HVAC Terminology

Reliant AC Unit

AFUE (Annual Fuel Utilization Efficiency):

This value measures your furnace's efficiency in heating your home by measuring fuel input versus energy output, and is given as a percentage. A higher AFUE percentage means that less energy is escaping as exhaust—and that translates to lower fuel bills. An AFUE value of 78% is the minimum allowed by the U.S. government, and furnaces are currently available up to 96.6%.

Air Handler:

An indoor, all-in-one unit that contains a fan (or blower), evaporator coil, electric heat strips, and may also include a filter. Air handlers are used either as all-electric heating alternatives to gas furnaces or as part of a heat pump system. (Note: "air handler" is often also used as a general term to describe the component of a system which moves air through the ductwork, regardless of the fuel used to heat the home.)

Auxiliary Heat:

See Supplementary Heat.

Balance Point:

The lowest outdoor temperature below which a heat pump is no longer sufficient to supply the heating needs of the home. This temperature is normally between 30° F and 45° F. Once the temperature outside dips below the balance point, auxiliary heat (usually electric heat strips in the air handler) is called on to aid the heat pump in providing heat to the home.

Btu (British Thermal Unit):

The amount of thermal (heat) energy required to raise the temperature of one pound of water by one degree Fahrenheit at sea level. One Btu is about equal to the amount of heat given off by a single wooden kitchen match. A typical home furnace can have a rating of anywhere between 40,000 and 150,000 Btu/h (Btu's per hour), depending on its tonnage and efficiency. Btu's are also used to measure the amount of heat extracted from your home by the air conditioner, with 12,000 Btu/h equaling one ton of cooling capacity.


The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in Btu's; for cooling, it is more commonly given in tons, though it too could be given in Btu's. Total capacity fluctuates with outdoor and indoor conditions: as the weather outside heats up (or inside cools down), the capacity drops.

Carbon Monoxide (CO):

A highly poisonous gas produced when carbon-based fuels (such as natural gas) burn without sufficient air nearby. CO is colorless and odorless, which means that it cannot be detected without a specialized carbon monoxide detector. Prolonged exposure to mid-to-high levels of CO can affect the lungs, heart, and nervous system, and can ultimately cause death. Though the gas itself cannot be observed without the aid of a detector, symptoms of CO poisoning can, which include shortness of breath, nausea, headaches, dizziness, impaired vision and reduced alertness.


The heart of an air conditioning or heat pump system, the compressor sits down inside the condenser and is responsible for applying pressure to the refrigerant, which both aids in condensing it from a gas to a liquid and also moves the refrigerant through the entire system.

Condenser / Condensing Unit:

As the outdoor component of a split system, the condenser is responsible both for pumping refrigerant through the evaporator coil and for dissipating heat from the system's refrigerant, causing it to change from a hot vapor back into a cool liquid. The condenser contains the compressor within it, though the two are often mistakenly thought of as one and the same. Along with the compressor, a condenser also includes a large copper condensing coil and a fan. Condensers are used primarily for cooling your home, though a heat pump is a specialized type of condenser which also functions in heating your home.

Condenser Coil:

Also sometimes referred to as the "outdoor coil," this is the part of a condenser which receives hot refrigerant from the compressor and allows it to cool by giving off heat to the outside. This part of the air conditioning process is where the hot, pressurized gas condenses into a liquid to be sent back to the evaporator coil. In a heat pump system, the condenser coil is also where heat from outside is absorbed by the refrigerant in a reversal of the process.


A type of "valve" used in ductwork that can be opened or closed to control the amount of airflow allowed through a particular duct. Most dampers are manual and can be adjusted by moving a simple lever, but some dampers (such as those used in zoned systems) are controlled by electronic motors. The adjustment of dampers to even out airflow within a home is called an "air balance."


A type of furnace that takes cool air from the top and blows warm air to the bottom. These are usually only found where furnaces must be located in a second-floor closet or utility area.

Dual Fuel:

See Hybrid Heat.

Ducts / Ductwork:

A system of pipes or other closed conduit through which air flows from the air handler to the supply registers in a home. Ducts are typically made of sheet metal or fiberglass board, and can be rigid or flexible depending on the material from which they are made. Decreased airflow in some areas of a home or increased dust in the air can sometimes be caused by crushed or torn ductwork.

EER (Energy Efficiency Ratio):

Measures the efficiency with which a cooling product uses energy to function when the outdoor temperature is at a set level of 95° F. In other words, whereas SEER rates a system's efficiency over an entire season, EER rates a system's efficiency at peak day conditions. As with SEER ratings, a higher EER number means a system is more efficient.

