The Trades AlmanacA homeowner's reference for every system in the house

Reference library

Trade Terms A to Z

130 plumbing, HVAC, electrical and carpentry terms, defined in a sentence or two the way a homeowner would use them. Every explainer on the site links into this page; every definition is drawn from a source listed at the bottom.

A

AeratorPlumbing
The small screened insert at the tip of a faucet that mixes air into the water so it flows in a smooth stream instead of splashing.
AFCI (arc-fault circuit interrupter)Electrical
A breaker (or receptacle) with sensing circuitry that tells harmless arcing, like a switch flicking, from the dangerous arcing of damaged or overheated wiring, and shuts the circuit off to prevent a fire. Standard breakers do not catch arcing. See also Arc fault, Circuit breaker.
AFUEHVAC
Annual fuel utilization efficiency: the share of a furnace's or boiler's yearly fuel that becomes heat for the house. An AFUE of 90 percent means 10 percent is lost up the chimney and elsewhere. The FTC requires new furnaces to display it.
Air handlerHVAC
The indoor unit of an air conditioner or heat pump system, holding the blower and the indoor coil, that moves air through the ducts.
Ampere (amp)Electrical
The unit of electric current. One volt pushing through one ohm of resistance produces one ampere.
Anode rodPlumbing
A sacrificial rod of aluminum, magnesium or zinc inside a steel water heater that corrodes so the tank does not. One manufacturer recommends inspecting it yearly; IBHS says a professional look at least every two years.
Arc faultElectrical
Dangerous sparking caused by damaged, overheated or stressed wiring or devices: frayed or cracked old wires, a nail or screw driven into a cable behind a wall, a damaged outlet. See also AFCI.

B

Backflow preventerPlumbing
A device that keeps water from flowing backward into the supply, so contaminated water cannot reach the drinking water.
Ball valvePlumbing
A shutoff valve with a pivoting ball inside; a quarter turn of its lever opens or closes it. Common as a main water shutoff.
BaseboardCarpentry
The trim board run around a room along the bottom of the wall, next to the floor.
Bibb washerPlumbing
The rubber washer fastened to the bottom of a compression faucet stem. Pressed into the seat it stops the water; lifted, it lets water flow. It is the usual cause of a two-handle faucet drip. See also Seat, Stem.
BlockingCarpentry
Short pieces of lumber installed between framing members to brace them or to give drywall or paneling something to nail to.
BlowerHVAC
The motor-driven fan in a furnace or air handler that pushes heated or cooled air through the ductwork.
Branch circuitElectrical
The wiring that runs from the last breaker or fuse protecting a circuit out to its outlets, lights and appliances.
BroachPlumbing
The shaped opening inside a faucet handle that grips the top of the stem. The stem should fit snugly inside it, which is why stems are matched by broach as well as length.
BTUHVAC
British thermal unit: the heat needed to raise one pound of water one degree Fahrenheit. Heating and cooling capacity is rated in BTU per hour.
Bus barElectrical
A metal bar inside the service panel, one per hot leg, that carries power from the main breaker to the individual breakers snapped onto it.

C

CapacitorHVAC
An electrical part that stores energy to kick-start the compressor and fan motors and then helps keep them running. A humming outdoor unit that will not start often means a failed capacitor; because capacitors hold charge after power is off, replacement is a technician's job.
CartridgePlumbing
A replaceable insert in a faucet or shower valve that controls water flow, used mostly in single-handle designs. Replacing it is the standard fix for a single-handle faucet or shower that drips.
CasingCarpentry
The trim molding around a door or window opening that hides the gap between the frame and the wall.
Circuit breakerElectrical
A switch you can operate by hand that also opens automatically when current exceeds its rating. It protects the wiring from overheating; it is not primarily a shock-protection device. See also GFCI, AFCI.
CleanoutPlumbing
A capped access point, usually about 4 inches across, where a drain or sewer line can be cleared or serviced.
Compression fittingPlumbing
A connector that seals tubing by squeezing a sleeve (ferrule) between a nut and the fitting body. Common on supply stops and supply lines.
CompressorHVAC
The pump in the outdoor unit that pressurizes refrigerant vapor into a hot, high-pressure gas and circulates it through the system.
CondensateHVAC
Water that condenses out of house air on the cold indoor coil during cooling. It collects in a drain pan and leaves through a condensate drain line; a clogged line can trip an overflow switch that shuts the system down.
Condenser (condenser coil)HVAC
The outdoor coil of an air conditioner that releases the heat collected indoors. In a heat pump the roles of the coils reverse in winter.
ConduitElectrical
A pipe, usually metal, that electrical wires are run through.
ContinuityElectrical
A complete, unbroken path for current to flow. A continuity test checks whether a wire, fuse or switch still provides one.
Cripple studCarpentry
A short stud above or below an opening, placed where a full-length stud would otherwise have gone, such as between a window's rough sill and the bottom plate.

