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HVAC Terms

Annual Fuel Utilization Efficiency. The AFUE percentage tells you how much energy is being converted to heat. The higher the number The Greater the efficiency.

Air Handler
The portion of your heating and cooling system that forces air through your home’s ductwork.


British Thermal Units. The amount of heat it takes to raise one pound of water one degree Fahrenheit. The higher the BTU rating, the larger the heating capacity of the furnace or air conditioner.


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


The motor/pump that drives the air conditioning unit. It is responsible for pumping refrigerant throughout the system.

Condenser Coil

Part of the outdoor portion of a heating or cooling system, that releases or collects heat from the outside air.


A valve or movable plate used in duct work that opens and closes to control airflow. They are used to direct air to specific areas of the home.


A type of furnace that takes cool air from the top and blows warm air to the bottom.


Hollow metal venting used to transfer air throughout your house.

Electronic Air Cleaner

An electronic device that filters out particles and contaminants in indoor air.

Evaporator Coil

Part of the heating or cooling system located indoors, that cools and dehumidifies the air by converting liquid refrigerant into gas.

Heat Exchanger

The major part of the furnace that transfers heat into your home.

Heat Pump

A unit that handles both heating and cooling. In some climates, a heat pump may handle your heating and cooling needs more efficiently than a furnace and air conditioner.

Horizontal Flow

A type of furnace, installed on its side, which draws air from one side, heats the air and then sends it out the other side.


Heating Seasonal Performance Factor. Measures the heating efficiency of a heat pump. The higher the number the more efficient the heat pump heats your home.


A piece of equipment that adds moisture to the air as it comes out of the furnace.


Heating, Ventilation and Air Conditioning


A chemical that cools the air as it evaporates.


Seasonal Energy Efficiency Rating. SEER measures a unit’s cooling efficiency. The higher the number, The Greater the efficiency.

Single Package

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

Split System

Refers to an air conditioner or heat pump that is combined with indoor components, such as an evaporator coil inside and a condenser coil outside your home.


A device that monitors and controls your temperature inside your home.


A type of furnace that draws cool air from the bottom and blows the warmed air out the top.


A ventilator captures heating or cooling energy from the stale indoor air and transfers it to fresh incoming air.


Zoning allows you to control the heating and cooling delivered to specific areas of your house for a custom solution. Zoning can increase efficiency and comfort in the areas of the house you use most often.

Plumbing Terms

Ambient Temperature

The average temperature of the atmosphere in the vicinity of an appliance.

Atmospheric Combustion

Takes place when gaseous, liquid or solid fuels react at an elevated temperature with oxygen by burning, thus releasing heat.

Back Flow

When water travels from one system back into any part of the main distribution system.

Boiling Point

The boiling point refers to the temperature at which liquid changes to vapor by the addition of heat.


A heat producing device where the flame or heat is produced.

Check Valve

To allow fluid to flow in only one direction in a pipe; also known as a backflow preventer.

Closed System

A system where the incoming cold water supply has a device that will not allow water to expand when heated (i.e. check valve, backflow preventer, some pressure reducing valves).

Combustion Gases

Gases released when a gaseous, liquid or solid fuel reacts at an elevated level that needs to be vented with gas or oil-fired water heaters.

Commercial Application

Water heaters used for more than a single-family dwelling.


Formed by more heat being absorbed into the water leaving less temperature to carry the water vapor produced in gas combustion to pass out of the venting system.

Conventional Venting

Atmospheric venting that utilizes the natural convective rise of hot flue gases through the system to release the products of combustion.

Degree Rise

Difference between the starting water temperature and the ending temperature after heating is complete.

Dip Tube

The tube inside the water heater that sends cold water to the bottom of the tank.

Direct Vent

Pulls outside air for combustion and vents combustion gases directly outside.

Drain Valve

A device designed to allow drainage of stored contents from a water heater.

Dual element heater

An electricalwater heater with an upper and lower element for heating water.


Energy Cut Off – a Safety device designed to shut the power off to the water heater and prevent high temperature.


Energy Factor – A measure of the overall efficiency rating of the water heater based on the model’s recovery, efficiency, standby loss, and energy input.

Efficiency Energy Guide Label

The Federal Trade Commission (FTC) requires that gas, electricaland oil residential automatic storage water heaters be labeled to show 1.) an estimated annual cost of operation for that particular model, based on a national average cost of fuel specified by FTC and 2.) how the efficiency of that model compares to all other comparable models.

