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 does not 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. When recycling water from a bath or shower 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.
Impact and cost
Heating water accounts for 18% of the average household utility bill. Standard units save up to 60% of the heat energy that is otherwise lost down the drain when using the shower. Installing a water heat recycler reduces energy consumption and thus greenhouse gas emissions and the overall energy dependency of the household. 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. A 2-year independent study of waste water heat recovery systems installed into residential houses in the UK found savings of 380kWh and 500kWh per person per year.
A heat pump can be combined with municipal sewage lines to allow a large building's HVAC system recycle the winter heat or summer cool of water flowing out of many homes and businesses. The reverse is also possible: heat from air conditioning and industrial chillers can be used to pre-heat water.
