In the past, heating water was typically the second- or third-largest energy user in homes—after space heating and, sometimes, air conditioning. As we’re doing a better job insulating our homes, though, the proportion of home energy used for water heating has steadily risen. In 1978, according to the U.S. Department of Energy (DOE), water heating accounted for 14% of the energy use in homes, compared with 66% for space heating. By 2005, those percentages had shifted to 20% and 41% respectively.
In today’s ultra-energy-efficient “passive houses,” which accomplish their space-heating needs primarily through passive solar gain and use high levels of insulation to minimize heat transfer through the envelope, it’s not unusual for water heating to be the largest energy consumer in a home—and it can be as much as twice that of space heating. A standard residential electric water heater is responsible for nearly half the carbon dioxide emissions of an average passenger car.
This article takes a broad look at the things you can do before you select a water heater, including reducing demand for hot water and distributing it more efficiently.
In seeking to reduce a water heater’s energy consumption, it makes sense to start by reducing demand. Strategies include installing low-flow showerheads and faucet aerators, and replacing older clothes washers and dishwashers with more efficient models. Fortunately, there have been dramatic advances in water-conserving products and appliances in recent years. Product directories, such as BuildingGreen’s GreenSpec Directory, can lead you to state-of-the-art products to help reduce hot-water demand (see Access).
After reducing the demand for hot water, then seek to eliminate water heating system inefficiencies, which include how the water is heated (combustion efficiency, standby losses, etc.) and distributed. A lot of attention is focused on how to heat water efficiently. Unfortunately, very little attention is paid to distribution losses.
“DOE views water heating as the province of an appliance—the water heater,” says researcher Dan Cautley of the Energy Center of Wisconsin. But distribution losses, due primarily to heat loss from pipes, are far from insignificant.
Gary Klein, a leading researcher on hot water distribution, estimates that distribution losses range from 10% to 30% in typical American homes. In homes with continuous-circulation hot water systems, no pipe insulation, and relatively little hot water use, he has seen distribution losses as high as 90%.
That’s why it’s important to insulate all hot-water pipe runs. Pipe insulation won’t eliminate heat loss from hot water pipes, but it will slow that loss considerably. And it will increase the likelihood that a homeowner won’t have to wait for hot water after a relatively short period of non-use.
During new construction, it’s easy to insulate pipes. Buy foam pipe insulation that’s designed for the diameter pipe you’re insulating, slip it on, miter-cut corners, and use quality tape to seal it. Retrofitting pipe insulation is a lot harder, and it usually doesn’t make sense to pull apart walls to do it—unless you needed to get into those walls anyway.
Then there’s the issue of wasting water as you’re waiting for hot water—in a typical U.S. home, that can add up to as much as 10,000 gallons of water per year. The amount of water wasted depends on the pipe diameter and the distance between the water heater and the tap. There can be a five-fold difference in the wait time between 3/8-inch-diameter and 1-inch-diameter pipe. The smaller the pipe, the more quickly hot water reaches the tap and the less the water and energy waste. Larger-diameter pipe wastes additional energy, because more hot water remains in the pipe after the tap is turned off, and there is more pipe surface area for heat transfer to occur.