D

DadoCarpentry
A groove cut across a board or panel to receive the edge of another board, as in shelf joints.
DamperHVAC
A movable plate in the ductwork that regulates or redirects airflow.
Dimensional lumberCarpentry
Lumber from 2 inches nominal up to, but not including, 5 inches nominal in thickness: the 2x4s, 2x6s and 2x10s of house framing.
Dip tubePlumbing
The extension of the cold-water inlet inside a tank water heater that carries incoming cold water down toward the bottom of the tank. A disintegrating plastic dip tube can shed bits that clog faucet aerators.
DiverterPlumbing
A valve that redirects water from one outlet to another, such as from the tub spout up to the showerhead. On a three-handle tub and shower valve it is the center stem.
Drain-waste-vent (DWV)Plumbing
The drainage piping that carries wastewater out of the house plus the vent piping that lets air in and out so the traps keep their water seals. See also P-trap, Vent stack.
Drywall (gypsum board)Carpentry
A manufactured panel of gypsum plaster faced with paper, commonly 1/2 inch thick in 4 by 8 or 4 by 12 foot sheets.
DuctworkHVAC
The metal or synthetic tubes that carry conditioned air from the equipment through the house and bring return air back.
Duplex receptacleElectrical
The common wall outlet with two plug-in positions on one mounting strap.

E

Equilibrium moisture content (EMC)Carpentry
The moisture content at which wood neither gains nor loses moisture in air at a given relative humidity and temperature. It is why interior wood moves with the seasons. See also Moisture content.
EscutcheonPlumbing
A decorative cover plate that hides rough plumbing, such as the trim ring around a faucet stem or a shower valve.
Evaporator coilHVAC
The indoor coil where liquid refrigerant evaporates and pulls heat out of the house air passing over it. DOE suggests checking and cleaning it yearly.
Expansion tank (thermal)Plumbing
A small tank on the water heater piping that absorbs the pressure rise when heated water expands in a closed plumbing system (one with a check valve, backflow preventer or pressure-reducing valve).

F

Fire blockingCarpentry
Material installed in the concealed spaces of the framing to cut off hidden air passages and resist the passage of flame between floors. In stud walls it is required at floor and ceiling levels and at horizontal intervals of no more than 10 feet.
Flame sensorHVAC
A furnace safety device that confirms a burner flame is present; if it cannot detect flame, the gas is shut off. A furnace that lights and shuts off a few seconds later, repeatedly, often has a dirty or failing one.
FlapperPlumbing
The rubber seal at the bottom of a toilet tank, attached to the lift chain, that opens to flush and closes to let the tank refill.
Frost-free sillcockPlumbing
An outdoor faucet whose shutoff valve sits deeper in the wall, inside the heated part of the house, so the water in it does not freeze. See also Hose bibb.

G

Gate valvePlumbing
A shutoff valve opened and closed by turning a round wheel handle, clockwise to shut the water off. Common on older main shutoffs.
GFCI (ground-fault circuit interrupter)Electrical
A device meant to protect people. It compares the current going out to a device with the current coming back; when they differ by a few thousandths of an ampere, it cuts power in a fraction of a second, far below what would trip a breaker. See also Circuit breaker.
GroundElectrical
A connection between a circuit or equipment and the earth. Intentional grounding creates a low-resistance path so that if a fault energizes a metal case, the current has somewhere to go other than through a person.
Grounding electrodeElectrical
The metal object that connects the electrical system to the earth: a ground rod driven at least 8 feet deep, the footing's reinforcing steel, or a metal underground water pipe.

