Water heat recycling

Water heat recycling (also known as drain water heat recovery, waste 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.

How it works


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 (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.

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 about 20% of overall energy demand. The energy savings can result in an average payback time for the initial investment of 2–10 years.

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.

Industrial scale and HVAC
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 (compared to the outside air) 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. Heat rejected by a chiller system for providing air-conditioning to larger buildings can be recovered by installing a heat-exchanger between the incoming domestic cold water, and condenser water return. A conventional chilled water system rejects heat gathered by the condenser water loop from the refrigerant to a cooling tower. By diverting a fraction of mass flow rate of condenser water away from the cooling tower, and circulating it through a heat-exchanger (usually a plate-and-frame configuration), incoming domestic cold water can be pre-heated before reaching the boiler. This reduces the required increase in temperature of the water before it can be supplied to the end user, and therefore lowering boiler fuel burn.