One of the most intriguing heating options available, especially for tight, well-insulated houses, is a type of air-source heat pump—a ductless minisplit. Just like conventional air-source heat pumps, minisplits have inside and outside components. The outdoor compressor pumps refrigerant to an indoor evaporator/fan coil, which blows hot (or cold) air to individual rooms. As many as eight indoor fan units can be connected to a single outdoor compressor, although most systems have fewer than that. Airtight houses can be heated with just a few wall-mounted heads, and because the indoor fan units are operated individually, these systems make it easy to zone a house for heating and cooling.
Inside and outside components are connected by a small refrigerant line, a power supply, and a condensate drain. In new construction, these lines can be hidden in an exterior wall; in a retrofit, installers bore a hole (3 inches or less) through an exterior wall to connect the equipment. A cover on the outside of the house hides the supply lines.
Conventional air-source heat pumps operate most efficiently in ambient temperatures of 40°F or higher. Below that, electrical resistance heat or some other supplemental heat source has to kick in, lowering overall efficiency. For this reason, air-source heat pumps are much less common in the northern tier of the country where winter design temperatures are typically well below 40°F. Minisplits, however, operate efficiently in much lower temperatures, some of them down to -13°F. They allow fans and compressors to run by variable speed motors—a more efficient option than the on–off cycling of conventional heat pumps. Fujitsu, a minisplit manufacturer, says that less than 1% of standard air-source heat pumps use variable speed motors while 75% of minisplits use them. Ductless minisplits do not have supplemental sources of heat. You’ll have to install a backup unit—wood, natural gas, or even electric (in a high-performance house)—if your ambient temperature is too low.
Heat pump efficiency is described by the coefficient of performance (COP), which is the ratio of energy consumed to heat delivered. If a heat pump has a COP of 5 it means it produces five units of heat for every unit of energy it uses. The National Renewable Energy Laboratory found COPs for minisplits it tested ranged from a 2 or less at -5°F to 7 when the outdoor temperature was 55°F.
Ductless minisplits also eliminate the heat losses of a ducted system, which, according to the U.S. Department of Energy, can waste 30% of the heat that otherwise would be circulated in the house. Heat losses are especially high when ducts pass through uninsulated crawl spaces or attics. Insulated refrigerant lines in minisplits are much smaller and incur losses between 1% and 5%.
Manufacturers say minisplits are often less expensive to install than conventional ducted systems. Although the equipment itself tends to be more expensive, there’s less labor because there are no ducts to build and install.
Minisplit indoor distribution units can be installed on an exterior wall.
However, they need to be in reach of electrical source circuits and the plumbing from the outdoor unit.