Drain-Water Heat Recovery

Drain-Water Heat Recovery Saves Energy

Bird's eye view

A simple device saves significant amounts of energy

Most of the energy that's used to heat water goes right down the drain as the water is used. Drain-water heat recovery is a way of capturing heat that would otherwise be wasted — the equivalent of 350 billion kWh of electricity each year in the U.S., according to the U.S. Department of Energy.

Installation costs are not especially high, and a simple system can save 16% or more of the energy used to heat water.

See below for:


Most systems have no tank

There are two main types of drain-water heat recovery systems. In both cases, warm drain water is used to heat the cold-water supply; where they differ is in their ability to store the heat.

  • Storage systems. These incorporate a dedicated storage tank. Water from the drain goes through a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. inside the tank, warming the stored water. A second heat exchanger at the top of the tank preheats water on its way to the hot-water heater or to its point of use. Because of the high cost of these systems, they are rarely installed.
  • Systems without storage. These systems, which are far more common than storage systems, require the installation of a simple drain-water heat-recovery device (for example, the GFX). The device consists of a length of copper drain pipe, usually 3 in. or 4 in. in diameter and 4 ft. to 6 ft. long, surrounded by a spiraling length of copper supply tubing. Installed vertically, the device heats cold water as it flows to the hot-water heater or the point of use. Non-storage systems are effective only when hot water is going down the drain at the same time that cold water is coming in, such as when someone is taking a shower. The devices will not save energy when installed in houses where occupants prefer baths to showers.
  • Design Notes

    Field studies point to savings of 16% to 30%

    Drain-water heat recovery devices are usually 4 to 6 feet long, and must be installed vertically. They can't be installed in a single-story home with a slab foundation, but they work well in a two-story home with upstairs bathrooms or in any home with a basement.

    Several studies have looked at potential savings in the energy required to heat water resulting from the installation of a drain-water heat recovery device. The data from three studies show that in a house where occupants prefer showers to baths, the following savings in energy used to heat water can be expected:

  • Study #1 (a residential study conducted by Northeast Utilities in Hartford, CT): 20% savings.
  • Study #2 (a study conducted at a multi-unit residential building in Duluth, MN by Oak Ridge National Laboratory): 25% to 30% savings.
  • Study #3 (a field study of 18 Manitoba homes conducted by Manitoba Hydro): 16% savings.
  • [Sources: Energy Design Update, September 1997, September 2000, March 2001, October 2006]

    Builder Tips

    Follow manufacturers' instructions

    Many plumbers are unfamiliar with the installation of a drain-water heat recovery device. However, manufacturers of the units provide detailed installation instructions.

    The Code

    The code

    While there are no specific code previsions dealing directly with drain-water heat recovery devices, all installations must comply with Water Supply and Distribution (Chapter 29) and Sanitary Drainage (Chapter 30) regulations. Installation must also be in accordance with the manufacturer’s installation instructions.


    Drain-water heat recovery is not standard in new construction, but it has obvious value in an effort to conserve resources, a cornerstone of green building.

    Like irrigating plants or lawns with gray-water, heat-recovery systems systems capture something of value that is usually thrown away without much thought. In the process, savings can be considerable.


    LEED-H The LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. program offers no points for drain-water heat recovery.

    NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 7 — Energy Efficiency: 2 pts. for drain water heat recovery system (703.5.3).


    Simple systems have speedy payback

    The energy department estimates that energy losses in hot water amount to 80% to 90%, a good argument for installing a drain-water heat-recovery (DHR) system.

    In a gravity-film heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. (GFX), drain water flows through a copper pipe that is wrapped in smaller diameter copper tubing. Heat from the drain water is transferred to incoming water. Heat exchangers are made from solid copper so they are durable, and there are no moving parts to wear out.

    Exchangers without storage. In the simplest type of system, the heat exchanger preheats cold water coming into the house. This arrangement does not include a way of storing the recovered energy so it's only really effective when hot water is being drawn and drained at the same time — as it would be during a shower, for example.

    The most efficient systems are balanced, which means all of the incoming water is piped through the heat exchanger. In one study reported by the energy department, electricity savings were between 800 kWh and 2300 kWh per year. Payback for the $500 investment was in as little as two years. (With recent increases in copper prices, a drain-water heat-recovery device is more likely to cost $800 or $1000 these days.)

    Systems with storage tanks. The main disadvantage of non-storage systems is that they won't recovery energy lost in bathwater or water drained from a washing machine or dishwasher because there's no incoming water moving through the heat exchanger.

    The alternative is expensive: a heat-recovery system with a storage tank. One heat exchanger inside the tank captures heat from the drain water while a second heat exchanger for incoming cold water picks it up.


    Manufacturers of drainwater heat recovery devices include:

    RenewAbility Energy

    ReTherm Energy Systems

    Watercycles Energy Recovery

    WaterFilm Energy

    Image Credits:

    1. ECO-GFX
    Tags: , ,
    Feb 11, 2012 4:51 PM ET

    Response to John Scime
    by Martin Holladay, GBA Advisor

    Here is one set-up sometimes used for slab-on-grade homes:

    I'm not necessarily advocating this approach -- it lacks the no-moving-parts efficiency of an installation that depends on gravity -- but you could do it if you want.

    Feb 11, 2012 12:04 AM ET

    Edited Feb 11, 2012 12:45 PM ET.

    How close to your heater for best efficiency?
    by John Scime

    I love the idea of the DWHR pipes - simple and effective. My trouble is that with a slab-on-grade build - where you cannot easily go through a floor joist to connect directly - you would end up running a lot of tubing in an less-than-direct fashion. I figure with my configuration, the DWHR outlet would be 18 - 20 feet from my on-demand water heater. Would my newly heated water cool down over that length of run, thereby deminishing the effiency? This would be a big concern given the capital cost.

    Any thoughts?

    John, near Ottawa

    Aug 26, 2009 6:51 AM ET

    Install Considerations / Details
    by Mike Guertin, GBA Advisor

    You have to consider the install details before buying a GFX. Vertical orientation is critical for the film to form on the drain pipe walls and the longer models are more efficient than the shorter models. So you have to locate the tallest vertical drop from the main drain line to the sewer / septic pipe exit for the best efficiency.

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