Electronic Air Cleaner (EAC):

An electronic device which filters out large particles and contaminants from indoor air before it enters the air handler to be cooled or heated. This is achieved by ionizing the tiny particles, such as dust and bacteria, and then drawing them into a collector plate.

Emergency Heat:

See Supplementary Heat.

Energy Star:

Energy Star is a government-backed program which sets industry standards for superior energy efficiency and environmental preservation. Products with the Energy Star approval will be efficient and will contribute to savings on energy bills.

Evaporator Coil:

In a split system, the evaporator coil is the indoor component through which refrigerant flows after being pumped from the condenser. As air passes over the copper tubes (coils) inside the evaporator coil, the refrigerant inside those tubes absorbs heat from the air. This process also removes excess moisture from the air, which increases the perceived cooling effect of the system. When paired with a heat pump condenser, the evaporator coil also works in reverse to heat the air that flows over the coils.

Fan Coil:

An indoor component of some split systems, the fan coil is a single unit used in place of a separate furnace and evaporator coil, but has the same functions as both. (See Air Handler)


See Refrigerant.

Fresh Air Controller:

See Ventilator.

Heat Exchanger:

The part of a furnace that transfers heat to nearby air. Heat exchangers are present in all furnaces regardless of the heating method used. In a heat pump system, the evaporator coil is not referred to as a heat exchanger.

Heat Pump:

A heat pump is a specialized type of condenser which functions in both cooling and heating your home. In cooling mode, a heat pump works exactly the same as a regular condenser, but the flow of refrigerant is reversed when in heat mode, absorbing heat from the outside air and transferring it to the indoor air handler. Because heat pump systems simply transfer heat instead of burning fuel to create it, they can work very efficiently. One disadvantage to this process, however, is that extremely cold temperatures make it difficult for the heat pump to absorb enough heat to transfer. For this reason, heat pumps are excellent options in generally moderate climates such as north Texas.

Horizontal Flow:

A type of furnace, installed on its "side," that draws air from one end, heats it, and sends it out the other end. These are most often found in attic or crawl-space installations.

HSPF (Heating Seasonal Performance Factor):

This number is a measurement of a heat pump's heating efficiency over the course of one season, with a higher HSPF number meaning better efficiency. By definition, the HSPF of electric resistance heaters is 1.0. The minimum HSPF standard was established at 7.7 in 2006, but a value of 8.2 or higher is needed for a heat pump to be considered "high-efficiency." The maximum currently available is between 9.5-10.


A piece of equipment that adds water vapor to heated air as it moves out of the furnace. This adds necessary moisture to protect your furnishings and reduce static electricity. According to ASHRAE standards, ideal indoor relative humidity is 35% during the heating season, and 40-60% during the cooling season.


General abbreviation for Heating, Ventilation, and Air Conditioning.

Hybrid Heat:

Also known as "dual fuel," these systems deliver exceptional performance by combining the strengths of two heating sources to provide the most energy-efficient comfort during heating conditions. A heat pump combined with a gas furnace is an example of a hybrid heat system.

Indoor Coil:

See Evaporator Coil.


Term for the copper piping which carries refrigerant from the condenser outside to the evaporator coil inside, then back again in a cycle. The lineset consists of a smaller liquid line for cool, high-pressure liquid refrigerant, and a larger suction line for hot, low-pressure gaseous refrigerant. The lineset for an HVAC system is closed, meaning that no refrigerant should enter or leave the system at any point unless there is a leak or the pressures are adjusted by a qualified service technician.

Load Estimate:

A series of calculations performed to determine the heating or cooling requirements of your home. A load analysis uses information such as the square footage or your home, window or door areas, insulation quality and local climate to determine the heating and cooling capacity needed by your furnace, heat pump or air conditioner. Load estimates are also used in determining the optimal placement of thermostats.

Matched System:

A heating and cooling system comprised of components which have been certified to perform at promised comfort and efficiency levels when used together, and used according to design and engineering specifications.

MERV (Minimum Efficiency Reporting Value):

This rating is used to tell how efficiently an air filter is able to retain particles from the air that passes through it. The MERV scale ranges from 1 (lease efficient) to 16 (most efficient), and measures a filter's ability to remove particles from 3 to 10 microns in size. A higher MERV generally creates more resistance to airflow, because the filter's material becomes denser as efficiency increases. The exception to this is in the case of high-efficiency ionized air purification systems such as those offered by Carrier, which offer high MERV's with minimal resistance.

