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Knowledge Database


A heat exchanger containing two diagonal coils that are connected together in a manner that resembles the letter “A”.


Abbreviation for alternating current, an electric current that reverses its direction many times a second.

AC or DC

Abbreviation for equipment capable of operating on alternating or direct current.


Air Conditioning Contractors of America.


Of or pertaining to sound.


Annual Fuel Utilization Efficiency. A measure of a gas furnace’s efficiency in converting fuel to energy. The higher the rating, the more efficient the unit. The unit is more efficient when the rating is higher.


Abbreviation for American Gas Association, Inc.

Air Cleaner

A piece of machinery that removes unwanted particles from moving air.

Air Conditioner

A system for controlling the humidity, ventilation, and temperature in a building or vehicle.

Air flow Volume

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

Air handler

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


Air Conditioning & Refrigeration Institute.


American Society of Heating, Refrigeration and Air Conditioning Engineer


Acrylonitrile-butadiene-styrene. Used for drain and vent lines.


The system of screens screwed into the spout of most faucets. The aerator helps control the flow of water and keep it from splashing all over. They frequently contain a baffle to help reduce the flow of water to the mandated 2.5 gallons per minute.

Air Admittance Valve (AAV)

A device that replaces a traditional vent to allow air to enter the pipe and equalize pressure, preserving the seal of water in the fixture trap.


In the drainage system, the unobstructed vertical opening between the lowest opening of a waste line and the flood level of the device into which it empties. Purpose is to prevent backflow contamination.

Anti-Scald Valve

See Pressure-balancing Valve


Abbreviation for ampere, the measure of the rate of flow of electric current.

Arc-fault Circuit Interrupter (AFCI)

Abbreviation for ampere, the measure of the rate of flow of electric current.

Armored Cable

This cable contains two insulated conductors and a thin aluminum or copper bonding strip inside a metal sheath. The metal sheath is the ground, not the bonding strip. Also called Armor-clad, AC, or BX.

British thermal unit. The standard of measurement used to guage the amount of heat required to raise the temperature of one pound of water by one degree (Fahrenheit).

British thermal units per hour. 12,000 BTUh equals one ton of cooling.

A instrument that uses fuel to support combustion.

Burner (sealed combustion)
A burner that acquires all air for combustion from outside the heated space.

Burner Orifice
The opening through which gas flows to the air/gas mixing chamber of the burner.

Backflow Preventer

A device that prevents wastewater and other contaminants from flowing into the potable water supply. Generally required for sprinkler systems, hand-held showers installed in bathtubs, faucets with pullout spouts, kitchen sprayers, and the like.


A valve operated by a float, such as a floatball or a float-cup in a toilet tank.


Any part of the drain system other than the main, riser, or stack.


A pipe fitting used to join two pipes of different sizes. A bushing is threaded inside and out.


The device that provides the current for fluorescent lights. Ballasts have become quite small, allowing the creation of compact fluorescent bulbs that can be used in place of incandescent bulbs.


Device for mounting electrical fixtures and their wiring in walls and ceilings. Common varieties include new work, old cork, single gang, two-gang, fan, and junction.

Branch Circuit

The portion of an electrical running between the breaker or fuse and the devices it powers. Can serve a single device or several.


The heavy-duty, rigid connector that connects the circuit breakers or fuses to the incoming power. Also called a Busbar.



A brand name of armored cable.

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

The metric temperature scale in which water freezes at zero degrees and boils at 100 degrees, designated by the symbol “C”. To convert to Fahrenheit, multiply a Celsius temperature by 9, divide by 5 and add 32 (25 x 9 equals 225, divided by 5 equals 45, plus 32 equals 77 degrees Fahrenheit).

The abbreviation for cubic feet per minute, commonly used to measure the rate of air flow in an air conditioning system.

Adding refrigerant to a system. Refrigerant is contained in a sealed system or in the sensing bulb to a thermostatic expansion valve.

The pump that moves the refrigerant from the indoor evaporator to the outdoor condenser and back to the evaporator again. The compressor is often called “the heart of the system” because it circulates the refrigerant through the loop.

Vapor that liquefies due to the lowering of its temperature to the saturation point.

Condenser coil (or outdoor coil)
A series or network of tubes filled with refrigerant, normally located outside the home, that removes heat from the hot, gaseous refrigerant so that the refrigerant becomes liquid again.

