Plumbing Terms in Plain Words

Plumbers name parts fast: a P-trap, a dip tube, a flapper, a cleanout. Knowing what each part does makes a repair easier to follow, and it helps when you describe a problem on the phone. “Water around the base of the toilet” and “a failed wax ring” often point to the same fix.

A house’s plumbing comes in four parts worth telling apart: the supply side, which brings water in under pressure; the drain side, which carries it out by gravity through traps and vents; the water heater and its valves; and the working parts inside toilets and faucets.

Questions that need more than a definition, such as whether a job needs a permit or when to flush a water heater, have longer answers.

Terms and Plain Meanings, A–Z

ABS

Black plastic pipe for drains and vents, joined with solvent cement. It does the same job as white PVC; where the two meet, the joint needs a fitting made for the change.

Aerator

The small screen in the tip of a faucet. It mixes air into the stream; when grit or scale clogs it, the flow drops at that one faucet.

Angle stop

The small shutoff valve under a sink or behind a toilet, where the supply line connects. Closing it stops the water to that fixture only, so the rest of the house keeps running.

Anode rod

A metal rod inside a tank water heater that corrodes in place of the tank; once it is used up, the tank starts to rust. A rotten-egg smell in the hot water alone can start at the rod.

Backflow preventer

A valve assembly that lets water flow only into the building. It stops water from a hose, a boiler or a sprinkler system from being pulled back into the drinking supply when the pressure drops.

Ball valve

A shutoff valve that goes from open to closed in a quarter turn. The handle lies along the pipe when the valve is open and across it when it is closed.

Cast iron

Heavy black pipe used for drains and the main stack in older houses. After many decades it can rust through, crack along the bottom or scale up inside until the drains slow down.

Check valve

A valve that lets water move one way only. On a sump pump’s discharge pipe, it keeps the water in the pipe from falling back into the pit when the pump stops.

Cleanout

A capped opening in a drain or sewer line that gives a drain cable a way in, often at the base of the main stack, in the basement floor or outside near the foundation.

Compression fitting

A fitting that seals when its nut squeezes a brass ring, the ferrule, onto the pipe. It is common at angle stops and needs no heat or glue, but overtightening can crack the ring.

Copper pipe

Rigid orange-brown pipe for water lines, joined by soldering or with press fittings. It lasts for decades, though corrosive water can pit it from the inside and cause pinhole leaks.

CPVC

Cream or tan plastic pipe for hot and cold water lines, joined with solvent cement. It turns brittle with age, so old CPVC can crack when it is bent or bumped during a repair.

Dielectric union

A fitting that joins two different metals, such as steel and copper, with an insulating washer between them. Without one, the joint corrodes fast, often at old water heater connections.

Dip tube

A plastic tube that carries incoming cold water to the bottom of a tank water heater. When it breaks, cold water mixes in at the top of the tank, and the hot water runs out much too soon.
Related: Water Heaters

Drain-waste-vent (DWV) system

Every pipe that carries used water out of a building, plus the vents that let air in: the fixture drains, the main stack, the roof vent and the main drain to the sewer.

Escutcheon

The round trim plate that covers the hole where a pipe or valve stem comes through a wall or floor, such as behind a toilet’s shutoff valve.

Expansion tank

A small tank on the cold line by the water heater, with an air cushion inside. Heated water expands, and the tank takes up the extra when a check valve or pressure-reducing valve keeps it from flowing back toward the street.

Fill valve

The valve inside a toilet tank that refills it after a flush and shuts off as the float rises. A worn one hisses or keeps running after the tank is full.

Flapper

The rubber seal at the bottom of a toilet tank that lifts for a flush and drops to hold the water in. A worn one lets water seep into the bowl, so the toilet refills by itself.
Related: For Homeowners

Floor drain

A drain in a basement floor, often near the water heater or the laundry. Its trap needs water in it; when the trap dries out, sewer gas can come up through the grate.

Frost-free sillcock

An outside faucet with a long stem that puts the shutoff inside the heated wall. A hose left on over the winter still holds water in the stem, which can freeze and split inside the wall.

Galvanized steel

Gray steel pipe with a zinc coating and threaded joints, found on water lines in many older houses. It rusts from the inside, narrowing the pipe, lowering the pressure and tinting the water brown.
Related: Pipe Repair

Main shutoff

The valve that closes the water to the whole building, usually where the water line comes in, by the meter. In a leak it is the first thing to close, so know where it is and that it turns.
Related: Burst Pipes

P-trap

The U-shaped bend under a sink, tub or shower drain. The water it holds blocks sewer gas, and the bend catches rings and other small things before they travel down the line.
Related: Drain Cleaning

PEX

Flexible plastic tubing for hot and cold water lines, often red, blue or white. It bends around corners with few fittings, so it suits repiping and remodels. Sunlight breaks it down.
Related: Pipe Materials

Pressure-reducing valve

A bell-shaped valve on the main water line that lowers high street pressure to a level easier on pipes, faucets and appliances. When it fails, the pressure can creep up or drop off.

PVC

White plastic pipe for drains, vents and sump pump discharge lines, joined with primer and solvent cement. It is not made for hot water lines.

Rough-in

The stage of a job when pipes, drains and valve bodies go in while the walls and floors are open, before drywall, tile and fixtures. Where a permit applies, the rough-in is usually inspected first.

Sediment

Minerals and grit that settle at the bottom of a tank water heater. A thick layer makes the tank rumble as it heats, slows the heating and can clog the drain valve. Flushing the tank clears it.

Sewer lateral

The pipe that carries a building’s wastewater from the foundation to the sewer main under the street. Roots, sags and broken joints in it cause backups that show first in the lowest drains.
Related: Drain Cleaning

Sump pit

The lined hole in a basement floor where groundwater from around the foundation collects. The sump pump in it starts on a float switch as the water rises and pushes the water out through a discharge pipe, away from the house.
Related: Seasonal Care

Supply line

Any pipe or hose that carries water under pressure to a fixture, such as the short flexible lines between a sink’s shutoff valves and its faucet. An old or kinked one can split without warning.

Sweat joint

A soldered joint on copper pipe: the pipe and the fitting are cleaned, coated with flux and heated until solder flows into the gap. The pipe has to be dry for the joint to take.

T&P relief valve

The temperature and pressure relief valve on a tank water heater. It opens if the water gets too hot or the pressure too high, sending water down its discharge pipe. One that drips means the valve has failed or the tank is building too much heat or pressure.
Related: Our Process

Vent stack

The pipe that runs from the drain lines up through the roof. It lets air into the drains, so water flows and traps are not siphoned dry; a blocked vent makes drains slow, gurgle or smell.

Water hammer

A bang in the pipes when a valve shuts fast, as a washing machine valve does. High pressure, loose pipe straps or a missing arrestor (a small shock absorber) cause it, and the shock can loosen joints.

Water meter

The meter the water utility reads to bill for water, usually near the main shutoff. With every faucet in the house closed, its small leak dial should stay still.

Wax ring

The ring of wax that seals a toilet to the drain flange in the floor. When it fails, water or sewer gas escapes at the base of the toilet; a toilet that rocks can break the seal.