H

The horizontal structural member over a door or window opening that carries the load from above across the opening. See also Jack stud, King stud.
Heat exchangerHVAC
The sealed metal chamber in a furnace that takes heat from the burning fuel and passes it to the house air flowing over its outside, keeping the combustion gases separate. Cracks or corrosion can let carbon monoxide into the house, which is why it is inspected yearly.
Heat pumpHVAC
Equipment that heats and cools by moving heat with a reversible refrigerant cycle instead of burning fuel. In winter it collects heat outdoors and releases it inside.
Hose bibb (sillcock, spigot)Plumbing
An outdoor faucet with a threaded outlet for a garden hose. See also Frost-free sillcock.
Hot wireElectrical
An ungrounded, energized conductor. Standard 120/240-volt home service has two, called leg A and leg B. See also Neutral.
HSPF2HVAC
A heat pump's seasonal heating efficiency: the heat delivered over the heating season, in Btu, divided by the electricity used, under the test procedure DOE adopted in 2023. Higher is better.
Humidifier (whole-home)HVAC
A device connected to the heating system that adds moisture to the air to keep indoor humidity steady through the house.

I

Inducer motor (draft inducer)HVAC
The small fan in a furnace that clears the burner area before ignition and pushes exhaust out the vent. It runs before the burners light.

J

Jack stud (trimmer)Carpentry
The stud under each end of a header that supports it and carries its load down to the bottom plate. See also Header, King stud.
JambCarpentry
The side and head pieces lining a door or window opening. The side jamb that holds the hinges is the hinge jamb; the other side is the strike jamb.
JoistCarpentry
One of a series of parallel framing members that support a floor or ceiling.
Junction boxElectrical
An enclosure that houses wire splices. Splices must be inside such a box, not loose in a wall.

K

Kiln-driedCarpentry
Lumber dried in a kiln with artificial heat to a controlled moisture content.
King studCarpentry
The full-length stud on each side of a door or window opening. See also Jack stud.

L

Level and plumbCarpentry
Level means truly horizontal; plumb means exactly vertical. A level is also the tool used to check both.
Limit switchHVAC
A furnace safety switch that shuts the furnace down if it senses overheating. A clogged filter that starves airflow is a common trigger.
LoadElectrical
Anything on a circuit that draws power when turned on: a lamp, an appliance, a motor.

M

Main breakerElectrical
The breaker that serves as the home's main service disconnect, shutting off power to everything downstream of it. Code requires a way to disconnect both incoming hot conductors.
MERVHVAC
Minimum Efficiency Reporting Value: a rating of how well a filter captures particles between 0.3 and 10 microns. The higher the number, the more it traps; MERV 13 and above must remove at least half of the smallest test particles.
Meter base (meter socket)Electrical
The box with jaws and connectors on the house into which the utility's meter plugs.
MiterCarpentry
A joint between two pieces cut at an angle that splits the corner angle evenly, as at the corners of door casing.
Moisture contentCarpentry
The amount of water in wood, expressed as a percentage of the wood's oven-dry weight. Interior woodwork in most of the country settles at about 8 percent. See also Equilibrium moisture content.

N

NeutralElectrical
The grounded center wire of 120/240-volt service. Code requires it to be connected to the earth. Current returns to the panel on it. See also Hot wire.
NM cable (nonmetallic-sheathed cable)Electrical
Factory-made cable of two or more insulated wires inside a moisture-resistant, flame-retardant nonmetallic sheath; the common house wiring cable.
Nominal sizeCarpentry
The size by which lumber is known and sold, which differs from its actual measured size: a 2x4 is not 2 inches by 4 inches.

