Short definition
A union is a three-piece serviceable fitting that lets a joint be unscrewed for service or replacement without cutting the pipe. Two threaded ends with hex shoulders, joined by a tightening nut that compresses a metal-to-metal seat or gasket between them. Loosening the nut disassembles the joint without disturbing the rest of the run.
What it is
The three pieces: each pipe end is threaded into one of the union’s threaded shoulders, then the central nut threads down to compress the two shoulders against each other. The seat between them — either precision-machined metal-to-metal (a “ground joint” union) or sealed by a gasket — is what holds water or gas tight.
Strategic placement is the value. Unions go wherever future service is anticipated: at water-heater inlet and outlet, at boiler pumps and valves, at gas-appliance connections, and at fixture risers in older galvanized systems.
Why it matters to a homeowner
The reason your plumber can pull and replace a water heater in two hours is unions. Unscrew the cold-inlet union, the hot-outlet union, the gas union if applicable, and the heater comes out without cutting any pipe. Without unions, the same job means cutting and re-soldering or re-threading both supply lines.
When a quote talks about “union at the heater connections” or “boiler-loop unions,” the work scope includes serviceability for next time.
Common variants and what a union is not
- Union vs. coupling. Coupling is fixed (cut to remove); union is serviceable (unscrew to remove).
- Union vs. flange. Flanges are bolted joints used on larger commercial pipe; unions are screw-together on smaller residential pipe.
- Dielectric union — a union with a built-in plastic isolator between the two metal sides. See its own entry.
Common failure modes
- Loose nut allows a slight gap, weeping leak.
- Damaged seat from over-tightening.
- Missing gasket on gasket-style unions.
- Misaligned pipes preventing the nut from threading evenly.