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HVAC Dictionary – Precision Air Conditioning & Heating

Terms of Interest If You’re Curious About Comfort

Click the term to see the definition.

AC (Alternating Current)

A type of current where the polarity is perpetually reversing, causing the directional flow in a circuit to reverse at regular intervals.


Air Conditioning Contractors of America.


Air Changes Per Hour. It is a way to measure the ventilation rate for a specific room or building. ACH tells you how many times the air in the room is completely different. A common unit of measure of ventilation rate for a space, or air leakage rate for a building, defined as the volumetric flow rate divided by the volume of the space considered.


A heat exchanger consisting of two diagonal coils that are joined together and looks like the letter “A”.


Of or pertaining to sound.


Annual Fuel Utilization Efficiency. Indicated as a percentage, your furnace’s AFUE tells you how much energy is being converted to heat in a perfect world. For example, an AFUE of 90 means that 90% of the fuel is being used to warm your home, while the other% escapes as exhaust with the combustion gases. The higher the AFUE, the more fuel efficient the unit is. The government’s established minimum AFUE rating for furnaces is 78%. The established minimum rating for a fossil-fueled boiler is 80%; and the minimum rating for a gas-fueled steam boiler is 75%.AFUE doesn’t include the real world heat losses of the duct system or piping, which the U.S. Department of Energy says can be 35% of the energy for output of the furnace when ducts are located in the attic.


American Gas Association, Inc.

Air Conditioner:

A device that changes humidity levels, temperature or quality of air.

Air Flow Volume:

The amount of air the system circulates through your home, expressed in cubic feet per minute (cfm). Proper air flow depends on the outdoor unit, the indoor unit, the ductwork and even whether the filters are clean.

Air Handler:

The portion of the central air conditioning or heat pump system that moves heated or cooled air throughout a home’s ductwork. In some systems a furnace handles this function.

Air Leakage:

Uncontrolled and/or unintended airflow through a building enclosure or between units of occupancy. Leakage from indoors to outdoors is known as exfiltration and leakage from outdoors to indoors is known as infiltration. Air leakage can cause indoor air quality problems, condensation, excess energy use, comfort complaints, and smoke transport.


Air Conditioning and Refrigeration Institute.


American Society of Heating, Refrigeration and Air Conditioning Engineers.

Balance Point:

An outdoor temperature, usually between 30° F and 45° F, at which a heat pump’s output exactly equals the heating needs of the home. Below the balance point, supplementary electric resistance heat is needed to maintain indoor comfort.

Blower Door/Blower Door Fan:

An instrument used in Home Performance to help best identify the sources of air leakage in your home. A blower door consists of the door and the door fan. The door fan is nothing like a regular fan because it is a very powerful tool. The door fan is a specially designed, calibrated fan, which is temporarily mounted in the blower door that we set up inside the opening for one of the outer doors of your home. The fan is used to blow air into or out of a room, home, or building to measure the total air leakage of the location being tested.

A blower door fan works by establishing a pressure differential between the inside and the outside of an enclosure. The fan puts the home under positive or negative pressure (depending on the type of testing being done). The pressure difference forces air to leak through all of the holes in the thermal boundary of the location being tested. The amount of air that is required to maintain a constant pressure difference is equal to the amount of air that is leaking from the enclosure. A specially designed gauge is then used to measure the amount of air flowing through the Door Fan, and the pressure difference, which can be used to determine the total size of all the air leaks.

With the home under the pressure achieved through the blower door/blower fan, we then use a thermal imaging camera and tracer smoke to locate the areas of leakage so they can be sealed.


Abbreviation for Building Performance Institute (see below)


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 equal to the heat given off by a wooden kitchen match. BTU also measures the amount of heat required to raise or lower the temperature of one pound of water 1° Fahrenheit.


British Thermal Units per hour. 12,000 BTUh equals one ton of cooling.

Building Performance:

See “Home Performance”

Building Performance Institute, Inc. (The):

The Building Performance Institute, Inc. (BPI) is an independent, non-profit, national standards development and credentialing organization for residential energy efficiency retrofit work (energy efficiency as applied to the already existing housing market, not new construction). BPI brings together leading building science experts from across North America to develop their standards using a consensus-based methodology. BPI offers professional certification examinations on the assessment and upgrade of residential buildings – both single-family and multifamily. BPI also provides accreditation to contracting companies committed to whole house home performance and quality assurance programs that ensure this work adheres to BPI’s nationwide technical standards. One of our owners, Larry Stewart, is a BPI Certified Building Analyst.


