What is a Mini-Split: Features and Inverter Technology

There are many situations where ducted heating and cooling solutions are not a viable option. Smaller and older homes are two examples where retrofitting ductwork into the building may not work because of space or noise concerns.  In situations like this, radiators, electric heating plates, and window air conditioning units used to be the only real options. The Japanese market, with restricted living space and constant cost concerns, led to the development of the ductless heating and air conditioning solution sometimes called a mini-split AC/heat pump.  The success of these systems in Japan has led to their use in other parts of the world and are now available in the United States.

What is a Ductless (Mini-Split) AC/Heat Pump?

A small condensing unit containing the compressor is placed outside the home. Rather than ducting treated air through various parts of a home, the ductless AC/heat pump splits the coolant line for distribution to fan coil units located in various rooms. This is why they are often called mini-split AC/heat pumps.

These units take up significantly less space than ductwork inside the home, making them an ideal solution for some small living spaces.

Parts of a Ductless AC/Heat Pump


The unit consists of four main parts, each of which plays a role in the energy efficiency of the entire system.

  • Condensing Unit – The condensing unit is placed outdoors, and contains the compressor that performs the actual heat exchange, i.e., the heating or cooling of the refrigerant.  That refrigerant is then pumped through refrigerant lines into the mounted units located in various parts of the home.
  • Refrigerant Lines – These are small tubes containing refrigerants pumped from the compressor after treatment into the indoor units in various parts of the home. They pass through small holes in the exterior wall of the home and split so they can be directed to different destinations. This is where the term mini-split heat pump originates.
  • Interior Mounted Units – Sometimes called a head when wall-mounted, or a cassette when ceiling-mounted, this is the unit that actually blows treated air into the living space. These units are almost always multi-zoned to allow more adaptability when only parts of the home require heating or cooling at any given time.
  • Control Unit – This panel controls the system, allowing selection of different modes, turning on and off various zones, and selecting temperature set-point. The control unit could be mounted but is often contained in a remote control.

Inverter Technology


One important advance in the design of ductless heat pumps is the use inverter technology. Inverters allow the compressor to operate at a variable range of speeds, adapting to the situation, rather than settling for a speed a bit too fast or slow for the situation. That improves energy efficiency, allowing just the right amount of heating or cooling based on demand.

Optional Features

Some units come with optional features that may be suited to your unique situation or just nice to have.

  • Advanced filtration, such as electrostatic, anti-bacterial, or activated carbon filters.
  • Moving louvers that direct air to different parts of a room, usually left and right, but some units also have vertical louvers.
  • Remote control with the temperature sensor, known as “I Feel.” This puts the sensor controlling the unit into a location of the occupant’s choice rather than restricting the sensor to a wall-mounted position.
  • Some units feature a quick-cool mode to run the system at maximum until the room reaches the set point temperature.
  • A fresh-air option can bring untreated air directly into the home. The disadvantage of this option is it requires some ductwork.
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