Like many specialty fields, the heating and air conditioning industry has a language of its own. As you do research on comfort solutions for your East Dundee, IL area home, please use this glossary of HVAC terms to help you clarify the things that matter to you and your family’s comfort when making this important purchase. If you have any questions, you can always call us at (630) 504-8688.

Glossary of HVAC Terms


Annual Fuel Utilization Efficiency. Indicated as a percentage, your furnace’s AFUE tells you how much energy is being converted to heat. For example, an AFUE of 90 means that 90% of the fuel is being used to warm your home, while the other 10% escapes as exhaust with the combustion gases.


See Evaporator Coil.

Air Handler

The portion of your air conditioner or heating system that forces air through your home’s ductwork. This is used when a furnace is not being used for heating your home.


British Thermal Unit. Used for both heating and cooling, BTU is a measure of the heat given off when fuel is combusted. Or for cooling, it’s a measure of heat extracted from your home. (One BTU is approximately equal to the heat given off by a wooden kitchen match.)


A British Thermal Unit (BTU) is the unit of heat required to raise 1 pound of water by one-degree Fahrenheit at sea level. BTUH is British Thermal Units per Hour.


Stands for Cubic Feet per Minute. A measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air is being forced through the system.


The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs. For cooling, it is usually given in tons.

Carbon Monoxide

A colorless, odorless, highly poisonous gas produced when carbon-based fuels, such as natural gas, burns without sufficient combustion air.

COP (Coefficient of Performance)

The rated heating capacity output provided by a heat pump to the energy consumed by the system under designated operating conditions. The higher the COP, the lower the operating costs.


Part of a split-system heat pump or air conditioner’s outdoor unit that controls the pressure applied to the refrigerant, necessary for taking in heat to warm your home with a heat pump or getting rid of heat to keep your home cool.


A heat exchanger in which hot, pressurized (gaseous) refrigerant is condensed by transferring heat to cooler surrounding air, water or earth.

Condenser Coil

Part of the outdoor portion of a split-system air conditioner or heat pump. By converting refrigerant that is in a gas form back to a liquid, the coil sends heat carried by the refrigerant to the outside.

Cycling Losses

The actual efficiency of a heating or cooling system is reduced because of start-up and shut-down losses. Oversizing a heating or cooling system increases cycling losses.


Decibels (dB) are a unit measuring the intensity of sound. (Click Here to see our sound chart) Insert link


A type of “valve” used in duct work that opens or closes to control airflow. Used in zoning to control the amount of warm or cool air entering certain areas of your home.

Downflow Furnace

A type of furnace that takes cool air from the top and blows warm air to the bottom-commonly used where furnaces must be located in a second-floor closet, utility area or slab foundation.


Refers to heating and cooling technologies that do not require ductwork to make your home comfortable. Whether you live in a new home or an older home without ductwork or you want more complete year-round climate control over a specific room or area of your home, ductless systems are an efficient way to expand your comfort and save money on your energy bills.


Hollow pipes used to transfer air from the Air Handler or furnace to the air vents throughout your home. Ductwork is one of the most important components of a home heating and cooling system.


Energy Efficiency Ratings (EER) measure the efficiency with which a product uses energy to function. It is calculated by dividing a product’s BTU output by its wattage. Used to access heat pump’s air conditioning cooling capacity output.

Electronic Air Cleaner (EAC)

An electronic device that filters out large particles and contaminants in indoor air. It then electronically pulls out tiny particles that have been magnetized, such as viruses and bacteria, drawing them to a collector plate.

Energy Star

Energy Star is a government-backed program helping businesses and individuals protect the environment through superior energy efficiency. Products with the Energy Star rating will be efficient and save cost on energy bills.

Evaporator Coil

Part of a split-system air conditioner or heat pump located indoors. The evaporator coil cools and dehumidifies the air by converting liquid refrigerant into a gas (or vice-versa). A blower motor, typically in a furnace, then moves air over the coil to either heat or cool your home.


