• Cozy Heating Inc.

HVAC Terminology

Updated: Jun 15

Have you ever had the experience of having an HVAC technician in your home and they start explaining what they are doing, or go over your system with you, but to you this is all sounding very foreign?



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We can help you understand what these HVAC terms mean, so you can have a more thorough understanding of what they are telling you when they come out and repair or service your equipment.

Language of the Trade


AC = Air conditioner (aka condensing unit)


AFUE = Annual Fuel Efficiency Ratio – Measures the energy efficiency of the heating system (Furnace or boiler) the measurement reflects how much fuel is being used to heat the home vs. how much energy is lost through exhaust systems.

e.g.: if a furnace has an AFUE rating of 90% - 90% of the heat it generates goes into the duct work, 10% goes out the flue or exhaust pipe = 90% losses 10% of its heat out the flue.


Air Handler = They move air through their coils and into the ductwork. They also have heat strips for backup or auxiliary heat Typically fitted to heating and cooling components such as heat pumps.


BTU (British Thermal Unit) = Measurement of energy or heat that is used to indicate the rate of cooling, dehumidifying, or heating in an HVAC system. One BTU is the amount of energy needed to raise the temp of one pound of pure water by 1 degree – This is how we measure heat.


CFM (Cubic Feet per Minute) = Common method to measure and quantify air flow. This is how we measure air flow or air delivery.

e.g.: 3 ton air handler delivers 1200CFM (400 CFM per ton).


Coil = Equipment that implements heat transfer to air when mounted inside an air handling unit or ductwork. It is heated or cooled by electrical means or by circulating liquid or steam within it.


A-coil = (Shaped like an A- 2 different coils, leaning against each other piped together) It’s an evaporator coil.


Evaporator Coil = Refrigerant evaporates, and it flows into the evaporator oil and the refrigerant evaporates and when it does it gets really cold inside the coil; air flows up through the furnace or air handler; through the coil as air flows through this coil it gives up its heat to the refrigerant flowing through the coil. That refrigerant, now, with that heat goes to that outdoor unit (condensing unit)


Condenser Coil = Removes heat from the refrigerant. A central HVAC’s condenser coils are usually located in an outdoor AC or HP unit. Refrigerant condenses heat and then the heat is extracted and given up to the outdoor air.


Compressor = HVAC compressor is responsible for moving refrigerant. Typically located on the outdoor unit. Compressor is like the engine for a car (mechanical pump) that pumps refrigerant through the unit (refrigerant cycle)


Conditioned Space = Space where the air is being cooled. You don’t always want to heat a conditioned space.


COP (Coefficient of Performance) = Expression of the efficiency of a HP in heating mode. Measures its efficiency.


Dehumidifier = Reduces the level of humidity from the air. Cools the air to the point where water turns to liquid from vapor form, which is then removed via an inset tray in the unit or through a drain. AC system is also a dehumidifier.


Delta T = Difference of two temps; difference between air on one side of the coil compared to the other end of the coil.

e.g.: temperature of air entering an evaporator coil and leaving the evaporator If you measure air going into the furnace and the air coming out of the evaporator coil and there is a 20 difference; that is the Delta T.


Diffuser = (aka- register) Placed over ductwork, the diffuser separates air with vanes going in differing directions. It evenly distributes airflow in the desired directions. Diffuses the air.


Duct System (Ductwork) = Specialized pipe or channel for the airflow (including supply air, return air, and exhaust air) within a home. Can be made from several different materials including galvanized steel (the most common), aluminum, fiber glass, polyurethane, and plastic (called flexible ducting) – Air delivery system


EER (Energy Efficiency Ratio) = Standardized measurement of the efficiency of an air conditioner. The higher either ratio is, the more efficient the unit is.


Heat Pump = The HP is a compressor that cycles both hot and cold air. It is designed to move thermal energy in the opposite direction of heat flow by absorbing heat from a cold space, which is released to warmer space. Same as an air conditioner but in reverse. Pumping heat inside and pumping cold air outside.


HEPA Filter (High Efficient Particulate Air) = A type of pleated mechanical air filter. This type of air filter can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns (µm). Has to meet certain and strict guidelines to be called HEPA.


HSPF (Heating Seasonal Performance Factor) = Used to measure the efficiency of a HP. The HSPF is a ratio of the heat output to electricity use over an average heating season. The higher the HSPF the greater the energy efficiency. Evaluating, judging, or labeling the efficiency of a HP. (AC is often measured by seer and eer, HP are SOP and HSPF)


HVAC (Heating, Ventilation and Air Conditioning) = Some HVAC companies also offer refrigeration work. This work is usually for the commercial locations such as food and beverage.


IAQ = A term used to describe the quality of the air within a conditioned space. Simple air filter, humidifier, UV light, is an IAQ


Load Calculation = This is also known as a heat gain/loss calculation. This is a process to determine how much heat a building or space gains or losses in one hour. This information allows designers to determine the correct size of an HVAC system and how big the heating and cooling system needs to be. Also, how much heat is introduced in the home in an hour and how much heat gets lost in an hour.


Mean Temperature = The average temperature. Measures over a period of time, inside the conditioned space. You record the temperature a few times, within an hour, and then average those times out.


OAT (Outside Air Temperature) = Measures the air temperature outside of a building.


Plenum = An enclosed space used for airflow. This is usually a box above or below the furnace or air handler. It will often house the evaporator coil (if used). It is normally in a box, that sits above or below the evaporator coil, it is the box that the main trunk lines attach too. May even house an evaporator coil (not usually).


Preventative Maintenance = The work performed by a technician, designed to reduce unplanned HVAC system failure and breakdowns.


Refrigerant = Substance that is used to absorb heat and release heat. This ability allows a cooling system to move heat from one location to another. The medium in which heat is moved from inside to outside or outside to inside.


Return Air = Air that has been conditioned (warmed or cooled) and distributed through the supply duct work. Return ducts bring back air to the furnace or air handler to be reconditioned.


Schematic Diagram = Representation of the elements of an HVAC system or specific unit. They use abstract and graphic symbols rather than realistic pictures. A tech might mention they need a schematic diagram to finish the service call.


Supply Air = Conditioned air that is distributed through your ductwork and supplied to various spaces.


Ton = The HVAC industry often uses this term to describe the approximate capacity of an air conditioner. One ton of cooling is equal to 12,000 BTUs of heat removal per 24 hours. (In HVAC we have: 1 ton, 1.5 ton, 2 ton, 2.5 ton, 3 ton, 3.5 ton, 4 ton, 5 ton)


Ventilation = Moving air through a space or movement of air.






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