With a growing percentage of electricity generated by non-fossil fuel sources such as wind and solar, sustainability advocates believe that all-electric homes will lead the way to reducing housing's carbon footprint.
Most of these homes use an air source heat pump for space conditioning. Unfortunately, the typical heat pump's inability to satisfy peak heating loads on the coldest days has always required an electric resistance backup heater, leading to unacceptably high winter bills. That has led many builders to stick with gas furnaces.
The good news is that improved heat pump technology, more efficient building envelopes, and evolving utility rates are making electric homes more practical. In some northern areas, gas-free homes are already cost-effective. In others, it makes more sense to use a hybrid system, where the home gets backup heat from a gas furnace, which costs less to run than electric resistance coils.
The prerequisites for either approach are top-notch mechanical equipment and a super-efficient building envelope, with air leakage at or below 1ACH50 and a HERS score in the 40's or lower.
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One company that has committed to an all-electric future is Thrive Home Builders, in Denver. For the past few years, the local builder has installed hybrid systems in their Zero Energy Now (or Z.E.N.) series of production homes, all of which are certified by the U.S. Department of Energy's Net Zero Ready program.
Thrive's commitment to energy efficiency has paid off. During follow-ups with homeowners the company learned that backup heat was seldom needed.
"We began to think that we were spending money on a furnace that wasn't needed," says Bill Rectanus, Thrive’s VP of Homebuilding Operations. Improvements in heat pump efficiency gave the builder confidence to eliminate it; their new Ultimate Z.E.N. series of homes use a super-efficient Mitsubishi heat pump with an electric backup coil that's only needed a few days a year. "We're confident this system will maintain the comfort that our customers expect on even the coldest days," he says.
Of course Denver isn't that cold. Average January temperatures are 30.2° F, and the thermostat only dips below zero five or six days per year. Things are tougher in Central Minnesota, where the January average is just 12.4° F, and where it's not unusual to have 30 or more days of sub-zero temperatures. In that climate, backup heat is needed way more often. That led St. Cloud, Minn., custom builder Werschay Homes, which also builds super-efficient homes, to opt for the hybrid approach, according to director of operations Jim Heckendorf.
Why not just stick with gas? Heckendorf cites financial incentives offered through tiered pricing structures and demand management programs implemented by a growing number of utilities. For instance, some utilities offer homeowners reduced rates if they let the utility shut off the heat pump for short periods during times of high electric demand. "That makes it cost-effective to use electricity as the primary heating fuel," he says. Although the home doesn't switch to gas during these peak times (it only does so when temperatures fall below a predetermined setpoint), they tend to be short enough for a very efficient home to coast through without calling on the furnace.
Hybrid systems should become even more attractive as electric utilities adopt time-of-use rates. To take full advantage of those rates, however, technology has to catch up.
The wait shouldn't be too long. According to Rob Buchanan, Product Portfolio Manager at Xcel Energy (which serves eight states, including Colorado and Minnesota), control systems are under development that will switch the heat source from gas to electric according to current electric and gas rates, as well as the current temperature. "The utility can communicate with the control system to make sure the home is using the optimum fuel at any moment." That should save homeowners even more money.
"It's bleeding-edge technology," says Buchanan, "but I wouldn't be surprised if it becomes widely available in the next couple of years."