Energy recovery ventilation (ERV) is the energy recovery process of exchanging the energy contained in normally exhausted building or space air and using it to treat (precondition) the incoming outdoor ventilation air in residential and commercial HVAC systems. This system not only captures heating/cooling energy from air exchanged but also Humidity

Fan Coil

An indoor component of an air conditioner or heat pump system, used in place of a furnace and evaporator coil, to provide change the refrigerant from a gas to a liquid (or vice-versa) and blow air over the coil to cool or heat your home.

Fossil Fuel

Any of several types of combustible fuels formed from the decomposition of organic matter. Examples are natural gas, propane, fuel oil, oil and coal.

Geothermal Heat Pump

A heat pump that uses the earth as a heat source and heat sink.


A guarantee is formal promise or assurance (typically in writing) that certain conditions will be fulfilled, especially that a product will be repaired or replaced if not of a specified quality and durability. Note: This type of assurance is NOT backed by an insurance policy and is typically not offered by Heating manufacturers. This type of assurances are made by heating contractors without financial backing. If the dealer goes out of business your guarantee is likely null and void


The Heating Seasonal Performance Factor is a measure of the heating efficiency of a heat pump. The higher the HSPF number, the more efficiently the heat pump heats your home.


Term used for Heating, Ventilation, and Air Conditioning.

Heat Exchanger

The part of a furnace that transfers heat to nearby air.

Heat Pump

A product that works just like an air conditioner in cooling mode; however, in heating mode, the refrigerant flow is reversed, and heat is extracted from the outside air too heat your home.

Heat Sink

The medium—air, water or earth—which receives heat rejected from a heat pump.

Heat Source

The medium—air, water or earth—from which heat is extracted by a heat pump.

Horizontal Furnace

A type of furnace, installed on its “side”, that draws in air from one side, heats it and sends the warm air out the other side. Most often used for installations in attics or crawl spaces.


A heat–recovery ventilator brings in fresh air while exhausting old stale air and uses the heat in the outgoing stale air to warm up the fresh air. A typical unit features two fans—one to take out household air and the other to bring in fresh air.


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 home furnishings, reduce static electricity and most importantly can improve the health of those living in the home.


Is a patented technology from Mitsubishi that allows your heat pump to operate in temperatures as low as 13 degrees Fahrenheit below zero still providing approx. 77-92% of rated capacity. This is designed to provide comfortable indoor temperatures in extreme cold-climate conditions while saving energy and money.

Indoor Coil

See Evaporator Coil.

Load Estimate

A series of studies performed to determine the heating or cooling requirements of your home. An energy load analysis uses information such as the square footage of 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. When referring to heating, this is often known as a Heat Loss Analysis, since a home’s heating requirements are determined by the amount of heat lost through the roof, entry ways and walls.

Low Boy

Low Boy is a type of furnace configuration in which the furnace is lower in height and occupies more floor space.


The Minimum Efficiency Reporting Value is the standard comparison of the efficiency of an air filter. The MERV scale ranges from 1 (least efficient) to 20 (most efficient) and measures a filter’s ability remove particles from 3 to 10 microns in size.

Matched System

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

Mini Splits

See Ductless


Mitsubishi is a worldwide leader in the designing and building of heat pumps and HVAC products. For more than 38 years Mitsubishi Electric Cooling and Heating has developed innovative technologies designed to enhance your home and your life.

Modulating Furnace

A modulating furnace operates with finesse to reduce temperature fluctuations, providing consistent indoor comfort from room to room. This technology is the quietest and most comfortable way to heat your home! Efficiencies range from brand to brand up to 98%. That means fuel is used more efficiently, reducing your energy cost. Plus, the home can be more easily and efficiently kept in the perfect comfort zone. This is the top of line furnace available today!

Operating Cost

The day-to-day cost of running your home comfort equipment, based on energy use.

Outdoor Coil

see Condenser Coil.

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.