Outdoor Coil:

See Condenser Coil.

Packaged System:

As the name implies, a packaged system is a heating and air conditioning system that has all of the components of a normal split system completely contained in one outdoor unit. Unlike a split system, ductwork takes air outside the home in order to travel through the packaged unit, where it is either heated or cooled before being sent back into the house. Packaged systems often cannot offer the high efficiency levels found in some split systems, but they do offer a benefit as space-savers when indoor space is at a premium.

Payback Analysis:

Overall measure of the efficiency and value of your home comfort system. By combining your purchase price and ongoing operating costs, a payback analysis determines the number of years required before monthly energy savings offset the purchase price of a new system.


See Refrigerant.


See Refrigerant.


See Refrigerant.

Reciprocating Compressor:

A type of compressor used in air conditioners that compresses refrigerant by using a type of "piston" action.


Returning used refrigerant to the manufacturer for proper disposal or purification and re-use.


Refrigerant is the chemical which produces a cooling effect by absorbing heat out of the air and expanding from a liquid into a vapor. Most residential air conditioning systems contain either R-22 (Freon) or R-410A (Puron). The Montreal Protocol, adopted in 1987 and amended in 1992, established a phase-out schedule for R-22 due to its status as an HCFC greenhouse gas. According to this schedule, all new HVAC systems manufactured after January 1, 2010, must use the newer R-410A refrigerant, and all production of new R-22 refrigerant must cease completely by January 1, 2020, though existing systems which use it may continue to do so past that date.

Refrigerant Lines:

Two copper lines that connect the condenser to the evaporator coil. One line is for liquid refrigerant, and the other is larger to accommodate heated refrigerant in its expanded gas form.

Return Air:

This is where air from within a conditioned space is drawn into the furnace or air handler to be cooled or heated before being blown back into the home. The return air grill is larger than the supply registers and is often where the system's air filters are found.

Scroll Compressor:

A specially designed compressor that works in a circular motion vs. an up and down piston action. Scroll compressors are usually touted as being quieter and more energy-efficient than reciprocating compressors.

SEER (Seasonal Energy Efficiency Ratio):

This is a measure of the cooling efficiency of an air conditioner or heat pump operating over an entire season. A higher SEER means that the system is more efficient at converting electricity to cooling power. Typical SEER ratings for new systems currently range from 13 to 21.

Split System:

Refers to a complete HVAC system which has components located both inside and outside the home. This is the most common design for new and recent homes.

Supplementary Heat:

Any heating source which provides backup or assistance to a primary heating source is considered supplementary. Most of the time, supplementary heat is present in heat pump systems as electrical resistance heat strips. When supplementary heat comes on to assist the primary heat source in achieving the desired temperature quicker, it is referred to as auxiliary heat. When supplementary heat comes on in the absence of the primary heat source due to a malfunction of some kind, it is referred to as emergency heat.

Supply Registers:

These are the vents throughout a home through which cooled or heated air flows back into the conditioned spaces. They are usually located in ceilings or floors, but can also be found high on some walls as well.


A temperature-sensitive device that monitors and controls your HVAC system products. Most thermostats are digital and programmable, and many have options to monitor indoor air quality factors (such as humidity) in addition to temperature.


A unit of measure for cooling capacity. One ton = 12,000

Two-Stage Compressor:

A compressor which is capable of two levels of operation: a low stage and a high stage. Properly-sized equipment will operate 80% of the time in its low stage, enhancing efficiency and comfort with lower humidity levels and quieter operation.

TXV (Thermostatic Expansion Valve):

A thermostatic expansion valve (TXV) is a precision device used to meter the flow of liquid refrigerant entering the evaporator coil at a rate that matches the amount of refrigerant being boiled off in the evaporator coil.


A ventilator is responsible for exchanging stale indoor air with fresh air from outside the home. Today's ventilators are capable of performing this task with minimal energy loss, regardless of the weather conditions outdoors.


A type of furnace that draws cool air from the bottom and blows the warmed air out the top into the ductwork. This type of furnace is usually found in a basement or first-floor closet.

UV Lamp:

Ultraviolet light lamps, when mounted in the evaporator coil, are extremely effective in killing mold and other microscopic organisms, thereby purifying the air and maintaining efficient operation of the coil.


A zoned system is one in which a single split system serves two or more distinct areas (or "zones") independently of each other. Each zone has its own thermostat which, when used with proper programming, can aid in increasing home comfort and energy efficiency. Individual zones are controlled by motorized dampers which open and close to direct airflow as it is called for.