HVAC Basics Glossary

The best choices come from educated decisions. This helpful guide can help you understand some of the terms used by Armstrong Air professionals.


This is a percentage measurement of a furnace's heating efficiency. The U.S. government's minimum efficiency level is 78%. The higher the AFUE, the more efficient the furnace. The initials stand for Annual Fuel Utilization Efficiency.

Air Handler

This is the indoor section of a split system. It can be a dedicated air handler, or could be your furnace. Also known as a fan-coil.


Air-Conditioning, Heating and Refrigeration Institute. AHRI is an industry trade association that develops standards for measuring and certifying product performance. For instance, AHRI Standard 270 provides guidelines for establishing sound levels for outdoor air-conditioning equipment.


The distribution or movement of air.

Ambient Temperature

This is the air temperature (usually the outdoor air temperature) surrounding the equipment.


Microscopic living organisms suspended in the air that grow and multiply in warm, humid places.


British Thermal Unit. This is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. You'll see this measurement when you look at heating and cooling capacities. For example, your dealer may recommend a 75,000 Btu furnace and a 24,000 Btu air conditioner for your home.

Condensing Unit

This is the outdoor section of a split-system system. You'll know it best as the air conditioner that sits outside your home.


Coefficient Of Performance. A ratio that compares a heat pump system's heating efficiency to that of electric resistance heat. For example, a heat pump system with a COP of 3.0 provides heat at 3 times the efficiency of electric resistance heat. A heat pump's system COP will decrease as outdoor temperatures drop, eventually providing little or no efficiency advantage over electric resistance heat—and that's when your auxiliary heat strips start to heat your home.

Dual Fuel

For even greater efficiency, this heat pumpcan be paired with a furnace. Called a dual-fuel system, this combination minimizes heating costs by seamlessly alternating between the two energy sources, depending on outdoor conditions.


Energy Efficiency Rating.

Energy Star

ENERGY STAR® is the international symbol of premium energy efficiency. Products that display the ENERGY STAR symbol have been tested according to prescribed procedures and have been found to meet or exceed higher energy efficiency levels without compromising performance.


Gallons Per Hour. You might see this rating if you are looking at an oil furnace. In addition to input and output, an oil furnace also has a rating of gallons per hour, the volume of oil a furnace is capable of burning in 60 minutes.

Heat Pump

A unit that both cools and heats your home. A heat pump system can be either a split system or a packaged system. A heat pump can be used in conjunction with a gas/oil/LP furnace (using the furnace instead of electric resistance heat when temperatures fall below about 35° F) with the addition of a fossil fuel kit.


This is a measurement of a heat pump's heating efficiency. There is no governmental minimum rating. The higher the HSPF, the more efficient the heat pump's heating performance. HSPF stands for Heating Seasonal Performance Factor.


HVAC (pronounced "H-V-A-C") is an acronym that stands for the closely related functions of "Heating, Ventilating, and Air Conditioning."

Packaged System

Packaged units provide both heating and cooling from one unit that is placed outside the home—on the ground, on the roof, or sometimes mounted through the walls of the building. Packaged units come in several combinations of fuel sources—gas heat/electric cooling; heat pump; electric heat/electric cooling; oil heat/electric cooling.


This is a measurement of the efficiency of cooling products. The U.S. government's minimum efficiency level is 13.0 SEER for split systems and 9.7 for packaged units. The higher the SEER, the more efficient the cooling product. SEER stands for Seasonal Energy Efficiency Rating.

Split System

This describes an air conditioning or heat pump system that is split into two sections—an outdoor section and an indoor section. It won't work without the outdoor section plus an indoor section to move the air.


You'll often see this as a measurement of the capacity of an air conditioning system. Don't panic, it doesn't measure weight! Just like gas and oil furnaces, air conditioners and heat pumps are rated in Btus. One ton of air conditioning is 12,000 Btus per hour. This means that a "one ton" air conditioning system has the capability of removing 12,000 Btus of heat per hour from your home.