R-22 Refrigerant

R-22 is a single component HCFC refrigerant with low ozone depletion potential. It has long been used in a variety of air-conditioning and refrigeration applications in a variety of markets. Production of R-22 will cease in 2020 per the Montreal Protocol. R22 has not been legal to use in newly manufactured Air conditioning units since January 2010.

R-410A Refrigerant

R-410A Refrigerant is an environmentally sound refrigerant designed not to harm the earth’s ozone layer. Federal law requires that all manufacturers phase out ozone depleting refrigerants in the next few years. R410A Refrigerant is approved by the US Environmental Protection Agency as a replacement in central air conditioners for R-22 Refrigerant. NOTE: you can NOT add R410A into a system designed to run on R-22. They are meant to be used in different equipment.

ocating CompreReciprssor

A type of compressor used in air conditioners that compresses refrigerant by using a piston to do the compression.

Reclaiming Refrigerant

Returning used refrigerant to the manufacturer for recycle or disposal.


Removing, cleaning and reusing refrigerant and other materials from your jobsite.

Refrigerant Lines

Two copper lines that connect the Condenser (Outdoor) Coil to the Evaporator (Indoor) Coil.


The Seasonal Energy Efficiency Ratio is a measure of the cooling efficiency of your air conditioner or heat pump. The higher the SEER number, the more efficient the system is at converting electricity into cooling power.

Scroll Compressor

A specially designed compressor that works in a circular motion vs. an up and down piston action.

Setback Thermostat

A state-of-the-art electronic thermostat with a built-in memory that can be programmed for different temperature settings at different times of the day.

Single Package Product

One outdoor unit that contains both a heating and a cooling system.

Split System

Refers to an air conditioner or heat pump that has components in two locations. Usually, one part of the system is located inside (evaporator coil) and the other is located outside your home (condenser coil). These systems can be conventional home comfort systems with ductwork or what the industry calls Mini Splits or ductless which do not require ductwork. See Ductless.

Supplemental Heating

A heating system used during extremely cold weather, when additional heat is needed to moderate indoor temperatures. May be in the form of fossil fuel heat pump or electric resistance heat and could be built in as a permanent heat source or something temporary like unit heaters.


Unit that monitors and controls your HVAC system. You can raise or lower temperature and may be able to control many other functions from your home comfort system with this device.

Thermostatic Expansion Valve

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

Ton of Air Conditioning

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

Two Stage Compressor

Two Stage Compressors are capable of two levels of operation, a low stage and a high stage. Properly sized equipment will operate 65% of the time in low stage, enhancing efficiency and comfort with lower humidity levels and quieter operation. It’s like getting two air conditioners or heat pumps in one system.

Upflow Furnace

A type of furnace that draws cool air from the bottom and blows the warmed air out the top into the duct work. This type of furnace is usually installed in a basement or an out-of-the-way closet.

Variable Speed Furnace

A variable speed furnace has a special blower motor which allows for the volume of air to increase or decrease automatically as needed. There are many reasons for choosing a variable speed furnace, but the main reason is comfort. The term “variable speed” refers to the furnace’s fan motor, which moves at different speeds to precisely control the flow of heated and cooled air throughout your home significantly improving comfort.


A ventilator circulates air such as in a parking garage where a ventilator will exhaust the car fumes and bring in fresh air.


A warranty is a written guarantee, issued to the purchaser of an article such as a furnace or air conditioner by its manufacturer, promising to repair or replace it if necessary within a specified period of time. By law a warranty must be insured meaning the manufacturer has to pay for an insurance policy to cover the warranty or put a specified amount of cash in an account to support all claims on the warranties. If your installing HVAC dealer were to go out of business your warranty would still be in effect from the manufacturer that is the main difference between this and a Guarantee.


A way to increase your home comfort and energy efficiency by controlling when and where heating and cooling occurs in a home. Programmable thermostats are used to control operating times of the equipment. Dampers are used to direct air flow to certain parts or “zones” of the home. This allows for different temperatures in different zones or rooms at the same time. Zoning is also available through the use of ducted/ductless VRF/VEV add Linear Metering.