Condenser Fan
The fan that distributes air over the aircooled condenser.

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

Crankcase Heater
This is the electric resistance heater installed on compressor crankcases to keep the crankcase oil at a temperature higher than the coldest part of the system to prevent migration. Many newer cooling systems do not require crankcase heaters, however heat pumps do require crankcase heaters. Crankcase heaters are used to overcome the problem of migration and condensation of refrigerant in the crankcases of compressors used in air conditioning and heat pump systems.

Canadian Standards Association.

Check Valve
A type of backflow preventer installed in a pipe run that allows water to flow in only one direction.

A pipe fitting with a removable plug for inspecting and cleaning out drain pipes.

Closet Bend
A round, flat fitting that attaches to the closet bend. The heads of closet bolts, used to secure the toilet in place, insert into slots in the closet flange.

A short fitting used to join two pieces of pipe.

Chlorinated polyvinyl chloride. Used for water supply.

Any connection or situation that may allow waste water to enter the supply system.

Typically, a group of individual conductors bundled together.

Branch circuit.

Circuit Breaker
The overcurrent protection device (OCPD) most commonly in residential use. The circuit breaker can be opened and closed repeatedly to allow or stop power from flowing along the branch circuit.

A device intended to carry electric current, typically wire or a busbar. May also refer to anything that carries current, intended or not.

The rate of flow of electricity.

Found in 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.

Direct current electricity. This type of electricity (as opposed to Alternating Current, or AC) flows in one direction only, without reversing polarity.

The process of removing ice or frost buildup from the outdoor coil during the heating season.

A computation that measures the amount of heating or cooling needed for a building. A degreeday is equal to 65 degrees Fahrenheit minus the mean outdoor temperature.

An air cooler that removes moisture from the air. This unit reduces water vapor in air by cooling the air below the dew point; removing water vapor from the air by chemical means, refrigeration, etc.

A grille over an air supply duct having vanes to distribute the discharging air in a defined pattern or direction.

Department of Energy

Downflow Furnace
A furnace that pulls in return air from the top and discharges warm air at the bottom.

Drain Pan
Also referred to as a condensate pan. This is a pan used to catch and collect condensate (in residential systems vapor is liquefied on the indoor coil, collected in the drain pan and removed through a drain line).

Dry Bulb Temperature
Heat intensity, measured by a dry bulb thermometer.

Dry Bulb Thermometer
An device that measures air temperature independently of humidity.

A pipe or conduit through which air is supplied. Ducts are typically made of metal, fiberboard or a flexible material. In a home comfort system, the size and application of ductwork is critical to performance and is as principal as the equipment.

Direct expansion. A system in which heat is passed on by the direct expansion of refrigerant.

Any pipe that carries wastewater or water-borne waste.

Drainage, waste, and vent.

Daisy Chain
When conductors run from one device to the next. Saves wire, but if one device fails, all downstream devices are affected.

The items installed in boxes that help control and distribute current, such as switches, receptacles, timers, thermostats, and dimmers.

Energy Efficiency Ratio (steady state). Means the ratio of the cooling capacity of the air conditioner in British Thermal Units per hour, to the total electrical input in watts under ARI-specified test conditions.

Evaporator Coil (or Indoor Coil)
The other half of an air conditioning system, this network of tubes filled with refrigerant which is located inside the home within the indoor unit, take heat and moisture out of indoor air as liquid refrigerant evaporates.

Expansion Valve
A refrigerant-metering valve with a pressure or temperature controlled orifice.

Curved fittings, usually 90° or 45°, used to change the direction of a pipe run. Also called “ells.”

A decorative or protective plate. In plumbing, the plate behind or under a fixture that covers the hole around the pipe or valve.

The temperature scale on which water freezes at 32 degrees and boils at 212 degrees; designated by the letter F. To convert Fahrenheit to Celsius, subtract 32 from the Fahrenheit number, multiply by 5 and divide by 9 (77 32 equals 45, times 5 equals 225, divided by 9 equals 25 degrees Celsius). This is the most commonly used scale of temperature measurement in the United States of America.

Any device that creates air currents.

Any device used to remove dust and other impurities from air for the purposes of reducing the load on the respiratory system and to protect the HVAC equipment through a straining process. Filters vary greatly in particle arrestant; the higher the MERV rating (Minimum Efficiency Reporting Value- a number from 1 to 16 that is relative to an air filter’s efficiency), the better the filter.

