Waste Water Heat Recovery or WWHR is the use of a heat exchanger to recover some of the heat from the waste water from activities such as dish-washing, clothes washing and especially showering. This is also referred to as water heat recycling, grey water heat recovery, drain water heat recovery ((DWHR) or sometimes shower water heat recovery.

Shower setup, no WWHR installed, with example temperatures

It is important to note that it is not the drain water itself that is re-used, only the heat that it carries is partially recovered. All WWHR systems work by preheating the incoming cold water using the heat contained in the drain water.

Shower setup, WWHR installed, with example temperatures

For the remainder of this discussion, we will focus on the application of WWHR in combination  with a shower, since this is most common and also gives the best opportunity for energy savings in a domestic setting. Also, because of the simultaneous flow of drain water from the shower and cold water from the utility, a larger variety of WWHR systems can be implemented without the need of storing the drain water.

WWHR makes a lot of sense, considering that domestic drain water still carries a lot of energy: water that is used in the shower has a temperature of 38-40°C (100-104°F) and enters the drain at 35-37°C (95-99°F). This means that approximately 90% of the energy that has been used by the water heater to warm up this water is wasted.

For more information about the types of domestic WWHR systems that are commercially available, see the page WWHR alternatives. Different ways of installation are presented on the page Installation Alternatives

The temperatures given in the accompanying figures for cold, hot and preheated water are meant as typical examples only. Cold water temperature varies with local conditions and time of year, hot water temperature depends on the settings and performance of the water heater. The preheated water temperature depends the effectiveness of the WWHR system itself, the shower flow, cold water temperature and installation details.