Tankless water heaters have one advantage over conventional storage units: no standby losses. Instead of keeping water hot around the clock, regardless of whether it’s actually needed, tankless units heat water only when a tap or an appliance is turned on. By rights, this should mean lower energy consumption, a decidedly green advantage.
But, as Ed Welch asks in a Q&A post, where are the savings when he can’t get his kids out of the shower? “I know we waste more water, as a result waste more energy heating that water,” he writes. “And the kids are not even teenagers yet!”
In addition to arguing the merits of tankless vs. tank heaters, Green Building Advisor readers had plenty of suggestions on the most economical ways of heating water and how to reduce consumption.
Pros and cons of tankless heaters
Welch writes that according to the Department of Energy, a tankless heater should save between $100 and $150 per year when compared to an Energy Star storage heater. But, he adds, the savings aren’t significant and they probably don’t factor in the long-shower problem. Moreover, tankless units cost two or three times as much as the best storage units, require a stainless steel flue, are difficult to install and cost more to maintain.
You got it, answers Robert Riversong. “You’re quite right that the super-sized burners on high-volume tankless heaters make no ecological sense,” he says. In addition to high initial costs and higher maintenance costs, Riversong adds, hard water can leave mineral deposits in the heat-transfer coils, which may force the purchase of a water softener.
Yes, says Michael Chandler, a builder in Chapel Hill, NC, on-demand hot water heaters are “more of a luxury than an energy conserving solution,” but keep in mind that most gas tank-style hot water hears are only about 60% efficient. Electric heaters can be even worse from an efficiency point of view. If the source of utility power is a coal-fired plant, only about a third of the energy potential of coal is actually available at the panel, making it “practically criminal” to use one of these appliances.
“One thing not mentioned in this discussion is that a tankless HWH is a great solution for some, not all,” writes Richard. He lives alone, is frequently away from home and doesn’t see the point of keeping 40 gallons of water hot around the clock. “I use cold water for laundry and quick hand washing,” he says. “My only hot water use is showers, dishwashing and washing up.”
First, limit hot water use
No matter how the water is heated, using less of it conserves energy. That’s a no-brainer. But posters differed on the best ways to accomplish that seemingly simple end, especially when children and teens live at home.
That’s simple, Riversong says: “The only ‘green’ way to save water heating energy is to use less hot water. Unfortunately, that requires imbuing our children with the old-fashioned ethics of forbearance and limits – and no piece of technology is going to do that. It’s part of the responsibility of parenting.”
Besides, he adds, people seem hung up on taking frequent showers in the first place. With water shortages expected to become a major problem, it’s better for health as well as the environment to bathe less often. After all, that strategy served our forbears just fine.
Lucas Durand came across an interesting conservation approach when he visited his brother in South Korea. The brother’s small apartment was served by a tankless hot water heater, but it could be activated only by pushing a button on a control panel. That got you 10 minutes worth of water. If you wanted more, you had to press the button again. The “big catch” was that you could press the button only five times in a 24-hour period, and that had to cover all hot-water needs, not just showers.
“This set-up may not have been typical of every home in Korea, but it does show that concepts of hot water use vary widely even within the developed world,” Durand wrote. “In other words there are many, many people living in civilized parts of the world that do just fine on what some North Americans might consider a water ration.”
Danny Waite had another suggestion: an $8 ball valve installed on the hot side of the water heater. It could be shut off whenever a shower went on too long. “My teenage sons quickly learned to limit shower times to under 5 minutes after instantaneously having their hot water eliminated,” he says. “Cold water seems to awaken the senses and get one to think ‘green.'”
Looking for economy water heating
If on-demand heaters are not a shoo-in for most economical, what is?
Riversong’s suggestion is an indirect hot water tank connected to a high-efficiency boiler. Indirect heaters have no heat source of their own but tap into the boiler via a heat exchanger. The arrangement, he says, provides nearly unlimited hot water as it heats the house with very low standby losses. Fuel consumption is a fraction of what a large tankless unit would use.
Chandler proposes using a tankless heater to heat water in a tank, in much the same way an indirect system uses a boiler, and adds a link to an illustration (with a warning that while he’s a licensed plumber, there’s still something of a “mad scientist experimentation” at work).
Solar hot water collectors
are another possibility, but here opinions were divided on whether the sizable investment they require is going to pay off.
While Chandler thinks solar collectors will reduce energy consumption, fellow GBA senior editor Martin Holladay writes that most people won’t see a payback for between 30 and 60 years. In particular, he cited a 2006 study by Steven Winter Associates that examined a $7,800 solar hot water system in Massachusetts and a $6,500 system in Wisconsin.
In the case of Massachusetts, annual savings were a measly $135 with a payback of 58 years; in Wisconsin, savings were even lower, $86 years, with a payback after 76 years. “Finally,” he adds, “it should be pointed out that the researchers assumed zero maintenance costs — and we all know that’s not going to happen.”
Stephane Boisjoli suggests the installation of a drain water heat-recovery
system, which captures transfer heat from the water draining from a shower to the incoming water supply. These passive devices are installed vertically to replace a section of conventional drain line. There are no moving parts, and no maintenance. Savings can be considerable.
Finally, there are on-demand hot-water circulation systems
in which hot water is pumped to its point of use after a button is pressed or a motion sensor in the bathroom goes off. As the water warms up, it’s recirculated so none of it is wasted. After a short wait, when the shower or tap is turned on, hot water is available right away. For long plumbing runs, such a system might make sense.