Expansion Tank

Designed to absorb excess pressure due to thermal expansion, e.g. closed system.

First Hour Delivery (Rating)

Combination of the usable stored volume of hot water in a tank, plus the recovery capacity for the first hour of operation.

Flow control valve

A device designed to reduce water flow (GPM) to a plumbing fixture (i.e. shower head at 5 GPM vs. 2.5 GPM); the use of flow control valves can be cost-effective in load calculations, reducing the water usage and the amount of energy used to heat water.

Flow rate

Rating in gallons per minute (GPM) or gallons per hour (GPH).


A passageway for products of combustion.

Foot Print

The area of space taken up by the water heater.


The pressure of water as measured at a stated point; it may be measured in feet or in pounds per square inch (PSI).

Heat Exchanger

A heat transfer system.

Heat Transfer

When heat is passed from one medium to another.

Heat Trap

Restricts heat loss through water connections to a tank.


The amount of fuel used by a water heater in a given period of time: generally rated in 1 hour.

Instantaneous Water Heater

A type of water heater that heats water as it flows through a heat exchanger coil.

Mixing Valve

Mixes hot and cold water to achieve a specified delivery temperature.


A thin coating or layer, usually calcium on the bottom of a tank or interior parts, that may prevent heat transfer.

Sealed Combustion

Sealing of combustion chamber to prevent spillage of combustion products.


A threaded opening on the water heater tank.

Storage Tank

A tank used to hold a specific volume of water

Electrical Terms

Alternating Current

An electrical current which reverses direction repeatedly due to a change in voltage which occurs at the same frequency. Often abbreviated AC or ac.


An electricalgenerator designed to produce alternating current. Usually consists of rotating parts which created the changing magnetic field to produce the alternating current.

American Wire Gauge

A standard measure which represents the size of the wire. The larger the number, the smaller the wire. Abbreviated AWG.


The maximum continuous current that a conductor can carry without overheating above its temperature rating.


The electricalcurrent produced by one volt applied to a resistance of one ohm. It is also equal to the flow of one coulomb per second. Named after French physicist Andre M. Ampère 1836.


The flow of electricity equal to one ampere for one hour. Commonly used to rate the capacity of batteries.


A measuring or display methodology which uses continuously varying physical parameters. In contrast, digital represents information in the discrete binary form using only zeros and ones.


A group of two or more cells connected together to provide electrical current. Sometimes also used to describe a single cell which converts chemical energy into electrical current.

Battery Cycle Life

The number of discharge and recharge cycles that a battery can undergo before degrading below its capacity rating.

Black Start

Refers to certain electricalutility generating units that can start upon demand without any outside source of electricalpower. These are often combustion turbines that have stationary battery banks to provide backup power to energize all the controls and auxiliaries necessary to get the unit up and running. In the event of a large area-wide blackout, these units are critical to restoring the utility grid. Most utility generating units do not have black start capability.


An electrical conducting path formed by the permanent joining of metallic parts. Intended to assure electrical continuity and the capability to safely conduct any likely current. Similar to bonding jumper or bonding conductor.


A device that stores electrical charge usually by means conducting plates or foil separated by a thin insulating layer of dielectricalmaterial. The effectiveness of the device, or its capacitance, is measured in Farads.

Circuit Breaker

A device designed to open a circuit either by manual action or by automatic action when the current exceeds a value longer than permitted. A circuit breaker can provide overcurrent protection.


Usually a metallic substance capable of transmitting electricity with little resistance. The best conductor at normal temperature ranges is silver. The most common is copper. Some other recently discovered substances called superconductors actually have zero resistance at extremely low temperatures.

Continuous Load

A sustained electrical load current for three hours or more.


The flow of electricity commonly measured in amperes.


An electronic semiconductor device that predominantly allows current to flow in only one direction.

Direct Current

An electrical current that normally flows in one direction only. Abbreviated dc.


A nonmetallic conductor of electricity usually consisting of a liquid or paste in which the flow of electricity is by ions.


The capacity for, or the ability to do, mechanical work. Electrical energy is measured in kilowatt-hours for billing purposes.


A short circuit.