Any vent or duct, pipe, or chimney for carrying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors.

That part of an environmental system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.

A fuse is a type of overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows, which breaks the circuit in which it is connected, thus protecting the circuit’s other components from damage due to excessive current.

Any pipe part used to join together two sections of pipe, such as elbows, couplings, bends, wyes, etc.

Accepts or discharges water or wastewater, such as faucets, sinks, toilets, etc.

Flapper Valve
The part on the bottom of the toilet tank that opens to allow water to flow from the tank into the bowl.

Float Ball
In the toilet tank, the hollow ball attached to a rod that rises as the tank refills and shuts the water inlet valve.

Substance applied to copper pipes and fittings before soldering to help the fusion process and prevent oxidation.


The conductors that feed panels other than the service panel.

Fish Tape
A coiled spring-steel line used for pulling, or fishing, cable and wire through enclosed spaces.

Accessories such as bushings and clamps that serve a mechanical rather than an electrical function.

Four-way Switch
A set of three switches wired to control the same fixture or group of fixtures.

An overcurrent protection device that contains a thin strip of metal that will melt and open the circuit in case of circuit overload. Must be replaced after a circuit overload.

Gas Appliance Manufacturers Association.

Gas Furnace Heat Exchanger
Found in the furnace, the heat exchanger transfers heat to the surrounding air, which is then transported throughout your home.

The measure of the size of a wire. The smaller the number, the thicker the wire and the higher its current-carrying capacity.

Goof Plate
An oversize cover plate designed to hide a rough patching job around a box. Note that drywall and plaster must be repaired to within 1/8″ of any box; larger gaps may not simply be hidden behind a goof plate.

A connector that runs between a device or circuit to safely conduct current to earth.

Ground Fault
The leaking of current to the grounding conductor.

Ground-fault Circuit Interrupter (GFCI)
A device designed to interrupt the flow of power when a minuscule imbalance is detected between the flow and return of current.

Heat Exchanger
An area, box or coil where heat flows from the warmer to the colder fluid or surface. A device for the transfer of heat energy from the source to the conveying medium.

Heat Gain
The amount of heat introduced to a space from all heat producing sources by appliances, solar energy, occupant respiration and lighting.

Heat Loss
The rate of heat transfer from a building interior to the outdoors.

Heat Pumps
An automated compression cycle refrigeration system that can be switched to either heat or cool the controlled space.

Heat Transfer
The movement of heat energy from one area to another. The means for such movement are convection , conduction, and radiation. Heat will flow naturally from a warmer to a cooler space or material.

Heating Coil
Any coil that serves as a heat source.

A measure of the number of cycles or wavelengths of electrical energy per second; U.S. electricity supply has a standard frequency of 60 hertz.

Heating Seasonal Performance Factor. This rating is used in measuring the heating efficiency of a heat pump by taking in account the variations in temperature that can occur within a season and is the average number of BTU of heat dispatched for every watt-hour of electricity used by the heat pump over a heating season. The higher the number, the more efficient the heat pump system.

A machine that adds water vapor to the air to increase humidity.

A humidity sensing device designed to regulate humidity input by recting to changes in the moisture content of the air.

The measure of the moisture content of air. Air conditioners can remove moisture for added comfort.

Humidity, Absolute
The ratio of the mass of water vapor to the volume occupied by a mixture of water vapor and dry air. It is measured in grams of water vapor per cubic meter of air.

Humidity, Relative
A measure of the percent of moisture actually in the air compared with what would be in it if it were fully saturated at that temperature. When the air is fully saturated, its relative humidity is 100 percent.

Heating, Ventilating and Air Conditioning.

Device used to support pipes.

Hose Bibb
An outdoor faucet, also used to supply washing machines.

Home Run
In communications and structured wiring, when conductors are run from each device back to the source.

Current is present. A hot lead is the one carrying current along a circuit. It usually has black or red insulation. A hot circuit is one in which the breaker is closed and current is present.


The lighting of a gaseous mixture to the temperature at which combustion takes place.


A material that is a poor conductor of current and therefore used to shield wires, cables, and connectors.


Junction Box

A box containing splices in cables. Has a removable cover that must be accessible (cannot be buried in ceilings and walls). Also called a J-box.

Kilowatt (kW)
1,000 watts. A common unit of electrical consumption measured by the total energy created by one kilowatt in one hour.