The device that facilitates the combustion of air and gas.

Burner Orifice:

The opening in the burner through which the gas or fuel passes prior to combustion.


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.


A temperature scale that registers the freezing point of water as 0°and the boiling point as 100° under normal atmospheric pressure.


Cubic Feet per Minute. A measurement of airflow volume. A ton of air conditioning should deliver about 400 CFM.

Charging a System:

Adding coolant, or refrigerant, to an HVAC system.

Combustion Analysis:

Using a combustion analyzer, our technicians measure combustion efficiency, stack temperature, O2 (Oxygen), CO2 (Carbon Dioxide), CO (Carbon Monoxide) and flue draft. In doing so, we determine what is needed to properly efficiency tune your gas furnace or boiler to the best efficiency level it can provide. Additionally, if Carbon Monoxide levels are too high or are unstable, this analysis provides the information we need to reduce Carbon Monoxide to a safe level.

Combustion Analyzer:

A tool used to perform a combustion analysis. It measures combustion efficiency, stack temperature, O2 (Oxygen), CO2 (Carbon Dioxide), CO (Carbon Monoxide) and flue draft. Proper use of this tool can save your life. Not all Combustion Analyzers are created equally. We use only the top of the line tools with the least chance for malfunction or error.


Things that are able to catch on fire and burn.


This is the heart of an air conditioning or heat pump system. It is part of the outdoor unit and pumps refrigerant in order to meet the cooling requirements of the system.


Vapor that is turned into a liquid as its temperature is lowered.

Condenser Coil:

Also an outdoor coil. In an air conditioner, a device that removes heat from the refrigerant, allowing the refrigerant to be converted from vapor to liquid. In a heat pump system, the coil absorbs heat from the outdoors.

Condenser Fan:

A fan that passes air over the condenser coil to facilitate the removal of heat from the refrigerant.

Condensing Furnace:

High-efficiency furnace. The flue gases are condensed in a second heat exchanger for extra efficiency and have sealed combustion. Instead of a metal flue pipe, condensing furnaces have a flue pipe made of white PVC. Condensing furnaces range from 90% to 97% AFUE.

Conditioned Space:

Conditioned space is any place where air conditioning and/or heating should be. For example, the inside of your home is conditioned space. It is the opposite of “Unconditioned Space.”


A switch that can repeatedly cycle, making and breaking an electrical circuit. When sufficient current flows through the A-coil built into the contactor, the resulting magnetic field causes the contacts to be pulled in or closed.


Canadian Standards Association.


Direct Current. A type of electrical current that only flows in one direction.


Found at the exit point of ductwork, this movable plate opens and closes to control airflow. Dampers can be used to balance airflow in a duct system. They are also used in zoning to regulate airflow to certain rooms. They can be manual or electric.


Calculated by subtracting the average outdoor temperature for an area from 65° Fahrenheit. This measurement is used to estimate the amount of heating or cooling a home or building will need.


A device that removes humidity (moisture) from the air. Your air conditioning system itself operates to remove humidity from the air; however, when it is not hot enough for your air conditioner to run very long (or at all) your home can still feel stuffy and icky. A whole house dehumidifier, installed through your central air conditioning system, can be especially helpful for your comfort during these times of high humidity but not necessarily high temperatures – as it can work to remove humidity even with the air conditioning unit off. Additionally, a whole house dehumidifier can help save energy during the entire cooling season because they make the indoor air less humid. Lower humidity (drier air) allows you to feel more comfortable at a higher temperature. As this equipment also has its own drain system, you benefit from lower humidity without having to do anything with the water.


A grille over an air supply duct with vanes that distribute the discharging air in a specific pattern or direction.


Department of Energy

Downflow Furnace:

A type of furnace that takes cool air from the top and blows warm air to the bottom–common where your furnace must be located in a second-floor closet or utility area.

Drain Pan:

Also a condensate pan. As the refrigerant vapor is liquefied, the drain pan collects the condensate and funnels it to the drain line.

Dry Bulb Temperature:

The temperature as measured without the consideration of humidity.


A network of metal, fiberboard or flexible material flowing throughout a space which delivers air from an HVAC unit to the respective zones of a home or office.


Direct expansion; a system in which heat is transferred by the direct expansion of refrigerant.


