Plumbing Ontology & Knowledge Base

Professional plumbing requires fluency in specialized terminology, component classifications, and material specifications. This comprehensive reference provides the structured vocabulary and technical definitions necessary for effective communication, accurate planning, and code-compliant installations.

Understanding plumbing ontology enables homeowners to read code requirements, communicate effectively with suppliers and professionals, and interpret manufacturer specifications. This knowledge base complements the practical guidance in our Technical Deep-Dive and Common Challenges sections.

General Plumbing Terminology

System Components

Fixture

A device that receives water and/or discharges waste, such as sinks, toilets, showers, and washing machines.

Fixture Unit (FU)

A measure of water supply or drainage load representing the probable discharge from a fixture. Used for pipe sizing calculations.

Branch

A pipe that connects fixtures to a main line. Supply branches deliver water to fixtures; drainage branches collect waste from fixtures.

Riser

A vertical pipe that extends through multiple stories of a building, supplying water or collecting drainage.

Stack

A main vertical drainage pipe, typically including soil (toilet waste), waste (other fixtures), and vent functions.

Service Line

The pipe connecting a building's plumbing system to the municipal water main or private water source.

Manifold

A distribution device with multiple outlets, commonly used in PEX systems to provide home-run connections to fixtures.

Measurements and Specifications

Nominal Size

The named size of pipe or fittings, which may differ from actual dimensions. For example, 1/2-inch copper pipe has an actual outside diameter of 5/8 inch.

IPS (Iron Pipe Size)

A sizing standard based on iron pipe dimensions, used for threaded steel and brass fittings.

CTS (Copper Tube Size)

A sizing standard based on copper tube dimensions, used for copper, CPVC, and PEX tubing.

Schedule

A wall thickness designation for PVC and steel pipe. Higher schedule numbers indicate thicker walls. Schedule 40 is standard for residential drainage; Schedule 80 for high-pressure applications.

SDR (Standard Dimension Ratio)

The ratio of pipe diameter to wall thickness. Lower SDR indicates thicker walls relative to diameter.

Pipe and Tubing Materials

Supply Materials

Copper (Types K, L, M)

Rigid copper tubing in three wall thicknesses. Type K (thickest) for underground service; Type L (medium) for general supply; Type M (thinnest) for above-ground supply only.

PEX (Cross-Linked Polyethylene)

Flexible plastic tubing manufactured through three processes: PEX-A (Engel/cold method), PEX-B (moisture/silane method), and PEX-C (electron beam irradiation). PEX-A offers greatest flexibility and expansion fitting compatibility.

CPVC (Chlorinated Polyvinyl Chloride)

Rigid plastic pipe for hot and cold water distribution. Higher temperature tolerance than PVC; joined with solvent cement.

Galvanized Steel

Steel pipe with zinc coating for corrosion resistance. Obsolete for supply lines due to corrosion and flow restriction; still found in older homes.

Drainage Materials

PVC (Polyvinyl Chloride)

White or cream-colored plastic pipe for drainage, waste, and vent applications. Lightweight, corrosion-resistant, and joined with solvent cement.

ABS (Acrylonitrile Butadiene Styrene)

Black plastic pipe used for drainage in some regions. Similar properties to PVC but cannot be joined directly to PVC without proper transition fittings.

Cast Iron

Traditional drainage material providing superior sound dampening. Hub-and-spigot or no-hub (NH) connections. Still used for main stacks and commercial applications.

Orangeburg (Fiber Conduit)

Bituminized fiber pipe used for drainage from 1860s-1970s. Prone to deformation and failure; replacement recommended where found.

Fitting Types and Terminology

Basic Fitting Shapes

Coupling

A short fitting that joins two straight pipe sections end-to-end.

Union

A fitting that joins pipe sections with a threaded or compression connection that can be disassembled. Used where future disconnection is anticipated.

Elbow (90°, 45°, 22.5°)

A fitting that changes pipe direction. Available in various angles; street elbows have one male and one female end.

Tee

A T-shaped fitting that creates a branch connection. Sanitary tees include a sweep for drainage applications.

Wye

A Y-shaped drainage fitting that creates a branch at 45 degrees, providing better flow characteristics than tees for drainage.

Reducer

A fitting that connects pipes of different diameters, transitioning between sizes.

Cap

A fitting that closes the end of a pipe, used for future expansion or testing.

Plug

A fitting that inserts into a fitting hub to close the opening, as opposed to a cap that fits over pipe ends.

Specialized Fittings

Adapter

A fitting that transitions between different connection types or pipe materials (e.g., male adapter, female adapter, sweat-to-threaded).

Dielectric Union

A fitting that joins dissimilar metals (copper to steel) while preventing galvanic corrosion through an insulating barrier.

Transition Fitting

A specialized adapter for joining specific material combinations, such as PEX-to-copper or PVC-to-cast iron.

Push-to-Connect (SharkBite)

A compression-style fitting that requires no tools for installation. Suitable for repairs and accessible locations; opinions vary on concealed use.

