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Ductless HVAC Glossary of Terms

Air Conditioner

An air conditioner is a device that changes the temperature and moisture content of indoor air.


A British Thermal Unit is the quantity of heat needed to increase the temperatureof water by one degree Fahrenheit. BTU/h gauges how much heating or cooling capacity is produced each hour.

Load or Capacity

Load or capacity refers to the ability of an HVAC system to heat or cool an area.


A compressor is a component in an air conditioner unit that pumps refrigerant from the outdoor unit to the indoor unit.

External Static Pressure

External static pressure is the amount of resistance to air flow that is created by dampers, grilles, registers and other air duct equipment.

Heat Pump

A heat pump is a piece of equipment that has the capability to provide heating as well as cooling by reversing the flow of refrigerant.


The HSPF (heating seasonal performance factor) measures the effectiveness of a heat pump to provide heat. HSPF is a comparison of the heat output in BTUs to the watt-hours of electricity used over a heating season.


HVAC stands for heating, ventilation and air conditioning.

Indoor Unit

An indoor unit is the air handler part of a split HVAC system; it contains a blower, heat exchange coil, air filters and sound absorbers. When the system is for air conditioning only, the indoor unit is sometimes referred to as the evaporator unit.

INVERTER Technology

INVERTER technology is a Mitsubishi Electric innovation that enhances indoor comfort. INVERTER technology allows the compressor to run at variable speeds rather than one constant speed. The capability to adjust speed to meet fluctuating load requirements brought about by changes in temperature means the system uses only the amount of energy needed to generate a steady, comfortable air flow.


A microprocessor is a memory chip that controls a heating and cooling system. It receives and saves information from sensors that measure temperature, humidity and air flow. The microchip processes the information and tells the system when to perform certain functions such as turn on or increase the air flow.

Outdoor Unit

The outdoor unit contains a compressor, heat exchange coil and fan; it pumps refrigerant to the indoor unit. When the system provides air conditioning only, the outdoor unit is sometimes referred to as the condenser unit.


A refrigerant is a substance, usually a chemical, which provides heating and cooling by absorbing and discharging heat. Mitsubishi Electric uses R410A refrigerant, which is environmentally friendly.

Refrigerant Lines

Refrigerant lines transport the refrigerant between the indoor and outdoor units. The lines are usually made of insulated copper tubing and are encased in a conduit that connects the two units.


SEER stands for seasonal energy efficiency ratio; it measures the effectiveness of an air conditioner. It is a comparison of the BTU cooling output capacity of a unit to the energy used as measured by watt hours over the course of a cooling season.

Split-Ductless System

A split-ductless system is a system in which the air handler (indoor unit) is separated from the compressor/condenser (outdoor unit.) Conditioned air is delivered directly into the space to be heated or cooled.

Split-Ducted System

A split-ducted system also has two units, but the conditioned air is delivered from the air handler to the space via ductwork.

Split-Zoning System

A split-zoning system has two units; the indoor unit has the capacity to heat or cool several rooms or zones inside a home or building. The temperature in each zone is independent of the other zones.


A zone is a space, room or multiple room-areas in one building that receives conditioned air. The temperature in each zone is independent of the other zones.