• ReVision Energy's Phil Coupe checks the solar exposure of a rooftop on Peaks Island, in Portland, Maine. Solar arrays combined with mini-split air-source heat pumps have helped some island residents drastically cut back their consumption of costly heating oil.
    ReVision Energy's Phil Coupe checks the solar exposure of a rooftop on Peaks Island, in Portland, Maine. Solar arrays combined with mini-split air-source heat pumps have helped some island residents drastically cut back their consumption of costly heating oil.

Air-source mini-split heat pumps make up half of the total worldwide heating and cooling market—at least according to some figures. In the U.S., mini-splits barely have a toe-hold, accounting for just 2% of the market by recent estimates. But that could be changing—and in northern New England, at least, it's changing fast.

Fortunat Mueller, co-founder of ReVision Energy in Portland, Maine, presented a three-hour educational session on mini-splits and photovoltaics last week at the Building Energy 14 conference in Boston, Massachusetts. It was one of at least four sessions dedicated primarily to mini-splits. Said Mueller, "I don't know about the rest of New England, but in Maine, mini-splits are off the charts. You talk to the guys who distribute these things, the Fujitsu and Mitsubishi reps in particular, and they cannot keep them on the shelf."

A few years ago, ReVision installed its first tandem system coupling a set of mini-splits with a rooftop solar array. In the years since, heat pump makers have been leap-frogging one another in the competition to boost efficiency and optimize low-temperature performance, even as solar panels have improved in output and dropped in price. Now, the heat-pump-and-PV strategy is part of ReVision's bread and butter and, given a moderately well-insulated house, the technique has become a viable path to creating a net zero energy house, even in cold Maine. Mueller's talk covered the basics of installing a grid-tied array of photovoltaic modules, then delved at greater depth into the selection, sizing, installation, and operation of mini-split equipment.

Other talks at the BE 14 conference covered the topic from other angles. In one session, Efficiency Maine's Andy Meyer presented some numbers from two broad-based programs to install mini-splits in homes heated with oil to reduce the oil consumption in the dwellings (while leaving the oil boilers in place for use during the coldest winter conditions). The program effectively cut oil consumption in single-family homes, Meyer said, providing a 5-year payback on the heat pump investment; 91% of participants said they would recommend a heat pump to their friends. But in low-income multifamily applications, a similar Efficiency Maine program yielded mixed results, mostly because of variations in occupant behavior.

Energy uber-nerd Marc Rosenbaum of Martha's Vineyard-based South Mountain Company took a data-heavy look at the performance of a couple of simple mini-split systems that aren't performing quite as expected: one system in a Passive House in Vermont that is using substantially more power than the models predict; and one system in Rosenbaum's own house that appears to be cycling on and off much more than it should. But Rosenbaum also looked at some clear success stories: an expensive vacation-home retrofit that achieved a major reduction in fuel oil use with a combination of air-to-water heat pumps and electric resistance water heating tanks; and a very successful retrofit of a Vermont school building. In the case of the school, a room-by-room replacement of oil heating equipment with mini-splits completely eliminated the oil heat from the building with no increase in the school's electric bills, despite the power draw of the heat pumps. Besides consuming oil, Rosenbaum noted, oil heating systems also use a lot of electricity for hydronic pumps and other add-ons—enough power, in this case, to run the heat pumps and heat the classrooms after the change-out.

And mini-split heat pumps also figured prominently in sessions that weren't focused mainly on that technology. For example, heat pumps were part of the strategy for a housing development in Devens, Massachusetts, described by developer and builder Carter Scott of Transformations, Inc. The Devens buildings generate on-site power from a long, south-facing saltbox roof covered with an 18-kilowatt solar panel array. After supplying power to run the heat pumps that heat and cool the super-insulated buildings, each home's photovoltaic array generates enough extra power to run a Nissan Leaf electric car for 30,000 miles, noted Scott — leveraging home power to help cut the carbon emissions of the transportation sector. Heat pumps are also the heating system of choice for the near-zero-energy modular homes that were the focus of a talk by architect Phil Kaplan, manager Parlin Meyer, and house-setting contractor Steve Adamczyk, who are ramping up production of Maine architectural firm Kaplan and Thompson's "BrightBuilt" line of high-performance modulars in New England.