Plumbing Jargon Explained: A Homeowner's Guide to Technical Terms

Plumbing and drainage contractors sometimes use technical terms that leave homeowners nodding politely while having no idea what’s been said. This glossary covers the most commonly used terms in residential plumbing and drainage.

Drainage terms

Benching: The sloped concrete or mortar surfaces at the base of an inspection chamber, around the central channel. Benching directs flow into the channel and provides a non-slip working surface for engineers accessing the chamber.

CCTV survey: A Closed-Circuit Television survey — a camera is inserted into the drain to record condition internally. The camera travels along the pipe, and the footage is used to assess the condition of the drain.

CIPP (Cured-in-Place Pipe): A method of repairing a drain from the inside without excavation. A resin-saturated liner is inserted into the pipe and cured (hardened) in place, forming a new pipe within the old one.

Drain: A pipe carrying wastewater from a single property to the sewer. Your private responsibility until it connects to the public sewer.

Exfiltration: Sewage leaking outward from a damaged or cracked pipe. Contamination risk. Shown on CCTV as wet soil visible through a crack.

FOG: Fat, Oil and Grease — the collective term for cooking waste that causes the majority of kitchen drain and sewer blockages.

Gully: An outdoor drain inlet, typically a square grate at ground level, that receives surface water and wastewater from external sources (downpipes, washing machine overflows).

Infiltration: Groundwater entering a drain through cracks or open joints. Appears on CCTV as water ingress from outside the pipe.

Inspection chamber / manhole: An access point to the underground drain, allowing inspection and clearance. Usually covered by a metal or plastic cover at ground level.

Invert: The bottom of the internal bore of a pipe. “Invert level” is the depth of the bottom of the pipe below the ground surface — a critical measurement for drainage design and for assessing how deep excavation needs to go.

Jetting / high-pressure water jetting (HPWJ): Clearing drains using a high-pressure water jet. A self-propelling nozzle is fed into the drain and water at 2,000–10,000 psi cuts through blockages and cleans the pipe walls.

Root ingress: Tree or plant roots that have grown into the drain through cracked or open joints. One of the most common causes of blockages in older clay pipe drains.

Sewer: The main underground pipe that collects waste from multiple properties. Public sewers are the responsibility of the water company; private sewers (shared between properties) were mostly transferred to water company ownership in 2011.

Soil pipe / soil stack: The large-diameter vertical pipe (usually 100mm) that carries toilet waste from the building to the underground drain. The soil stack typically runs internally and vents above the roof level.

Soakaway: An underground pit or drainage field that allows water to infiltrate into the surrounding soil. Used for surface water run-off or for septic tank effluent.

WinCan: Industry-standard software used to produce structured CCTV drain survey reports. A WinCan-compliant report uses standardised defect codes and formats accepted by insurers, lenders and local authorities.

Central heating and plumbing terms

AAV (Air Admittance Valve): A mechanical valve fitted at the top of a soil pipe run that allows air in but not out. Used to vent drainage systems without extending a pipe through the roof.

Combi boiler: A combination boiler that provides both central heating and domestic hot water on demand from a single unit.

Diverter valve: On a combi boiler, the internal valve that switches water between the heating circuit and the domestic hot water circuit.

Expansion vessel: A pressurised vessel within (or adjacent to) a sealed central heating system. Contains a rubber diaphragm that separates water from pressurised nitrogen, accommodating the expansion of water as it heats.

Filling loop: A flexible braided hose connecting the cold mains supply to the sealed heating circuit, used to repressurise the system.

Inhibitor: A chemical additive to central heating water that prevents corrosion and scale formation. Should be maintained at the correct concentration.

Lockshield valve: The valve on the return side of a radiator (opposite side from the TRV). Set at commissioning to balance flow through the system. Requires a special key to adjust.

Magnetite: Black iron oxide sludge that forms in central heating systems from corrosion of steel components. Settles in radiators and blocks flow.

Powerflush: The process of forcing clean water and chemical agents through the central heating system at high flow to remove sludge and scale.

PRV (Pressure Relief Valve): A safety valve that opens to release pressure if the boiler or system pressure exceeds a safe level. Normally discharges via a pipe to an outside drain.

TRV (Thermostatic Radiator Valve): A self-regulating valve fitted to the flow side of a radiator that limits the radiator’s heat output based on room temperature.

Zone valve: A motorised valve that opens and closes to control flow to a specific zone of the heating system (e.g., upstairs radiators, hot water cylinder). Controlled by the programmer or thermostat.

General terms

Bar: The unit of pressure used for water supply and boiler pressure. 1 bar ≈ 10 metres of water head. Normal domestic supply is 1–3 bar; normal combi boiler pressure (cold) is 1–1.5 bar.

Flow rate (litres per minute): The volume of water that passes a point in the pipe per minute. Relevant to shower performance, hot water delivery rate, and drain self-cleansing.

Gradient / fall: The downward slope of a pipe, expressed as a ratio (e.g., 1:40 means 1mm fall per 40mm of horizontal distance). Drainage pipes need sufficient gradient to carry waste without leaving deposits.

Part H: The section of UK Building Regulations covering drainage and waste disposal.

Stop tap / stopcock: A valve on the mains cold water supply that can be turned to isolate the supply to the property.