Drain Water Heat Recovery When we shower, we heat cold water and then run it over ourselves and down the drain. A drain water heat recovery (DWHR) unit captures up to 60% of that heat and puts it back into the shower water or the hot water tank. These units can also triple the recovery rate of a water heater, making hot water available much sooner and extending the life of the heater. Drain water heat recovery units consist of a central core, through which warm wastewater flows, wrapped with copper heat exchanger tubing, through which the cold incoming water flows. Fresh cold water is thus preheated before it goes to the water heater—or better yet, to the cold-water side of the shower. (Preheating the cold shower water means you need less fuel-heated hot water.) It’s a relatively simple system with no moving parts and no maintenance, but it does require sufficient room below the drain to allow gravity to work; it may not be usable in a single-story home without a basement. Some brands are Power Pipe, ThermoDrain, and Retherm. Utility rebates are available in several states, paying up to 40% of the cost of these units.