Drain water (or greywater) heat recovery (DWHR, sometimes DHR) devices are an efficient method to recapture waste heat from water flowing down drains. This energy can be used to preheat cold water before it enters a water heater or a dedicated storage tank. DWHR systems reduce the energy needed for heating water which can increase the capacity of hot water heaters. Between 80 - 90% of the energy used to heat water eventually goes down the drain and a drain water heat recovery unit can collect up to 60% of that energy.
Drain water heat recovery devices use a heat exchanger to capture the energy as the water flows down the drain line. There are several residential drain water heat exchangers commercially available in the U.S. and models vary by the pipe size, orientation of the drain line, and heat exchanger design. The costs and the amount of energy saved vary by design.
DWHR system configuration
Types of DWHR Systems
Drain water heat recovery systems can be installed to store preheated water or send it directly to the hot water heater while it is in use. Units with storage capacity can recover heat from any waste hot water such as from a dishwasher, washing machine, or shower and bathtub which drains through the heat recovery device. Without storage capacity, DWHR units provide useful energy recovery only during simultaneous flow of cold supply water and warm drain water such as when showering.
Storage-type units utilize a tank containing a reservoir of clean water. Drain water 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 drain water is transferred to the cold supply water without mixing. The system is also designed not to lose stored energy when cold wastewater runs through the drain water heat exchanger.
Another system is a demand 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 drain, cold supply water flows through a copper tube spiraled tightly around the copper section of waste drain. Energy is transferred from the warm drain water to the cold supply water, which is then directed to the cold side of the fixtures and/or to the water heater. Although the water heater effectively stores some of the useful energy collected by this type of DWHR, it is not classified as a storage system.
How it Works
Drain water heat recovery technology works well with all types of water heaters, especially with demand and solar water heaters.
Water enters the home between 45-55 ºF and is heated to 120 ºF by the hot water heater. The showerhead temperature is approximately 105 ºF and this water goes down the drain at 98 ºF. DWHR units can preheat the cold water coming into the home by 20 ºF using the energy from the drain water.
DWHR units which are double-walled heat exchangers meet most local plumbing codes for potable water. Depending on the location of the drain line and water supply lines, you can have a plumber install the unit or a competent do it yourselfer can handle the job. Always check to see if a permit is required before altering any plumbing.
When selecting a DWHR unit, you will need to determine the length of your drain line between fittings for vertical configurations. Some of the larger units are slightly over 6 feet in length.
Most DWHR 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 line. Both horizontal and vertical configurations are equally efficient as water flowing down a vertical drain line clings to the sidewall of the pipe.
Benefits and Costs
Available systems can vary in cost between $300 and $800 depending on type and capacity. All DWHR units can be used for retrofit or new construction. However, installation will usually be less expensive in new home construction. Installing a DWHR system costs $1,000 to $1,250 which is typically recovered within six years.
DWHR units can be very effective when used in conjunction with tankless water heaters where the capacity can be reduced during winter months when the inlet water temperatures are much colder.
DWHR unit installed with a tankless water heater
You can save 25 to 40 percent on heating, depending on hot water usage. DWHR systems can lower your family’s annual greenhouse gas emissions by nearly one ton if you use natural gas to heat your water.
Energy savings will vary considerably, depending on location of waste drain pipes, location and orientation of the DWHR system, water heaters, bathrooms, and number and lifestyle of occupants.