Drain Water Heat Recovery or DWHR is the use of a heat exchanger to recover some of the heat from the drain 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, or sometimes shower water heat recovery.
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 DWHR systems work by preheating the incoming cold water using the heat contained in the drain water.
For the remainder of this discussion, we will focus on the application of DWHR 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 DWHR systems can be implemented without the need of storing the drain water.
DWHR 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 DWHR systems that are commercially available, see the page DWHR Categories. 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 DWHR system itself, the shower flow, cold water temperature and installation details.