Full load amperes, also sometimes abbreviated RLA for full load amperes. This is the current in amperes that a motor requires to produce rated nameplate horsepower output when rated nameplate voltage and frequency is provided to its terminals.


The number of complete alternations or cycles per second of an alternating current. It is measured in Hertz. The standard frequency in the US is 60 Hz. However, in some other countries, the standard is 50 Hz.


A rotating machine which converts mechanical energy into electrical energy. In the automotive industry, traditional terminology uses a generator to refer to only those machines designed to produce dc current through brushes and a commutator (as opposed to the alternator).


In the electrical arena, a term used to refer to the electrical utility distribution network.


A conducting connection between an electrical circuit or device and the earth. A ground may be intentional, such as in the case of a safety ground, or accidental which may result in high overcurrents.


A sine wave which is an integral multiple of a base frequency. For example, the third harmonic on a 60 Hz system is a frequency of 180 Hz. Certain types of electrical equipment generate harmonics which interfere with the proper functioning of other devices connected to the same system.


Unit of frequency. One Hertz equals one complete cycle per second of an ac source. Abbreviated Hz. Named after the German physicist Heinrich R. Hertz 1894. This unit replaces the former “cycles-per-second.”


A unit of power equal to 746 watts.


Abbreviation for Hertz.


The total effects of a circuit that oppose the flow of an ac current consisting of inductance, capacitance, and resistance. It can be quantified in the units of ohms.


The characteristic of an electricalcircuit by which a voltage is induced in it by a variation of current. This can be a variation of the current in the circuit itself (self-inductance) or in a nearby circuit (mutual inductance). The magnitude of the characteristic is measured in the units of Henries.


An electrical device which is designed to convert direct current into alternating current. This was originally done with rotating machines which produced true sine wave ac output. More recently this conversion has been performed more economically and efficiently using solid-state electronics. However, except for the most expensive models, these devices usually do not produce perfect sine wave output. This sometimes can result in electromagnetic interference with other sensitive electronic devices.


A positively or negatively charged atom or molecule.


A unit of work or energy equal to one watt for one second. One kilowatt hour equals 3,600,000 Joules. Named after James P. Joule, an English physicist 1889.

Joule’s law

Defines the relationship between the current in a wire and the thermal energy produced. In 1841an English physicist James P. Joule experimentally showed that W = I2 x R x t where I is the current in the wire in amperes, R is the resistance of the wire in Ohms, t is the length of time that the current flows in seconds, and W is the energy produced in Joules.


Unit of power equal to 1000 watts. Abbreviated kW or KW.


Unit of energy or work equal to one kilowatt for one hour. Abbreviated as kWh or KWH. This is the normal quantity used for metering and billing electricity customers. The price for a kWh varies from approximately 4 cents to 15 cents. At a 100% conversion efficiency, one kWh is equivalent to about 4 fluid ounces of gasoline, 3/16 pound LP, 3 cubic feet natural gas, or 1/4 pound coal.


Aan electrical device or material that has been tested by a recognized organization and shown to meet appropriate standards. Many local governmental authorities require that installed electrical products be listed. A well-known listing organization is Underwriters Laboratories (UL).


A device which consumes electrical power and is connected to a source of electricity.


Locked rotor amperes. This is the current that a motor would require if the rotor were locked in place and prevented from rotating and rated nameplate voltage and frequency were applied to its terminals. This is also the current that could appear briefly during motor starting. While the motor comes up to speed this current gradually drop off and when the speed approaches running rpm this current rapidly falls to the RLA value. Often the starting current is less than the LRA value because the voltage at the motor terminals dips during starting. This LRA value is important when sizing a generator because the generator’s surge rating must be large enough to handle it.


Minimum circuit amperes. This is the minimum current rating allowed for the wiring and circuit breaker or fuse protection for the equipment. It is used by the installer and electrician to size the branch circuit to feed the equipment.

National Electrical Code

A code for the safeguarding of people and property from hazards related to the use of electricity. Compliance with this code along with proper maintenance will result in an installation essentially free from hazard. Abbreviated NEC. The NEC was first developed in 1897 as a result of the efforts of various insurance, electrical, architectural, and allied interests. It is sponsored and regularly updated by the National Fire Protection Association.


Abbreviation for the National Electrical Code.


