ASHRAE Journal - November 2011 - 62

EMERGING TECHNOLOGIES
Energy Consumed (kWh) Water Inlet Temperature 35.24°F (1.8°C) 69.08°F (20.6°C) Hot Water Tank Without GWHE 16.5 10.6 Hot Water Tank With GWHE 13.6 9.4

Table 2: Energy consumed by hot water heaters with and without graywater heat There is less water being heated in the unbalanced ﬂow conﬁgurations (Table 1, Page 60).3 At least ﬁve manufacturers of DWHR units exist. Most are based in Canada and distribute their products to North America. DWHR units can also be bought at common hardware stores. However, it is recommended that a professional install the product.

exchanger.7 water heaters to provide for above their rated capacity or allow for water heaters to be undersized in new construction or during a retroﬁt.3 An average on-demand heat recovery unit costs $500 plus installation fees. It has been calculated that the payback time for one unit is two to ﬁve years.5 Depending on the home owner and number of appliances that could beneﬁt from DWHR units, one or more may be installed. Regardless, the technology will eventually pay for itself.

Energy Savings
Drain water heat recovery has been shown to save signiﬁcant amounts of energy. A study conducted in England in 1975 showed 31.5% savings on fuel used for water heating.

Another study conducted in 1998 found 35% to 50% savings depending on the conﬁguration (balanced or unbalanced) of the heat exchanger. A 1997 study of three manufacturers of DWHR units showed 16% energy savings on the energy load of the water heater.7 A study conducted in Montreal concluded that 40% of the energy load for hot water is consumed by simultaneous ﬂow, i.e., draining and demanding hot water at the same time. Thus, the reductions of hot water heater electricity demand with and without GWHE units were studied. Ten simulated electric hot water heater demand proﬁles were created, compared to 600 sets of experimental data, and averaged by hour of use. The experimental data sets conﬁrmed the energy expenditures seen in the simulation data, and a daily peak of 1.3 kW was recorded. Table 2 summarizes the results of the study. It shows that colder inlet water temperature results in more energy being recovered. The same study demonstrated a 10.4% reduction in domestic water heater electrical demand in the morning and a 21.5% reduction in the evening.7 A study conducted for the DOE found that DWHR units can save 800 kWh/yr to 2,300 kWh/yr per house based on the number installed.3

References
1. DOE. 2010. 2009 Buildings Energy Data Book. Table 2.1.6. http://tinyurl.com/ table2-1-6. 2. DOE. 2010. 2009 Buildings Energy Data Book. Table 3.1.5. http://tinyurl.com/ table3-1-5. 3. DOE. 2005. “Heat Recovery from Wastewater Using a Gravity-Film Heat Exchanger. Federal Energy Management Program.” http:// tinyurl.com/gravity-ﬁlm. 4. DOE. 2011. “Drain Water Heat Recovery.” http://tinyurl.com/drain-water. 5. Tomlinson, J.J. 2001. “Heat Recovery From Wastewater Using a Gravity-Film Heat Exchanger.” Federal Energy Management Program. Oak Ridge National Laboratory. http://tinyurl.com/tomlinson2001. 6. National Association of Home Builders. 2001. “Drainwater Heat Recovery.” http:// tinyurl.com/nahb2001. 7. Eslami-nejad, P. and M. Bernier. 2009. “Impact of grey water heat recovery on the electrical demand of domestic hot water heaters.” Eleventh International IBPSQA Conference. http://tinyurl.com/eslami.

Market Factors
On-demand drain water heat recovery units are durable and easily integrated into existing and new construction. There are no moving parts, they replace existing segments of pipe, and ﬁt into studded walls. These units provide beneﬁts beyond energy and monetary savings. They extend the life of water heaters. Also, by increasing the capacity of the water heater they either allow for undersized

www.info.hotims.com/37992-48

Alissa Cooperman is a technologist and John Dieckmann is a director in the Mechanical Systems Group of TIAX LLC, Lexington, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C.
November 2011

ASHRAE Journal

ASHRAE Journal - November 2011

Table of Contents for the Digital Edition of ASHRAE Journal - November 2011