There are two main types of grey water heat recovery systems: storage and non-storage.  Storage systems drain the water through a spiral pipe at the bottom of a water heating tank and heat the water within, causing it to rise to the top and be circulated throughout the home.  Non-storage systems use a copper coil that is placed around the existing drain pipe.  The cold water runs through this coil and is heated by the draining hot water and then circulates to showers, sinks, etc.  Though non-storage systems are more popular and less expensive, they typically work best only when simultaneous hot and cold water is flowing (for example during a shower when hot  water is draining and cold water is being pumped to through the heater).  Non-storage systems are typically easier to install because they just require a replacement of part of the drain line with a copper coil rather than a more integrated system replacement.

Estimated Cost Savings:
Typical grey water heat recovery systems cost between $300 and $500, depending on the brand and size.  As professional installation is recommended, there will be some installation costs in addition to the initial purchase costs, but these can usually be minimized if rolled in with new home building or larger renovations.  Installing a grey water heat recovery system can help reduce water heating bills by up to 40%.  Given that water heating makes up about 20% of a typical home's energy usage, this means you can reduce your total energy consumption by 5-10%.  Depending on how often you use hot water, the return on investment can be anywhere from two to eight years.  If there are a large number of people in your family and you use significant amounts of water for showering, laundry, dishes, etc., then your system is likely to pay for itself in closer to two to four years.