"Mini-split" air-source heat pumps are establishing a strong foothold in northern states, based on the advancing capability of heat pump technology. The newest generation of mini-splits boasts cooling efficiencies of SEER 25 and better, with heating coefficient of performance (COP) ratings near 3 (meaning the equipment can supply heat equivalent to three times the electric power it consumes). Better yet, the latest equipment can heat homes even as outdoor temperatures plunge below zero, making heat pumps viable as the sole heat source for a well-insulated house — even through a harsh Northern winter.

JLC visited a jobsite in Maine in April to see technicians from Portland-based ReVision Energy install a system in an existing home, where the unit will "unload" the home's existing oil boiler during much of the year, limiting the boiler's heating role to the coldest part of the season when the heating demand is greatest (which is also the period when the boiler will operate at its highest efficiency). During swing seasons, the heat pump will take over much of the home's heating requirement — an effective use of the heat pump, which operates most efficiently when its outdoor-air heat source is relatively warm.

The heat pump system shown here, a Fujitsu Halcyon AOU12RLS2, retails online for about $1,600 (not counting professional installation), and is rated at 12,000 Btu for heating. When outdoor temperatures drop below 20°F, the heating capacity decreases. But the unit can still supply some heat at outdoor temperatures of -5°F. The system comes with outdoor unit containing the compressor, controls, and a reserve tank containing enough R-410a refrigerant to charge 49 feet of refrigerant lines as well as the wall-mounted indoor unit's heating and cooling coil.

Four lines connect the indoor unit to the outdoor unit: two refrigerant lines (one for liquid coolant and one for vapor); a power line for the indoor unit's fan; and a condensate line that drains any water vapor that may condense on the indoor unit's coil when the system is operating in cooling mode.

A system like this takes the crew one or two days to install, including the wiring and refrigerant line installation. The two technicians shown in this slideshow are master plumbers with additional training in refrigeration technology. They also have the skill to run the rough wiring for the system, but a licensed electrician will be needed to inspect the installation and hook up to the home's electrical panel.