Knob and Tube
A system of wiring in which individual, loom-covered hot and neutral conductors were run using porcelain knobs to support the wires along framing members and porcelain tubes to protect wires passing through framing members.

Knockout (K.O.)
A partially prepunched opening in a box that is removed to allow the entry of cable. A knockout that is mistakenly opened or is open because a cable is removed must be filled with a knockout seal.



Latent Heat
A type of heat, which when added to or taken from a substance, does not change the temperature of the substance yet enables the heat energy to change its state.

Industry term for bathroom sink.


Used to terminate a wire.



The material in a filter that traps and holds the impurities. These HVAC filter media include fiberglass filter media, polyester filter media, dog hair filer media, blue/green filter media, charcoal filter media, and others.


The primary artery of the supply or drain system to which all the branches connect. Referred to as the Main Vent in the vent system.


A fitting that connects a number of branches to the main; serves as a distribution point

National Energy Council / National Electric Code.

National Electrical Manufacturing Association.

A short length of pipe installed between couplings or other fittings.

No-hub Connector
A connector for no-hub iron pipe consisting of a rubber sleeve and a stainless-steel band secured by hose clamps. A variation, a neoprene sleeve with two adjustable steel bands, is used for connecting dissimilar materials, as when connecting new plastic pipe to an existing cast-iron drain pipe.

Commonly, the return conductor in a circuit. It usually has white insulation. More properly called the grounded conductor because it returns current to ground at the service panel. Note that this is different from the green-sheathed or bare copper grounding conductor that does not carry current except in case of equipment fault.



Original equipment manufacturer.


The packing material used before sealing a hubbed cast-iron fitting with lead.


The measure of electrical resistance.

Open Circuit

A circuit in which the flow of current is interrupted due to an open breaker or fuse. May be intentional or unintentional (as caused by a short).


To run equipment or wire in excess of its normal full-load rating.




Package Unit
A heating and cooling system contained in one outdoor cabinet. This package unit is typically installed on the roof, beside, or sometimes in the attic of a home.

Pounds per square inch, absolute.

Pounds per square inch.

Pounds per square inch gauge.

Polyvinyl chloride; a type of plastic. In recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas.

Cross-linked polyethylene. PEX tubing, commonly used for hydronic radiant floor heat, is increasingly used for water supply lines.

Pipe Dope
Slang for pipe-joint compound. Substance applied to threaded fittings to create a watertight seal.

Water that suitable for consumption.

Pressure-balancing Valve
A mixing valve that monitors the water pressure in both the hot and cold supply lines and compensates for a drop in either one.

A short length of wire attached to an existing wire or wires.

A system in which the slots/blades for the hot leads are narrower than those for the neutral leads.

Reciprocating Compressor
A compressor that uses pistons driven by a crankshaft to deliver gases at high pressure.

A chemical substance that produces a refrigerating effect while expanding and vaporizing.

Refrigerant Charge
The required amount of refrigerant in a system.

A fitting that allows pipes of different sizes to be joined together.

Relief Valve
A valve that opens to relieve excess temperature and/or pressure in the system. See T&P Valve.

A supply line pipe that rises from one story to the next; also the short vertical pipes that bring water from the branch to the fixture.

Installation of the drain, vent, and supply lines in the structure prior to installation of the fixtures.

A plastic or metal channel used to conduct wires or cables from one point to another.

A brand of nonmetallic-sheathed cable.

Seasonal Energy Efficiency Ratio. A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It takes into account the variations in temperature that can occur within a season and is the average number of BTU (British thermal units) of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling season. The higher the SEER, the more efficient the unit. The more efficient the unit, the lower the operating cost. The U.S. Government’s minimum SEER rating is 10.

Self Contained System
A refrigerating system that can be moved without disconnecting any refrigerant lines; also know as a package unit.

Sensible Heat
That heat which, when added to or taken away from a substance, causes a change in temperature.

Any device that responds to a change in the conditions being measured, permitting the condition to be controlled.

The temperature or pressure at which a controller is set for desired comfort level.

Split System
The combination of an outdoor unit (air conditioner or heat pump) with an indoor unit (furnace or air handler). Split systems must be paired for optimal efficiency. This is the most common type of system installed in a home.

Split System
The combination of an outdoor unit (air conditioner or heat pump) with an indoor unit (furnace or air handler). Split systems must be paired for optimal efficiency. This is the most common type of system installed in a home.

