Question: What are ductless mini splits and how do they work?


Ductless split systems (also oftentimes referred to as ductless mini-splits) are a type of terminal in-room direct-expansion (DX) air conditioning system. In a ductless mini split system, typically a single outdoor condenser or compressor serves multiple indoor units (each containing a refrigerant coil and blower). Cooled (or heated) air is blown into each room or zone by fans in each evaporator unit.

While these systems have been popular in Asia for a while, use of ductless split systems in the U.S. has been primarily limited to unoccupied spaces such as data rooms, elevator machine rooms, and electrical closets. This is due to the fact that without ductwork, introduction of outside air for ventilation is not possible with ductless mini split systems.

These units could be implemented in a commercial building in conjunction with a dedicated outside air system, but such a configuration would be an expensive (and equipment-intensive) type of installation that would also require control and interfaces between many independent systems… obviously this wouldn’t make much sense from a design or operational standpoint. Therefore, ductless mini split system usage is most popular in residential applications (which include apartments or condos), where individual control is required and ventilation air could be provided by operable windows.CERN_Server150

In a residential application, ductless split systems are a good retrofit option to add air conditioning when adding ductwork for a central forced air system is cost-prohibitive.

The condenser only requires a 3-4” hole in the wall for connection to the indoor unit, and is definitely more aesthetically pleasing than a window air conditioner. Indoor units can also be grouped together, with up to four zones (and their associated units) connected to a common outdoor unit. The outdoor condensing units associated with these types of systems are significantly different than what we’re used to seeing in residential applications as well (typical dimensions of 30”x35”x12” wide coupled with a rectangular shape).

One major benefit of ductless mini-splits is, as their name may suggest, no ductwork. A common problem with ductwork is that energy loss through unwanted thermal transfer occurs with air transmission in central forced-air systems. So split systems are a good choice for small projects where energy-efficiency is a priority, with SEER ratings around 14, and are cost-competitive to traditional residential DX cooling systems, in the range of $1,000-$1,500 per ton of cooling.

Many ductless split systems are also Energy Star rated, which could potentially make them eligible for rebates or other incentives from your local utility.

While some units include electric heat, if a ductless mini-split is being utilized to condition an occupied area (residence) as opposed to a cooling-only application, heat pump type ductless split systems are available as well. These units operate similar to an air-source heat pump and can still be provided with electric backup for months where outside temperatures fall below freezing. Heating capacity from these type of units is not usually the primary function, so if you live in a very cold climate other HVAC systems should be considered.


imageSarah Gudeman

Sarah Gudeman is a mechanical design engineer and licensed EIT in the state of Nebraska. Ms. Gudeman also is a LEED Accredited Professional (BD+C) and an active member of the USGBC Nebraska Flatwater Chapter’s board of directors.
She specializes in building energy modeling and audits, sustainable design and environmentally-friendly practices.

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