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Ductless HVAC Glossary of Terms

Air Conditioner

An air conditioner is a piece of equipment that controls temperature and humidity in an enclosed area.


A British thermal unit measures energy. It is the amount of heat required to increase the temperature of one pound of water by one degree Fahrenheit. BTU/h, or BTU/hr, stands for British thermal unit per hour. The BTU rating of an air conditioner states the cooling capacity of the unit.

Load or Capacity

Load or capacity is the power output generated by a piece of equipment. In HVAC, it refers to the heating or cooling capability of a furnace, heat pump or air conditioner and is measured in BTUs. For air conditioning, it may also be stated in tons of refrigeration or watts.


A compressor is a device that increases pressure on a fluid. In an air conditioner or heat pump, the compressor changes the refrigerant from a gaseous state back into to a liquid state.

External Static Pressure

External static pressure is the amount of resistance, or friction that affects the air flow in an HVAC system. Resistance can be created by registers, dampers and other components in the supply and return ductwork. The greater the ESP, the less air can be moved. The fan in an air handler has to be big enough to overcome ESP in order for the system to function efficiently.

Heat Pump

A heat pump is a piece of equipment that moves heat from one area to another via a refrigerant. In cooling mode, heat is removed from indoor air and transferred outside; in heating mode, the refrigerant is reversed and the heat is removed from the outdoor air and transferred indoors.


HSPF stands for heating seasonal performance factor. The HSPF denotes the efficiency of a heat pump; the higher the HSPF, the greater the heating efficiency. It is calculated as a ratio between the heat production over a heating season and the watt-hours of electricity used.


HVAC stands for heating, ventilation and air conditioning.

Indoor Unit

The indoor unit is the part of an air conditioning system that sits inside the room or structure and distributes heated and cooled air to the indoor space. It contains a fan, filters, motor, coil and an air filtering device; the air filtering device is often called the air handler.

Inverter Technology

Inverter technology is an innovative technology from Mitsubishi Electric. Inverter technology allows a compressor to run at adaptable speeds rather than one constant speed. Unlike a regular compressor that draws full power when it runs, inverter technology adapts to temperature changes and draws only the amount of power needed to maintain the desired level of heating or cooling. Inverter technology saves money and reduces energy consumption by up to 40 percent.


A microprocessor is a memory chip that assimilates and stores information. It serves as the control for an HVAC system. It receives information from temperature, air flow, moisture and pressure sensors and tells the system when to perform functions such as turn on and off or decrease air flow.

Outdoor Unit

The outdoor unit is the part of an air conditioning system or heat pump that sits outside of the room or structure. It serves as a heat exchange point; in summer, it removes the heat from the indoor air and dispels it to the outside. In winter, it removes heat from the outdoor air and delivers it indoors. The outdoor unit contains a compressor, fan and coil.


Refrigerant is a chemical used in heating and cooling systems. It changes from a gas to a liquid and back to a gas as it absorbs and discharges heat during the heating and cooling cycle. Mitsubishi Electric uses R-410A, which is environmentally friendly and does not deplete the ozone.

Refrigerant Lines

Refrigerant lines carry the refrigerant between the indoor and outdoor units. They are usually made of copper.


SEER stands for seasonal energy efficiency ratio and indicates the efficiency of air conditioning equipment. It is calculated as a ratio: the amount of BTU cooling output to the cost of the watt-hours of electricity needed to run the system.

Split-Ductless System

A split-ductless system is a cooling system where the condenser (outside unit) is separated from the air handler (indoor unit); the two are connected via electrical and refrigerant lines. The air handler delivers conditioned air directly to a room or space.

Split-Ducted System

A cooling system with outdoor and indoor units; however, the air handler delivers conditioned air throughout a structure via ductwork.

Split-Zoning System

The same as a split-ducted system. Split-zoning allows each space to be controlled separately.


A zone is a room or space to be heated or cooled. Zoning provides ultimate comfort control because the temperature in each zone is unrelated.