For some who are trying to maximize energy savings, waste water heat recycling may be an attractive option. A waste water heat recovery system collects heat from warm water sources like tubs and showers or clothes washers and uses it to add heat to incoming cold water and reduce the load on the water heater.
A waste water heat recovery system is simple with no moving parts. It is basically a copper coil wrapped around the waste discharge pipe. Cold water circulates through the coil and picks up heat from the waste water flowing through the drain. The warmed water can then be supplied on the cold water side of the faucet where it is added at the mixing valve. Because the incoming cold water is warmer it takes less hot water to maintain the right temperature. The heat recovery line can also be attached to the intake line at the water heater; or with proper design, to both water heater and faucet.
Depending on the system a waste water heat recovery system can recover up to 60% of the waste heat. These coils are available for different waste pipe sizes and in different lengths. The length of recovery coil needed can make it difficult to retrofit an existing system so it is primarily used in new construction.
A waste water heat recovery system can cost from $300 to $1,000 depending on the size and efficiency. The U.S. Department of Energy suggests energy savings provide a payback in three to ten years, depending on how much hot water is used. Some sources suggest it can take up to twenty years to recover the cost for some residential conditions. A British study found a recovery system with an electric hot water heater can save from 380 to 600 kWh per year.
The heat recovery system can also be combined with a grey water recycling system where the waste water flows into a collection tank where a coil removes the heat and the filtered waste water is piped to toilets, saving both heat and water.