What is a union nut (pipe union)?
A pipe union is a detachable pipe joint made of two halves that are pulled together by a loose, rotating nut: the union nut. In Dutch it is called a “wartel”. Its big advantage is that you can remove a section of pipe, a valve or an appliance without turning the pipe itself.
How does a union work?
A union has a male part and a female part. The union nut turns freely on one half and screws onto the thread of the other, pressing the two mating faces together. The seal is made at those faces, not in the thread:
- Flat-faced: with a flat washer of fibre, rubber or another gasket material.
- Conical (metal to metal): a tapered or spherical seat that fits tightly into its counterpart, as on many radiator valve tails.
Where is it used?
- Radiators: the radiator valve and lockshield are connected to the radiator with unions, so the radiator can be taken off the wall for decorating.
- Water and gas meters: the meter sits between two unions and can be swapped without cutting pipe.
- Pumps, cylinders and filters: unions make replacement quick.
Fitting tips and common mistakes
- No sealant on the union nut thread. PTFE tape or hemp there does nothing, because the thread does not seal. Use it only on the fixed thread that screws into a valve or fitting.
- Always hold back with a second spanner, or you will twist the pipe or valve.
- Fit a new washer every time you undo a flat-faced union.
- Do not overtighten: too much force damages the seat or washer and causes the leak you wanted to avoid.
- Work on gas pipework should be left to a registered gas engineer.
Related terms
- Compression fitting
- Radiator
- Expansion vessel
- Pressure pipe
- Water hammer
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