Compression Fitting

A fitting that seals through compression of a ferrule or ring against pipe, requiring no soldering or cement.

Flare Fitting

A fitting that seals against a flared (expanded) pipe end, commonly used for gas lines and high-pressure applications.

Valve Classifications

Shutoff and Control Valves

Gate Valve

A valve that uses a wedge-shaped gate to stop flow. Provides straight-through flow when open but should not be used for throttling. Common as main shutoffs.

Ball Valve

A quarter-turn valve using a spherical ball with a bore through it. Excellent for on/off service; most reliable valve type for residential shutoffs.

Globe Valve

A valve designed for throttling service, using a disc that moves perpendicular to flow. Creates more pressure drop than gate or ball valves.

Butterfly Valve

A quarter-turn valve using a disc that rotates within the flow stream. Compact but creates some flow restriction.

Stop Valve

A compact shutoff valve for fixture supply connections. Angle stops have 90-degree outlet; straight stops align with supply.

Compression Stop

A stop valve that connects to supply pipe via compression fitting, allowing installation without soldering.

Sweat Stop

A stop valve with solder (sweat) connection to copper tubing, providing permanent fixture connections.

Specialized Valves

Check Valve

A valve that allows flow in one direction only, preventing backflow. Swing check, spring check, and silent check varieties.

Pressure Reducing Valve (PRV)

A valve that reduces incoming water pressure to a safe, consistent level. Required when street pressure exceeds 80 PSI.

Pressure Relief Valve (T&P Valve)

A safety valve that releases water when temperature or pressure exceeds safe limits. Required on all water heaters.

Thermostatic Mixing Valve

A valve that blends hot and cold water to deliver water at a safe, consistent temperature, preventing scalding.

Backflow Preventer

A device that prevents contaminated water from flowing backward into the potable water supply. Various types for different hazard levels.

Atmospheric Vacuum Breaker (AVB)

A simple backflow preventer for low-hazard applications, such as irrigation systems. Cannot be subjected to backpressure.

Reduced Pressure Zone (RPZ) Assembly

A complex backflow preventer for high-hazard applications, providing protection against both backsiphonage and backpressure.

Drainage System Terminology

Drainage Components

Trap

A U-shaped pipe section that holds water to prevent sewer gas entry. Required at each fixture.

P-Trap

A trap with a horizontal outlet, shaped like the letter P. Standard for most fixtures.

S-Trap

A trap with a vertical outlet, shaped like the letter S. Prohibited by most modern codes due to siphonage risk.

Trap Arm

The pipe section between the trap weir and the vent connection. Length limited by pipe diameter to prevent siphonage.

Cleanout

An access point in drainage piping for cleaning and inspection. Required at base of stacks and at direction changes.

Fixture Drain

The drain from a fixture's trap to the junction with another drain pipe.

Soil Stack

A vertical drainage pipe carrying discharge from water closets (toilets).

Waste Stack

A vertical drainage pipe carrying discharge from fixtures other than water closets.

Vent Stack

A vertical vent pipe that provides the primary venting for the drainage system.

Venting Terms

Individual Vent

A vent pipe connected to a single fixture drain.

Common Vent

A vent serving two fixture drains connected at the same level.

Wet Vent

A pipe serving as both drain and vent, carrying waste from upper fixtures while venting lower fixtures.

Circuit Vent

A vent that serves multiple fixtures on a horizontal branch, connected between the first two fixtures.

Relief Vent

A vent that provides additional protection for trap seals in high-rise buildings or long horizontal runs.

Studor Vent (AAV)

An Air Admittance Valve that opens under negative pressure to admit air, serving as a mechanical vent.

Vent Terminal

The point where a vent pipe terminates above the roof line.

Industry Standards and Organizations

IPC (International Plumbing Code)

Model plumbing code developed by the International Code Council (ICC), adopted by many U.S. jurisdictions.

UPC (Uniform Plumbing Code)

Model plumbing code developed by the International Association of Plumbing and Mechanical Officials (IAPMO), prevalent in western states.

NSF (National Sanitation Foundation)

Organization that develops standards and certifies products for public health and safety. NSF/ANSI 61 certifies drinking water system components.

ASME (American Society of Mechanical Engineers)

Professional organization developing standards for boilers, pressure vessels, and plumbing components.

ASTM (American Society for Testing and Materials)

Organization developing material specifications and testing standards for pipe, fittings, and joining materials.

CSA (Canadian Standards Association)

Canadian standards organization whose certifications are also recognized in the U.S. for gas and plumbing products.

Conclusion

This ontology provides the structured vocabulary necessary for professional plumbing work. Mastery of these terms enables effective communication, accurate interpretation of codes and specifications, and informed material selection.

For homeowners, this reference supports confident interaction with suppliers, effective research, and professional-level project planning. Combined with the practical guidance in other sections, this knowledge base empowers serious DIYers to achieve results that meet professional standards.