Mini-split heat pumps are generally used in homes without central ducting. This makes them particularly useful in retrofit scenarios for homes.

Ductless Heat Pumps

Ductless heat pumps, also commonly referred to as ductless mini-splits, are the most common variant of mini-split heat pumps. They are composed of an outdoor unit and typically one to four indoor units; however, up to eight indoor units (or heads) can be used with a single outdoor unit ^c. The outdoor and indoor units are connected with refrigerant lines. Exhibit 2 depicts the basic setup of single zone and multi-zone ductless heat pump systems operating in heating mode.

Mini-Ducted Heat Pumps

Mini-ducted heat pumps, also known as ducted mini-splits or compact ducted heat pumps) are a relatively new variant of mini-split heat pumps but they are becoming increasingly popular. Their indoor air handler is typically located in a concealed space (e.g. below the floor or above the ceiling), and short runs of ducting are used to distribute conditioned air to a number of spaces in the home. Since it is necessary to install ducting as part of the installation of these systems, they are best suited to larger home retrofit projects or homes with unfinished basements. Ducting can be installed in the unconditioned spaces as well (e.g. attic spaces) but extra care is required to avoid issues in these scenarios (e.g. need to ensure no leakage and insulate ducting to building code requirements for exterior wall/attic).

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^c As noted in the System Design section, there are several disadvantages to coupling a larger number of indoor units to each outdoor unit. In these cases, a larger number of outdoor units should be considered.