Hot-water recirculating schemes aren't so green afterall

May 3, 2013
Researchers examining recirculating hot water systems found previous studies validating the 'green' credentials of these schemes overlooked factors which probably lead to more energy use than in ordinary hot water heating.

A paper published in a recent issue of Journal of Green Building casts doubt on the use of hot water recirculating schemes as a means of saving energy.

The thought behind recirculating systems is that they eliminate the need to use a lot of water while waiting for hot water to arrive at the hot water faucet. Estimates are that average consumers use as much as three gallons of water while waiting for the proper temperature. A recirculating system eliminates the wait by incorporating a pump and return lines that circulate hot water to outlets continually so there is no waiting time.

But researchers Randi Brazeau and Marc Edwards found that recirculation systems can use 20% more energy than ordinary systems, even when the water pump is only on for a few seconds before use. The situation is worse when the pump is always on. Here a recirculation system might use double the energy of conventional setups.

These conclusions differ from those of several others that found energy savings with recirculation systems. But prior studies often made unreasonable assumptions and suffered from the use of small sample sizes, the researchers say. For example, one study by the Oak Ridge National Laboratory (ORNL) in conjunction with the City of Palo Alto asserted that water wastage can be largely eliminated by hot water recirculation, but only in an “ideal” situation where user behavior encourages immediate use of the hot water. Water saving estimates for this study ranged from 900 to 3,000 gallons per point-of-use per year.

But the Palo Alto study had problems, the two researchers say. Its sample size was small, study parameters were inconsistent, and the results were inconclusive. Moreover, it only looked at homes of a narrow age range not comparable to the current age distribution of homes in the U.S.

Another problem with the Palo Alto study was that the pump systems were consumer-activated just before use, and a heat sensor was employed to turn off the pump when water hit the desired temperature. This sort of behavior is probably not likely among average consumers in their day-to-day activities. Worse, the assertion that the system would save energy failed to consider energy demands to run the pump and possible increased heat loss from the recirculation system, with a pump that runs continuously, the researchers say. And it has been suggested that intermittent use of the pump consistent with reduced energy use could damage the pump.

“These are really consumer comfort and convenience devices, a luxury really, masquerading as ‘green’ or environmentally conscious devices,” Edwards said.

Other findings: On-demand electric systems operate with nearly 100% energy efficiency, but cannot be used in many circumstances dependent on scaling and incoming water temperature, and may require expensive upgrades to home electrical systems and use of low or ultra-low flow showerheads.

The research is noteworthy in that hot water recirculating systems are now required in some new homes and businesses in the U.S. But their energy savings, and therefore lessened environmental impact claimed by manufacturers “do not hold water,” Brazeau added.

The two researchers calculated that a typical consumer with an electric water heater would pay as much as $158 more annually compared to those having systems without recirculation. More research, though, is necessary to better inform policy and decision-making by regulators, public health officials, manufacturers, and consumers, Edwards and Brazeau said.

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