Plumbing Dictionary: Common Terms Explained

A plumber talks in parts and fittings, and an estimate reads the same way. This page translates the words you are most likely to meet: on a written estimate, in a sewer camera report, in a home inspection, or on the phone when you describe what is going wrong. It is a dictionary, not a repair manual. When a term below describes your house, the next step is still a call.

The terms, A to Z

Access panel

A removable panel in a wall or ceiling that lets a plumber reach a valve, a tub drain or a shut-off without opening the wall.

Anode rod

A metal rod inside a tank water heater that corrodes on purpose so the tank does not. Once it is used up, the tank starts to rust from the inside.

Backflow preventer

A valve that keeps water from flowing backward into the clean supply, for example from an irrigation system or a boiler. Many towns require one and an annual test.

Backwater valve

A one-way valve on the main drain that stops sewage from coming back up into the house when the city sewer surcharges in a storm.

Ball valve

A shut-off valve with a lever handle. A quarter turn closes it fully, which is why plumbers prefer it for main shut-offs.

Cleanout

A capped fitting on a drain or sewer line that gives a plumber access to clear a clog or run a camera without cutting pipe.

Closet flange

The ring that anchors a toilet to the floor and connects it to the drain. A cracked or corroded flange is a common reason a toilet rocks or leaks at the base.

Compression fitting

A fitting that seals by squeezing a small ring onto the pipe as the nut tightens. Common under sinks and behind toilets.

CPVC

A rigid plastic supply pipe rated for hot water, common in homes built from the 1990s on. It turns brittle with age.

Cross-connection

Any place where clean water and a contaminated source could meet, like a hose left sitting in a bucket. Backflow devices exist to guard them.

Dielectric union

A fitting that joins copper to steel while keeping the two metals apart, so they do not corrode each other.

Drain, waste and vent (DWV)

The system that carries waste out of the house and lets air in behind it, so drains flow and traps keep their water seal.

Expansion tank

A small tank near the water heater that absorbs the pressure created as water heats and expands. Required in many homes with a closed supply system.

Fill valve

The part inside a toilet tank that refills it after a flush. A worn one causes constant running or slow filling.

First-hour rating (FHR)

How many gallons of hot water a tank water heater can deliver in its first hour of heavy use. The number to match to your busiest hour of the day.

Flapper

The rubber seal at the bottom of a toilet tank. When it stops sealing, the toilet runs between flushes.

Flow rate (GPM)

Gallons per minute. Used to size tankless water heaters and to describe faucets and showerheads.

Frost-proof sillcock

An outdoor faucet built with its shut-off deep inside the warm wall, so the exposed part holds no water in winter.

Gate valve

An older style of shut-off with a round handle that turns several times. It tends to stick or fail to close fully with age.

Grease trap

A device that catches fats and grease before they reach the drain line. Standard in commercial kitchens, occasional in homes.

Hard water

Water with a high mineral content. It leaves scale in water heaters, fixtures and pipes and shortens appliance life.

Hose bib

The outdoor faucet a garden hose connects to. Also called a spigot or a sillcock.

Hydro jetting

Clearing a drain or sewer line with high-pressure water instead of a cable. Used for grease, scale and roots.

Lateral (sewer lateral)

The underground pipe that carries waste from the house to the public sewer main. The homeowner usually owns it to the property line or to the main, depending on the town.

Main shut-off valve

The valve that stops all water coming into the house. Usually where the supply enters, near the meter or in the basement.

Mixing valve

A valve that blends hot and cold water to a set temperature, so the tank can run hot for capacity while the taps stay safe.

P-trap

The U-shaped bend under a sink or in a floor drain. The water it holds blocks sewer gas from coming up the drain.

PEX

Flexible plastic supply pipe, now the usual choice for repipes and new homes. Joined with crimp, clamp or expansion fittings.

Pressure-reducing valve (PRV)

A valve on the main supply that lowers street pressure to a safe level for the house. A failing PRV shows up as pressure that swings or creeps up.

PSI

Pounds per square inch, the unit of water pressure. Most homes sit comfortably between roughly 40 and 60.

Recirculation pump

A pump that keeps hot water moving through the pipes so it arrives at the tap quickly.

Recovery rate

How fast a water heater reheats a tank after it has been drawn down.

Repipe

Replacing most or all of a home’s supply pipes, usually because of repeated leaks in old galvanized steel or polybutylene.

Sanitary tee

A drain fitting shaped so waste enters a vertical line smoothly. The wrong tee in the wrong place is a classic cause of a slow drain.

Scale

The mineral crust hard water leaves behind. In a water heater it insulates the burner from the water and causes rumbling.

Sediment

Loose mineral grit that settles at the bottom of a water heater tank. Left alone it shortens the tank’s life.

Sewer camera inspection

Running a camera through the lateral to see cracks, roots, offsets or a low spot in the line before deciding on a repair.

Slab leak

A leak in a supply or drain line that runs under a concrete slab foundation. Signs include a warm spot on the floor, a jump in the water bill or the sound of running water with everything off.

Snake (auger)

A cable tool that breaks up or pulls out a clog. Sized for sinks, toilets or main lines.

Sump pump

A pump in a pit in the basement or crawlspace that pushes groundwater away from the foundation. It fails most often in the storm it was bought for.

Supply line

The short flexible line that connects a fixture, like a toilet or faucet, to its shut-off valve.

Tankless water heater

A unit that heats water on demand as it passes through, instead of storing it. Sized by flow rate and the temperature rise it has to deliver.

Temperature and pressure relief valve (T&P valve)

The safety valve on a water heater that opens if the tank gets too hot or the pressure too high. Dripping from it is a warning, not a nuisance.

Thermal expansion

Water grows in volume as it heats. In a closed plumbing system that pressure has to go somewhere, which is what an expansion tank is for.

Thermocouple

A safety sensor on a gas water heater that shuts off the gas if the pilot light goes out.

Trap seal

The water sitting in a P-trap. A dry trap, from a drain that is rarely used, lets sewer gas into the room.

Vent stack

The vertical pipe that carries sewer gas up through the roof and lets air into the drain system. A blocked vent causes gurgling and slow drains.

Water hammer

The bang in the pipes when a valve closes fast and moving water stops suddenly. Arrestors and secure strapping quiet it.

Water heater (tank)

The common storage water heater, gas or electric, in sizes from about 30 to 80 gallons. Typical service life is around ten years, less with hard water.

Water main

The public pipe under the street that supplies the neighborhood. The utility owns it; the line from it to your house is the service line.

Water softener

A unit that removes hardness minerals by ion exchange, protecting the water heater and fixtures from scale.

Wax ring

The wax seal between the toilet and the closet flange. When it fails, water seeps out at the base with every flush.

Well pump

The pump that draws water from a private well. Pressure that drops or cycles is often a pressure tank or switch problem before it is the pump.

When a word on the estimate worries you

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