Energy Efficiency Ratio.

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. Electronic Air Cleaners should NEVER be used by those with asthma, allergies or sensitivities to Ozone as all Electronic Air Cleaners do emit some Ozone as part of its regular process.


Environmental Protection Agency.


The unintentional and/or uncontrolled transfer of air (air leakage) from inside a building, including a home, to the outside. For Home Performance purposes, we say that it is the unintentional or uncontrolled air leakage from conditioned space to unconditioned space (e.g., from your living space into your attic) or from inside a building, including a home, to the outside. Air leakage can cause indoor air quality problems, comfort issues, condensation, and higher energy use.

Expansion Valve:

A valve that meters the levels of refrigerant through a temperature or pressure control.

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, which absorbs the heat from the air. The warmed refrigerant is then carried through a tube to the outdoor unit (condenser coil).


A temperature scale in which water freezes at 32 degrees and boils at 212 degrees at normal atmospheric pressure.


A device that creates air flow.

Fan Coil:

An indoor component of a heat pump system, used in place of a furnace, to provide additional heating on cold days when the heat pump does not provide adequate heating.


A device that acts like a strainer to remove dirt or undesired particles.


A vent that removes the by-products of combustion from a furnace.


The major component in heating a home. It is a device that facilitates the combustion of fuel and air to create heat. Technically, “furnace” also means that the heating device is a gas appliance. Technically, “heater” means electric heat. However, most people say “heater” so “heater” is fine with us.


A delicate metal strip connecting two parts of an electrical circuit. This strip breaks, or melts, in the event of excess electrical charge, breaking the electrical circuit.


Gas Appliance Manufacturers Association.

Gas Furnace Heat Exchanger:

Located in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped throughout your home.

Heat Exchanger:

Located in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped throughout the home.

Heat Gain:

The amount of heat added or created in a designated area.

Heat Load:

See “Load Estimate”

Heating Coil:

A coil that acts as a heat source for a heating system.

Heat Loss:

The amount of heat subtracted from a designated area.

Heat Pump:

An air conditioner that contains a valve that allows it to alternate between heating and cooling by transferring heat between two reservoirs. A heat pump is perfect fit for the Memphis area as it allows the heat pump to provide the heating for your home as long as it is 35 degrees outside. A back up heat source is usually also present (a gas furnace, boiler or electric heat strips).

Heat Transfer:

Moving heat from one location to another.

High Efficiency Air Cleaner:

An air filter installed in your central system that collects up to 99% of particles 5microns or larger. In comparison your standard air filter only picks up about 8% of these same particles.

Home Performance:

(AKA “Building Performance”) A comprehensive whole-house approach to testing, identifying and repairing a home’s safety, comfort, moisture, indoor air quality and energy efficiency problems. Those most qualified to work in Home Performance have made an investment in: HVAC industry experience; extensive training; certifications; and the equipment needed for testing and providing solutions. The tools of home performance allow us to see what is not clear to the naked eye. These tools most often include: blower door, air flow capture hood, smoke puffers, thermographic (infrared) camera, combustion analyzer and differential pressure manometer in addition to special software.
Using Home Performance guides us in: sealing air holes and gaps in the building envelope; adding insulation where needed; sealing ducts; adding heat recovery ventilation or other means to control and upgrade fresh air ventilation (if needed); and controlling moisture. Further, the Home Performance approach can identify whether it is appropriate to suggest upgrading the air conditioning and/or heating systems, appliances and/or lighting by replacing them with more energy efficient equipment such as ENERGY STAR rated appliances, devices, compact fluorescent lamps (CFL), or LED lamps.

Horizontal Flow:

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.


Heating Seasonal Performance Factor. This factor rates the efficiency of the heating portion of the heat pump. The higher the HSPF, the more efficient the product. There is no government established minimum HSPF rating for heat pumps.


A device that adds humidity (moisture) to the air. This may be necessary to add moisture to protect your furnishings and reduce static electricity; however, these should be used only as a last resort. New building science knowledge has taught us that provided someone takes a shower or cooks in the home daily, your home usually produces enough humidity for most needs. If you do these activities and your air still feels dry or has static electricity, properly sealing your home against air infiltration and insulating properly may resolve these issues entirely. It is really the migration of dry, cold outdoor air into the living area through the openings in the living space either from outside or through the attic, basement, crawl space and/or ductwork that dries out a home the most.