O

OhmElectrical
The unit of electrical resistance. One volt across one ohm drives one ampere.
On center (o.c.)Carpentry
Spacing measured from the center of one framing member to the center of the next; wall studs are commonly 16 inches on center.
OSB (oriented strand board)Carpentry
A wood panel made of strand-type flakes deliberately aligned to make it stronger, often used in place of plywood for sheathing and subfloor.
OverloadElectrical
Running wiring or equipment above its rated capacity long enough to cause damage or dangerous overheating. A circuit's lights and appliances regularly demanding more current than it can safely deliver.

P

Packing nutPlumbing
The nut that tightens packing material around a faucet or valve stem to stop water leaking out at the handle. A leak at the handle is a packing problem; a drip at the spout is a washer or seat problem.
PEXPlumbing
Flexible water-supply tubing made of cross-linked high-density polyethylene, approved for hot and cold drinking water.
PlenumHVAC
The duct box attached to the furnace or air handler. The supply plenum feeds conditioned air to the ducts; the return plenum collects the air coming back.
PlywoodCarpentry
A glued wood panel of thin veneer layers with the grain of each layer at right angles to the next.
Pressure-balance valvePlumbing
A shower valve that senses the hot and cold supply pressures and compensates when one changes, so the shower does not scald when a toilet flushes. ASSE 1016 calls it a Type P valve. See also Thermostatic valve, Rotational limit stop.
Pressure switchHVAC
A furnace safety switch that responds to the suction created by the inducer fan, proving the inducer is working before ignition proceeds.
P-trapPlumbing
The curved section of drain under a sink or tub that stays full of water to keep sewer gas out of the house. The plumbing codes require the water seal to be 2 to 4 inches deep. See also Trap seal, Trap arm.

R

RafterCarpentry
One of a series of sloped framing members that support the roof.
RefrigerantHVAC
The working fluid in air conditioners and heat pumps that carries heat as it changes between liquid and gas. Federal rules require anyone who services equipment that could release it to be EPA Section 608 certified.
RegisterHVAC
The grille in a wall, floor or ceiling where conditioned air blows into a room.
Return airHVAC
House air pulled back through the return ducts and the filter to the air handler to be heated or cooled again.
RevealCarpentry
In trim work, the narrow strip of jamb edge left showing when the casing is set back from it; one trim guide marks it 3/16 inch from the jamb edge.
Reversing valveHVAC
The valve near a heat pump's compressor that flips the direction of refrigerant flow between heating and cooling, and to defrost the outdoor coil in winter.
Ridge boardCarpentry
The board at the peak of the roof to which the upper ends of the rafters are fastened.
Rotational limit stopPlumbing
The adjustable stop in a single-handle shower valve that limits how far the handle can turn toward hot, capping the maximum temperature. ASSE 1016 requires valves to have one capable of holding the outlet to 120 F; it is set at installation and may need re-setting.

S

Seat (valve seat, bibb seat)Plumbing
The smooth brass (or rubber) ring inside a faucet body that the washer or stem seals against. Friction gouges it over time, so a new washer may not seal on an old seat; replaceable seats come out with a seat wrench. See also Bibb washer.
SEER2HVAC
A cooling efficiency rating: the heat removed over the cooling season, in Btu, divided by the electricity used, under DOE's 2023 test procedure. Higher is better, and SEER2 numbers are not directly comparable to older SEER figures.
Service dropElectrical
The overhead utility wires running from the last pole to the house, where they connect to the service-entrance wires at the weatherhead. An underground service is called a service lateral.
Service panelElectrical
The box where incoming power enters the home's wiring and is divided into branch circuits protected by breakers or fuses. CPSC: do not remove the panel cover; that is a job for a qualified electrician.
SheathingCarpentry
The structural covering, usually plywood or OSB panels, fastened over the studs, joists or rafters.
ShimCarpentry
A small, usually wedge-shaped piece of wood slipped behind a framing member or door jamb to force it into position.
Short circuitElectrical
Electric current taking a shorter or different path than intended. The sudden surge trips a breaker's magnetic (instantaneous) element.
Shutoff valve (stop)Plumbing
A valve that turns off water to one fixture without shutting off the rest of the house: the small valves under a sink or behind a toilet.
Sill plate (mudsill)Carpentry
The bottom horizontal framing member of an exterior wall that rests on top of the foundation and is anchored to it.
Sole plate (bottom plate)Carpentry
The bottom horizontal board of a wall, attached to the subfloor, on which the studs stand.
StemPlumbing
The part inside a faucet or valve that moves when you turn the handle to start, stop and regulate the flow. On a compression faucet, its threads lift the washer off the seat. See also Bibb washer, Seat, Broach.
Strike plateCarpentry
The metal plate on the door jamb that the latch or deadbolt engages.
StudCarpentry
One of the repeating vertical framing members in a wall, running from the sole plate to the top plate. See also On center.
SubfloorCarpentry
The structural deck, usually wood panels, laid over the floor joists; the finish floor goes on top of it.
SubpanelElectrical
An additional breaker panel, typically fed by a double-pole breaker in the main panel.
Sump pumpPlumbing
A pump set in a pit below the basement floor that sends groundwater away from the house through a discharge pipe.
Supply linePlumbing
The flexible connector that carries water from the shutoff valve to a faucet, toilet or appliance.
Surge protectorElectrical
A device that protects equipment from sudden unwanted increases in voltage. Plug-in units protect only what is plugged into them; whole-home units protect the whole system at the panel.

