Unlike resistance heat, which uses electric elements to generate heat, a minisplit heat pump (MSHP) moves heat from one location to another using refrigerant, a compressor, heat exchangers, and an expansion valve. During the summer, an MSHP moves heat from inside the building to the outside. During the heating season, the unit operates in reverse, capturing heat from the outside air and moving it into the home. Since the heat source for these units is air, they are commonly referred to as air-source (or air-to-air) heat pumps.
These systems are referred to as “split” systems because they use two units—an outdoor unit (the condenser) and an indoor unit (the evaporator)—to transfer heat. They are referred to as “minisplits” to differentiate them from larger heat-pump systems, such as commercial HVAC systems that utilize large roof-mounted units and often ductwork.
Most MSHP systems are ductless, making them a versatile option for retrofits. Heated or cooled air is distributed via a fan in the indoor unit, which may be mounted on a wall, on a floor, or in the ceiling. Since airflow is resistant to constrictions, such as doorways, these units have the greatest impact in open spaces that permit broad distribution. In homes that are heavily partitioned, multiple indoor units may be needed to provide comfort throughout the home. For some applications, small ducted indoor units may be used. Because ductless MSHPs are point sources of heating and cooling, this often makes them best suited to reducing the energy demands of a central heating system rather than replacing the system entirely. In a new, well-insulated and sealed home, many designers combine heat pumps with strategically located electric resistance heat in bathrooms and other critical areas to avoid the need for a central heating system. In addition to heating and cooling modes, MSHPs commonly have a dehumidification mode and a fan mode. Dehumidification mode is similar to cooling mode—the unit removes moisture from the air, which condenses on the indoor heat exchanger and is drained to the outside. Dehumidification mode is less precise than cooling mode, in that it runs at a low fan speed to constantly cycle the air through the unit and a cooling capacity that is a fraction of the unit’s maximum output. The fan mode can be used to cycle the air in the room through the unit to provide a more even distribution of temperature in the room, without turning on the heat pump. This may help distribute heat from a wood heater in the winter, for example, or simply provide more comfortable air movement in the summer. The indoor unit also contains filters that limit the amount of dust and other airborne contaminants that accumulate on the heat exchanger, and help purify the air. These filters need to be cleaned regularly and are easily vacuumed with a soft-brush attachment or rinsed with tap water. The filters may need to be cleaned only every few months in homes with good air quality. In homes with smokers, where candles are used regularly, or that have airborne contaminants like pet hair, the removable filters need to be cleaned more frequently. If they are not cleaned, the airflow will be reduced, reducing the heat output. Pros: It’s most often used in a situation where a window AC unit or baseboard heating would be considered, such as a new addition to a house. They’re also exceedingly energy-efficient. In the average house, you’re losing 25 percent or more of your energy to ductwork Cons: To maintain your system, you’ll have to wash each unit’s filter monthly (more often if you smoke or have pets) Many homeowners are hesitant to install ductless because the units aren’t necessarily design-friendly. They come in standard white or beige and can’t be covered.