The device that measures humidity and turns the humidifier on and off.


Dampness in the air caused by water vapor.


Heating, Ventilation and Air Conditioning.


The lighting of a fuel to make it burn.


The unintentional and/or uncontrolled transfer of air (air leakage) from outside a building, including a home, to the inside of the building/home. For Home Performance purposes, we say that it is the unintentional or uncontrolled air leakage from unconditioned space to conditioned space (e.g., from your attic into your home) or from outside a building, including a home, to the inside of that building/home. Air leakage can cause indoor air quality problems, comfort issues, condensation, and higher energy use. If your home is dry and/or has static electricity issues, before you buy a Humidifier, call us for a Blower Door Test. These problems are most often caused by infiltration of dry, cold, outside air into the home.

Latent Heat:

A type of heat that when added to an area produces an effect other than an increase in temperature.

Load Estimate:

Also “Heat Load”. 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, window and door areas, insulation quality and local climate to determine the heating and cooling capacity needed by your furnace, or air conditioner. When referring to heating, this is also called “Heat Loss Analysis,” since a home’s heating requirements are determined by the amount of heat lost through the roof, entry ways and walls.

Matched System:

A heating and cooling system comprised of products that have been certified to perform at a set efficiency and output level when used together, and used according to design and engineering specifications.


The fine material of a filter that traps dirt, dust, mildew or bacteria.


National Energy Council/National Electric Code.


National Electrical Manufacturing Association.

Operating Cost:

The day-to-day cost to run your home comfort system, based on the energy required to run it.


An opening or hole.

Outdoor Coil/Condensing Unit:

The portion of a heat pump or central air conditioning system that is located outside the home and functions as a heat transfer point for collecting heat from and dispelling heat to the outside air.

Package Unit:

A heating and cooling system contained in one outdoor unit.

Payback Analysis:

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


A permit is required for every new equipment installation, every new gas line and also for modifications to ductwork among other improvements in most jurisdictions where Precision works. We always pull a permit when it is required.

Programmable Thermostat:

An electronic thermostat with a built in memory that can be programmed for different temperature settings at different times of day. These are one of the best ways to reduce energy consumption and costs. Programmable thermostats of the past mostly differed in their ability to set several different temperatures that change daily to ones that allowed you to keep a weekday schedule and a weekend schedule. Today, programmable thermostats can do everything from displaying in high definition to communicating with us (or you) over the internet.


Pounds per square inch.


Pounds per square inch, absolute.


Pounds per square inch gauge.


Polyvinyl chloride; a type of plastic.

Reciprocating Compressor:

A type of compressor used in cooling systems to compress refrigerant by using a piston action.


A chemical that condenses from a vapor to liquid and, in the process, decreases in temperature.

Refrigerant Charge:

The amount of refrigerant in a system.

SEER - Seasonal Energy Efficiency Ratio:

A rating system developed by the U.S. Government to indicate the efficiency level of cooling equipment (the minimum allowed is 10). The higher the SEER number, the more efficient the system is at converting electricity into cooling power.

Self-contained System:

A package unit.

Sensible Heat:

Heat added or subtracted that causes a change in temperature.


A device that reads to a change in conditions.

Split System:

An outdoor unit (condenser coil) combined with an indoor unit (evaporator coil).


Sensors that monitor and control the output of an HVAC system.

Thermostatic Expansion Valve:

A device that creates a constant evaporator temperature.


Unit of measurement for determining cooling capacity. One ton is12,000 BTUs per hour.

Unconditioned Space:

Unconditioned space is any place where air conditioning and/or heating should not be. For example, most attics should be unconditioned space. It is the opposite of “Conditioned Space.”

Upflow Furnace:

A furnace that pulls in air from the bottom and releases it through the top. This type of furnace is usually installed in a basement or an out-of-the-way closet.


A space where the pressure is significantly below that of standard atmospheric pressure.


A unit of electro-motive force.


The force pushing electrical current along wires and cables.


The unit of electrical power equal to the flow of one amp at a potential difference of one volt.

Wet Bulb Thermometer:

A thermometer that measures the relative humidity in the air.


A system that divides a home, office or space into different regions to better control the temperature and effectiveness of a heating and cooling system. If you have some parts of your home you want to stay warmer or cooler than other parts, zoning can achieve that. If you have a two story house sharing one air conditioning and heating system, zoning can make you more comfortable.

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