Technology at Work
Drain Water Heat Recovery

By Gerald Van Decker

Domestic hot water is the second highest energy cost in homes, accounting for 20 to 30 per cent of energy consumption. It can even equal space heating. Because of this, domestic hot water is also one of the largest contributors to residential energy costs and greenhouse gas emissions. Furthermore, showering accounts for 50 to 70 per cent of total hot water loads.
Hot water energy consumption can be reduced by decreasing shower time, using lower flow showerheads and doing laundry with cold water. Each of these options require lifestyle changes that not all homeowners are willing to accept; none are permanent.
Fortunately, we can reduce hot water energy consumption by installing an efficient water heater, a solar water heating system and/or a Drain Water Heat Recovery device. All of these technology options can be combined to substantially reduce hot water energy consumption but builders usually need to consider cost-effectiveness, maintenance and reliability.

DWHR Technologies
Drain Water Heat Recovery (DWHR) technologies are a class of heat exchangers that offer many benefits for home builders and homeowners. Well over 16,000 DWHR units have now been installed in Canadian homes.
Simply put, DWHR technologies work by using outgoing warm drain water (from the shower and fixtures) to pre-heat incoming cold freshwater so that the primary water heater does not have to work as hard to meet the hot water energy load.

There are three generations of DWHR technology used. All three generations rely on the fact that, as a result of surface tension, falling drain water clings to the inside wall of the drain pipe in a very thin film. The heat-laden film of drain water transfers its heat to the inner wall of the pipe very readily. In turn, the reclaimed heat flows from the pipe to the coil(s) wrapped tightly around the outside and then heats the incoming cold freshwater circulating up through the coil(s).
In homes, DWHR units become a part of the vertical drainage stack below the showers, by connecting to the plastic drainpipe using standard drain connectors. The performance differences between the three generations of DWHR technology are that the first generation has highest pressure loss, the second generation has lowest efficiency and the third generation has the highest efficiency with very low pressure loss. (Reference: Natural Resources Canada Ottawa, Drain Water Heat Recovery Characterization and Modeling, July 19, 2007)

Residential Market Uptake of DWHR
Last year alone, more than 6,000 DWHR units were installed in new Canadian homes. Many units were also installed in the U.S. and Europe. Builders can also receive substantial credits for using DWHR in the building energy codes of a growing number of jurisdictions, including Ontario, France and the UK, and in home labelling programs such as R2000 and ENERGY STAR for New Homes.
The table below was created using Provincial and Federal government reports and data. DWHR comes in very well at only $88 for each annual Gigajoule (GJ) saved. In contrast, upgrading above grade walls from R19 to R24 costs three times more than DWHR (at about $232) per annual GJ saved.

What does this mean for a builder? According to Drain Water Heat Recovery and the 2012   Ontario Building Code, an independent study released March 31, 2011 by Mindscape Innovations, it has been estimated that efficient DWHR technology can reduce the cost of building each average-sized home by $500 to $3,900 when complying with existing building energy codes, upcoming building energy codes, new home labelling programs and/or as an upgrade offering for home buyers. Furthermore, DWHR is one of the most cost-effective energy saving options for homeowners both in retrofit and new construction resulting in a typical tax-free return on investment range of 10 to 50 per cent annually, depending upon home occupancy and energy cost.

Additional Benefits and Features
Offering a 50+ year, maintenance-free life, DWHR is one of the most obvious and visible energy saving technologies that a builder can use for compliance to Building Energy Codes, labelling programs, or as an upgrade option for homeowners; people can even feel the heat pickup on the unit when hot water runs down the drain.
There is also the added benefit of increased hot water capacity. A typical unit installed in a residential home, when combined with a standard natural gas tank water heater, can supply a shower with a continuous supply of hot water that will not run out. In fact, the total primary domestic hot water system can typically be downsized by 40 per cent with confidence that all hot water needs will be met. A happy consequence is an increased life of primary water heaters, simply because the water heater doesn’t have to work as hard.
Last but not least, installation is straightforward and can be performed by any plumber with standard tools. Simple, safe, proven, practical and affordable: It’s easy to see why more than 200 builders now include DWHR as a standard item in their energy efficient homes.

The inventor of the Power-Pipe, Gerald Van Decker, M.A.Sc., P.Eng., is the President and CEO of the Kitchener-Ontario based RenewABILITY Energy Inc. He can be reached at gerald@RenewABILITY.com