T

Tamper-resistant receptacleElectrical
An outlet with built-in shutters that open only when a two-blade or grounded plug is inserted, blocking objects pushed into one slot. Required by the NEC since 2008.
ThermostatHVAC
The control that monitors indoor temperature and signals the heating or cooling equipment to hold the set point. A furnace cycle begins when its heat contacts close.
Thermostatic valvePlumbing
A shower valve that senses the mixed water temperature at the outlet and adjusts to hold it steady, so it also corrects for seasonal changes in incoming water temperature. ASSE 1016 Type T. See also Pressure-balance valve.
Three-way switchElectrical
A switch used in pairs so a light can be controlled from two locations.
Ton (cooling)HVAC
A unit of cooling capacity equal to 12,000 BTU per hour.
Top plateCarpentry
The top horizontal member of a framed wall, supporting the ceiling joists or rafters. Code calls for wood stud walls to be capped with a double top plate, with the splices in the two layers offset at least 24 inches.
TPR (temperature and pressure relief) valvePlumbing
The water heater safety valve that opens automatically to release excess heat and pressure. Code requires its discharge pipe to drain by gravity, with no traps, sags, valves or tees, ending no more than 6 inches above the floor or a drain.
Trap armPlumbing
The section of fixture drain between the trap and its vent. The codes cap its length by pipe size. See also P-trap.
Trap sealPlumbing
The depth of water held in a trap, measured from the crown weir (where water spills over) down to the top of the dip. The codes require 2 to 4 inches. See also P-trap.
TrimCarpentry
Finish materials such as moldings applied around openings and where walls meet floors and ceilings.
TrussCarpentry
An assembly of members joined into a rigid framework of triangles, commonly used to support a roof.

V

Vent stackPlumbing
A vertical vent pipe that lets air circulate to and from the drainage system, protecting trap seals from siphoning and back pressure. See also Drain-waste-vent.
VoltElectrical
The unit of electric potential; voltage is the difference in potential between two conductors or between a conductor and ground.

W

Water hammerPlumbing
The bang in the pipes when moving water is suddenly stopped by a closing valve, such as a washing machine's solenoid.
WattElectrical
The unit of electrical power: one ampere flowing under a pressure of one volt.
Wax ringPlumbing
The seal between the base of a toilet and the floor flange that keeps sewer gas and water from leaking out.
Wire gauge (AWG)Electrical
A number describing wire size. The lower the gauge number, the heavier the wire and the more current it can safely carry.

Z

ZoningHVAC
A setup that uses duct dampers and separate thermostats to heat or cool different areas of the house independently.

Sources for the definitions

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Every fact and number on this page comes from one of the sources above, read during the writing of this page. Codes and standards vary by jurisdiction and edition; your local code and your equipment's manual control.