometips.com/images/watercycle.jpg 
Heat Recovery from Greywater
When recycling water from a bath (100-150 litres) or shower (50-80 litres) the waste water temperature is circa 20-25°C. An In-house Greywater recycling tank holds 150-175 litres allowing for the majority of waste water to be stored. Utilizing a built in Copper heat exchange with circulation pump the residual heat is recovered and transferred to the cold feed of a combi-boiler or hot-water cylinder, reducing the energy used by the existing central heating system to heat water.
- "Drain-water Heat Recovery". Eco Home Tips. Retrieved 2/3/12.