What the words for the parts actually mean
Every term is a link: an address with # sends someone straight to one specific definition.
Connectors and adapters
Union (detachable coupling)
A joint with a swivel nut: lets you disconnect it without rotating either pipe. Put one on anything you'll ever service — pumps, boilers, meters, filters. It seals on a flat gasket or a metal-to-metal cone, and in both cases the swivel nut's thread is never wrapped.
Male / female (M / F)
Male thread is on the outside of a part, female — inside. A joint is always M into F. Catalogue shorthand: F/F, M/F, M/M — the two ends of the fitting.
Nipple
A short piece with male thread on both ends (M/M). Joins two female-threaded parts — say, a valve straight into a filter.
Barrel nipple
A nipple stretched into a barrel: male threads at both ends and clean pipe between. Bridges a gap between two female-threaded parts a plain nipple can't reach across.
Long-thread connector (running nipple)
A pipe piece with a short thread at one end and a long one at the other, carrying a coupling and a locknut. The long thread lets you wind the coupling fully onto it and break the joint without turning any pipe — the classic way into a steel riser where there's no union.
Locknut
The nut that backs up the coupling on a long-thread connector and, together with the sealant, holds the joint. It goes tight last, once the coupling has landed where it belongs.
Extension piece
M on one end, F on the other, various lengths. Brings a thread out of a wall or moves a valve away from an obstacle without extra joints.
Reducing bush
Screws inside a larger female thread and offers a smaller female one: 1″ → 3/4″, 3/4″ → 1/2″. The fastest way down a size without a coupling.
Coupling
A sleeve with female thread on both ends (F/F): joins two male ends. The reducing version has two different sizes.
Adapter
The umbrella word for parts that change size, thread gender or material: F/M reducers, plastic-to-brass transitions, PEX-to-thread fittings.
Tee
Splits one line into two. Named by its three ends in order: “tee 1″×1″×3/4″” — run × run × branch. Ends can differ in size and gender.
Eccentric connector
An adapter with offset axes: it screws into the thread coming out of the wall and shifts the outlet by a couple of centimetres. Fitted in pairs under a tap when the wall outlets don't match the standard 150 mm centres.
Flexible hose
A rubber hose in a braided sleeve with a swivel nut. Handy for taps, cisterns and appliances, but it's a consumable: its life is limited, and it gets replaced during servicing rather than when it bursts.
Valves and protection
Full bore
A valve whose open passage matches the pipe bore — no narrowing. For main shut-offs always take full bore; standard-bore valves choke the line.
Check valve
Lets water flow one way only. Standard after the meter, on boiler feeds and pump outlets — so heated or metered water can't push back.
Pressure reducer (PRV)
Drops unstable street pressure to a set value and holds it. Protects appliances from spikes; installed after the strainer, gauge port on top.
Strainer (Y-filter)
A coarse mesh in an angled housing that catches sand and scale before meters, reducers and appliances. The cap unscrews for cleaning — mount it cap down.
Fine filter
A housing with a replaceable cartridge downstream of the strainer: it stops what got through the mesh. The cartridge must be changed on schedule, or the filter itself becomes the reason your pressure dropped.
Air vent and radiator bleed valve
An automatic air vent sits at the high points of a system and bleeds air by itself. A radiator bleed valve is its manual counterpart on the radiator: with air trapped inside, the top of the panel stays cold.
Safety group
On a boiler — a block of pressure relief valve, air vent and gauge on the flow, right after the boiler. On a water heater — its own block of relief and check valves on the cold-water inlet. In both cases there must be no valve of any kind between the group and the appliance.
Assemblies and layouts
Manifold
A comb with one inlet and several valved outlets: each fixture or heating loop gets its own line. More pipe, but any branch can be shut off without touching the rest.
Bypass
A parallel section around a device (pump, meter, radiator) with its own valve. Lets the system run while the device is serviced or removed.
Tee and manifold layouts
Tee layout — one main run with fixtures teed off it along the way. Manifold layout — a separate pipe from the comb to every fixture. Which suits where — on the water inlet page.
Flow and return
Flow is the pipe carrying hot water from the boiler to the emitters; return brings the cooled water back. They can't be swapped: pumps, valves and heat exchangers all have a direction.
Expansion vessel and accumulator
A tank with a rubber diaphragm and air behind it. In heating it absorbs the expansion of hot water; on a well supply it smooths out pump starts; on a water heater it stops the relief valve dripping. Check the air pressure at least once a year (on a well accumulator — every six months), with the water side depressurised.
Installation and tools
Riser
The vertical pipe running through every floor. It's communal property: your side starts after the first isolating valve inside the flat, and a leak on the riser itself is the emergency service's job, not yours.
Compression fitting
A joint where a swivel nut squeezes a split ring around the pipe. It goes together with two spanners, comes apart and can be re-tightened — which is why it never goes into a wall: press or compression.
Press fitting
A sleeve crimped permanently with press jaws. The joint can't be undone, but it can be buried. The jaw profile (TH, U, H) must match the fitting's.
PTFE tape
Thin PTFE tape for sealing threads. Wound with the thread under tension, usually 6–10 turns — more for thin tape and larger threads. It doesn't tolerate backing off: undo the joint and the tape lifts, so the wrap gets redone from scratch.
Calibrator, facing tool, shaver
A calibrator restores the roundness of a multilayer cut and chamfers it. A facing tool and a shaver remove the aluminium layer of reinforced PP-R before welding — mid-wall and outer respectively. What to use when is covered under PP-R and multilayer.
Procedures and phenomena
Pressure testing
Testing the finished system above working pressure before it disappears into the walls. The standard calls for half an hour at 1.5× working pressure; before a screed installers often leave it for 24 hours. Watch the gauge: a falling needle means a leak somewhere, even if nothing is dripping yet.
Water hammer
A pressure spike when flow stops abruptly — a washing machine valve snapping shut, for instance. You hear it as a knock in the pipes, and it punishes hoses, diaphragms and seals far more than steady pressure does.
Common questions
What is a union fitting?
A demountable joint with a swivel nut: it lets you take the joint apart without rotating either pipe. Put one on anything you will ever service — pumps, boilers, meters, filters. The nut thread is never sealed with tape: a flat gasket or a metal-to-metal cone does the sealing.
What is the difference between a nipple, a barrel nipple and a running nipple?
A nipple is a short part with a male thread at both ends, joining two female threads. A barrel nipple is the same thing stretched out, with plain pipe between the threads. A running nipple has one short thread and one long one for a socket and a lock nut, so the joint can be taken apart without turning the pipes.
What is a reducing bush?
An adapter that screws into a larger female thread and gives you a smaller female one: 1″ to 3/4″, 3/4″ to 1/2″. The shortest way to drop a size without a socket.