Pump union gasket sizes —
what to measure before
you order a replacement
A 15 mm, 22 mm or 28 mm pipe size is only a clue. The gasket must fit the recess on the pump union face. The reliable specification is OD, ID and thickness from the old gasket or the union face — not the pipe size written on the fitting.
Why pipe size does not equal gasket size
In domestic central heating, pipework is normally described by copper tube size: 15 mm, 22 mm or 28 mm. A pump union then adapts that pipework to the pump connection, usually through a BSP-threaded union body. That description is useful for identifying the fitting family, but it is not the gasket specification.
The gasket seals on the flat face of the union, usually in a shallow machined recess. That recess has its own outside diameter, inside diameter and depth. Those dimensions come from the union design, not directly from the pipe outside diameter. Two unions used on the same nominal pipe size can therefore need different gaskets.
Common size clues — not a buying specification
The table below is deliberately written as a clue sheet, not a definitive size chart. It helps you recognise the sort of gasket family you may be dealing with, but the final ordering dimensions still come from the old gasket or the union recess.
| Copper pipe size | Common union body thread (examples only) | Common gasket OD x ID clues | Notes |
|---|---|---|---|
| 15 mm | 1″ BSP | around 33 x 21 mm | Treat as a clue only; older and replacement unions can differ enough to matter |
| 22 mm | 1 1/4 in or 1 1/2 in BSP | around 44 x 30 mm or 50 x 36 mm | Very easy to mis-order if you choose by pipe size alone; measure the face |
| 28 mm | 1 1/2 in or 2 in BSP | around 50 x 36 mm or 57 x 43 mm | Less common domestically; confirm directly before ordering replacements |
These examples are representative only. They are not a standard, not a guarantee and not a substitute for measuring. Pump manufacturers and union suppliers can use different face recesses even where the pipe and thread descriptions look similar.
The union design, not the pipe size, determines the gasket dimension. If the pump is replaced and the unions are changed with it, the old gasket size may no longer be correct. Measure the new union face before assuming the old gasket belongs on the new joint.
What to measure — and in what order
Measure the old gasket first — if it is still intact
If the existing gasket can be removed in one piece, measure its outside diameter (OD), inside diameter (ID) and thickness with a calliper. The OD and ID of the gasket tell you the dimensions of the recess it was sitting in. Thickness: most domestic pump union gaskets are 1.5 mm or 2 mm. A gasket that has been compressed in service will appear thinner than its original dimension — do not assume replacement thickness from the compressed gasket alone; confirm against the union recess depth before specifying the replacement.
If the gasket is damaged or missing — measure the union face
With the union disconnected, look at the face of the union that receives the gasket. There will be a machined recess — a shallow step ring in the face. Measure the outer diameter of that recess (this gives you the gasket OD) and the inner diameter of the recess (this gives you the gasket ID). The gasket should sit flat in this recess, flush with or just above the face surface.
Check the recess depth
Recess depth is only one input. The uncompressed gasket thickness must provide the intended stand-off and squeeze before any metal stop, using the OEM value or a verified equivalent. Measure the land, closure geometry and existing clearance; do not set thickness equal to recess depth by default.
Note the union face condition before ordering
While the union is apart, inspect the face for corrosion, scoring or deposits from the old gasket. A heavily corroded or deeply scored face may not seat a new gasket reliably. Light surface deposits can usually be cleaned with a cloth. Deep scoring or corrosion that cannot be cleaned may indicate the union body itself needs replacing rather than just the gasket.
Common mistakes when replacing pump union gaskets
Material selection — what the gasket has to survive
Domestic central heating pump unions seal against heating system water — inhibited, treated water at temperatures typically between 70°C and 90°C in normal service, with brief excursions higher in some boiler-fired systems. The gasket material must be compatible with:
- Heating system water including corrosion inhibitors
- Continuous service temperatures in the range of the central heating flow temperature
- Repeated thermal cycling between ambient and operating temperature
EPDM is a common choice for pump union gaskets in domestic heating because it handles hot water service and repeated temperature cycling well when the grade is suitable. Compressed fibre materials are also used in some heating union applications, especially where the original equipment or service conditions call for a fibre gasket.
The practical rule is simple: match the original specification when it is known. If it is not known, choose a gasket material that is explicitly suitable for heating system water, inhibitor compatibility, operating temperature and the way the union face compresses the gasket. A generic plumbing washer that happens to fit the diameter is not the same thing as a pump union gasket.
Do not use PTFE tape on pump union flat faces. PTFE tape is a thread sealant designed for threaded pipe joints. A pump union relies on a flat gasket compressed between two flat faces — not thread engagement — for its seal. Applying PTFE tape to a flat face joint or a union face does not improve the seal and may prevent the gasket from seating correctly. A correctly sized and correctly specified flat gasket fitted between clean flat faces is the correct assembly.
FAQ
Can I choose a pump union gasket by pipe size alone?
No. Pipe size describes the copper tube, not the gasket. Use pipe size only as a clue, then measure the old gasket or the union face recess to get OD, ID and thickness.
What dimensions matter on a pump union gasket?
The critical dimensions are outside diameter, inside diameter and thickness. OD and ID must suit the recess. Thickness must suit the recess depth so the gasket is compressed enough to seal without being crushed out of shape.
Can I reuse a pump union gasket?
Normally no. Once the gasket has been compressed in service, especially through heating cycles, it may not recover enough to seal reliably after the union is disturbed. Removed intact does not mean reusable.
Measure the recess, not the pipe. Replace the gasket, not just the pump.
Pump union gasket size is determined by the union face recess: OD, ID and depth. Pipe size, BSP thread designation and brand name are not reliable substitutes for a direct measurement. When replacing a central heating pump, measure the union face of the new pump before assuming the old gasket size still applies. Use a gasket material rated for heating system water and temperature. Removed intact does not mean reusable.