A conductor of an electrical system which usually operates with minimal voltage to ground. Depending on the type of system, it may carry little current or only unbalance current. Systems that have one conductor grounded use the neutral for this purpose.


A unit of electrical resistance. A circuit resistance of one ohm will pass a current of one ampere with a potential difference of one volt. Abbreviated using the Greek letter omega (W ). Named for the German physicist George Simon Ohm 1854.

Ohm’s Law

Defines the relationship between voltage, resistance, and current. In 1828 the German physicist George Simon Ohm showed by experiment that the current in a conductor is equal to the difference of potential between any two points divided by the resistance between them. This may be written as I = E / R where E is the potential difference in volts, R is the resistance in Ohms, and I is the current in amperes.

Open Circuit Voltage

The maximum voltage produced by a power source with no load connected.


Any current beyond the continuous rated current of the conductor or equipment. This may be valued slightly above the rating as in the case of an overload or may be far above the rating as in the case of a short circuit.


Operation of electrical equipment above its normal full-load rating or of a conductor above its rated ampacity. An overload condition will eventually cause dangerous overheating and damage.


The rate at which work is performed or that energy is transferred. Electric power is commonly measured in watts or kilowatts. A power of 746 watts is equivalent to 1 horsepower.

Power Factor

The ratio of real power to the apparent power delivered in an ac electrical system or load. Its value is always in the range of 0.0 to 1.0 or 0% to 100%. A unity power factor (1.0) indicates that the current is in phase with the voltage and that reactive power is zero.

Reactive Power

The mathematical product of voltage and current consumed by reactive loads. Examples of reactive loads include capacitors and inductors. These types of loads, when connected to an AC voltage source, will draw current, but since the current is 90o out of phase with the applied voltage they actually consume no real power in the ideal sense.

Real Power

The rate at which work is performed or that energy is transferred. Electric power is commonly measured in watts or kilowatts. The term real power is often used in place of the term power alone to differentiate from reactive power. Also called active power.


The characteristic of materials to oppose the flow of electricity in an electricalcircuit.


Running load amperes, also sometimes abbreviated FLA for full load amperes. This is the current in amperes that a motor requires to produce rated nameplate horsepower output when rated nameplate voltage and frequency is provided to its terminals.

Short Circuit

A low resistance connection unintentionally made between points of an electrical circuit which may result in current flow far above normal levels.


An ac electricalsystem or load consisting of at least one pair of conductors energized by a single alternating voltage. This type of system is simpler than three-phase but has substantial disadvantages when large amounts of power have to be delivered.

Surge Capacity

The ability of an electrical supply to tolerate a momentary current surge or inrush imposed by the starting of motors or the energizing of transformers.


An ac electricalsystem or load consisting of three conductors energized by alternating voltages that are out of phase by one-third of a cycle. This type of system has advantages over single-phase including the ability to deliver greater power using the same ampacity conductors and the fact that it provides a constant power throughout each cycle rather than a pulsating power, as in single-phase. Large power installations are three-phase.


A device that converts one ac voltage and current to a different voltage and current. Constructed using two or more coils of wire around a common magnetic core. The energy is transferred from one coil, usually considered the primary winding, to the other coil, the secondary winding by means of mutual induction in the magnetic core. Transformers are an efficient and economical means of transferring large amounts of ac electricalpower at high voltages. This is the primary advantage of ac systems over dc systems.

Underwriters Laboratories

A non-profit organization that was established by the insurance industry to test electrical devices for safety.

Uninterruptible Power Supply

A device that provides a constant regulated voltage output in spite of interruptions of the normal power supply. It includes filtering circuits and is usually used to feed computers or related equipment which would otherwise shutdown on brief power interruptions. Abbreviated UPS.


The electrical potential difference or pressure across a one-ohm resistance carrying a current of one ampere. Named after Italian physicist Count Alessandro Volta 1745-1827.


A unit of apparent power equal to the mathematical product of a circuit voltage and amperes. Here, apparent power is in contrast to real power. On ac systems, the voltage and current will not be in phase if reactive power is being transmitted. Usually abbreviated VA.

Voltage Drop

A voltage reduction due to impedances between the power source and the load. These impedances are due to wiring and transformers and are normally minimized to the extent possible.


A unit of power equal to the rate of work represented by a current of one ampere under a pressure of one volt. Named after the Scottish engineer James Watt, 1819.

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