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Drainwater Heat Recovery

A sealed coil that removes the heat from waste hot water for reuse in preheating hot water

Diagram shows how hot water from a shower can be recovered and sent to plumbing fixtures or the water heater.

Drainwater heat recovery (DHR) devices recover heat energy from drainwater. This energy can be used to preheat cold water going to showers, a water heater, or a dedicated storage device. DHR systems reduce the energy needed for heating water and, in essence, increase the capacity of water heaters.

Heat exchangers for use in DHR vary in terms of pipe sizing, orientation of the drain line (horizontal, vertical, or other), and heat exchanger design. They also vary in cost and the amount of energy savings achieved.

There are several residential drainwater heat exchangers commercially available in the U.S. Most have the ability to recover energy for later use. Units with storage capacity can recover heat from any waste hot water (e.g., from a dishwasher, clothes washer, bathtub, or shower) which drains through the heat recovery device. Without storage capacity, DHR units provide useful energy recovery only during simultaneous flow of cold supply water and warm drain water (for example, during showering).

Some DHR systems are storage-type units that have a tank containing a reservoir of clean water. Drainwater flows through a heat exchanger within the storage tank, causing the tank water to warm. The cold water supply to the water heater is preheated by circulation through a coil also inside the storage tank. Heat from the warm drainwater is transferred to the cold supply water without mixing. The system is also designed so it does not lose a lot of stored energy when cold wastewater runs through the drainwater heat exchanger.

Another system is a non-storage unit in which a copper heat exchanger replaces a vertical or horizontal section of a main waste drain pipe. As warm water flows down the waste drain, cold supply water flows through a copper tube spiraled tightly around the copper section of waste drain. Energy is transferred from the warm drainwater to the cold supply water, which is subsequently directed to the cold side of the fixtures (i.e., shower) and/or to the water heater. Although the water heater effectively stores some of the useful energy collected by this type of DHR, it is not classified as a storage system.


Energy Efficiency

Drainwater heat recovery units reduce the energy needed to heat water. The DHR systems provide greater potential for energy savings as the number of concurrent hot water uses increases.


Not-so-easy

DHR devices are commercially available. Many plumbers are not familiar with DHR installation. However, most systems use common equipment and tools, and installation is relatively simple.


DHR devices typically cost about $300 to $500 or more plus installation. Depending on the type of device used and the plumbing layout, more than one device may be necessary to provide energy recovery for all the hot water uses in the home.


Overall savings for heating hot water to satisfy all household needs will be much less.


DHR units described above are double-walled heat exchangers that meet most local plumbing codes for potable water. Double-walled heat exchangers are covered by the Uniform Plumbing Code, Section 603.3.4; 1995 CABO (Council of American Building Officials) One and Two Family Dwelling Code, Section 3402.3.1; 1998 ICC (International Code Council) One and Two Family Dwelling Code, Section 3402.4.2.1; and the 2000 and 2003 ICC International Residential Code, Section P2902.4.2 - Heat Exchangers.


Not Applicable


Most DHR systems are installed in the main waste drain of the house by an experienced plumber. Depending on the system, it is placed vertically or horizontally in the main waste drain. All DHR units can be used for retrofit or new construction. However, installation will usually be less expensive in new home construction.


Not Applicable


DHR can save energy and increase the capacity of undersized water heaters. Available systems can vary in cost between $300 and several hundred dollars, depending on type and capacity.

Energy savings will vary considerably, depending on location of waste drain pipes, location and orientation of the DHR system, water heaters, bathrooms, and number and lifestyle of occupants.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.