Shutoff Valve
Most commonly refers to the angle stops installed under sinks and toilets, but valves are also installed on branch lines and alongside the meter.

Another name for hose bibb.

A metal alloy that is melted to create a fused joint between metal pieces.

The vertical main in the DWV system extending one or more stories.

The vertical main in the DWV system extending one or more stories.

Short lengths of pipe installed during rough-in to which fixtures and drains will eventually be installed.

Slang for soldering.

The eased change of direction in a drain fitting that allows for smooth passage of waste. Fittings with abrupt changes of direction, such as a vent tee, may only be used for vents.

Service Entrance
The point where the electrical service enters the house, becoming your responsibility instead of the electric company’s.

Service Panel
Usually installed near the service entrance, this panel contains the main switch or breaker to disconnect the house system from the power source as well as circuit breakers.

Short Circuit
An accidental connection between two conductors or between a conductor and ground or some other unintended surface.

Single-pole Switch
A standard light switch with off and on positions for controlling flow of current to one or more devices.

Split Receptacle
A receptacle in which each of the two outlets is wired on a different circuit or in which one outlet is always live and the other is switched. Also called split-wired.

A series of sensors and relays that monitor and control the functions of a heating and cooling system by turning the device on or off when a specified temperature is reached.

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. Also called a thermal expansion valve.

A unit of measurement used for determining the cooling capacity of a system. One ton of cooling is based on the amount of heat needed to melt one ton (2000 lbs.) of ice in a 24 hour period. One ton of cooling is equal to 12,000 BTU/hr.

Two-stage cooling
The air conditioner/heat pump has a compressor with two degrees of operation: high for hot summer days and low for cooler days. While the low setting is adequate to meet household-cooling demands 80% of the time, a two-stage unit runs for longer periods and produces more balanced temperatures.

T&P Valve
Temperature and pressure valve. A valve that opens to release excess pressure and temperature in a system.

The section of pipe that runs between a fixture outlet and the trap.

Tee Fitting
Fittings that allow another pipe to be joined at a 90-degree angle. Those for the drain system have the changes of direction eased. See Sweep.

A fitting or portion of a fixture that, when properly vented, holds water to prevent entry of sewer gases.

Three-way Switch
A pair of switches wired to control the same fixture or group of fixtures.

The factor amounting to the resistance of heat flow through various building materials.

Underwriters Laboratories.

Upflow Furnace
A furnace in which air is drawn in through the bottom or sides and expels warm air out the top.

A pressure below atmospheric pressure. 30 inches Mercury (periodic symbol “Hg”) is a perfect vacuum.

Variable speed motor(s)
The fan motor inside Trane’s variable speed air handlers is designed to vary its speed based on your home’s heating and air conditioning requirements. Working in conjunction with the thermostat, it keeps the appropriate temperature air (e.g. warm air on cold days) circulating throughout your home, reducing temperature variances in your home. It also provides greater air circulation and filtration, better temperature distribution, humidity control, higher efficiency, and quiet performance.

A unit of measure of electrical force given to the electrons in an electric circuit. A single volt is the electromotive force that will cause a steady current of one ampere to surge through a resistance of one ohm. Abbreviated by the symbol “v”.

The force that pushes electrical current along wires and cables. Term voltage used to indicate the potential difference in a circuit, voltage is also known as the pressure which causes current to flow.

A device that regulated the flow of water.

Valve Set
The immovable portion of a valve. Water flow is stopped when the movable portion of the valve comes in contact with the valve seat.

A pipe that allows air into a drain system to balance the air pressure, preventing the water in the traps from being siphoned off.

The unit of electrical power equal to the flow of one amp at a potential difference of one volt. A single watt is equivalent to the work done at a rate of 1 joule (the SI unit of energy measuring heat, electricity and mechanical work) per second

Wet Bulb Thermometer
A thermometer whose bulb is covered with a piece of watersoaked cloth. It is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor.

Water Hammer Arrestor
A device installed near a fixture to absorb the hydraulic shock that happens when a fixture’s supply is suddenly shut off, causing water hammer, a loud banging noise in the pipes.

Wye Fitting
A drain fitting that allows one pipe to be joined to another at a 45-degree angle.

Wire Nut
A brand of twist-on wire connector.

Zoning System
A method of sectioning a home into different comfort zones so each zone can be individually controlled depending on use and need. An air conditioning system capable of maintaining diverse conditions for various rooms or zones.