Pump union leaking —
how to identify
the real source
First confirm where the water starts. A gasket-face leak, a pump body leak, a valve leak and water tracking down the pipework can all look similar once the area is wet. The repair path changes completely depending on the source.
Scope: this is a diagnostic reference for flat-face pump union joints in heating pipework. It is not a substitute for qualified heating-system work, and it does not cover gas-side boiler work.
First — confirm it is actually the union
Before anything else, confirm the leak is actually from the union face and not from somewhere else that is dripping onto the union. Dry the entire pump area completely. Bring the system to cold working pressure. Watch for where moisture first appears — not where it drips, but where it originates.
A union that weeps at the face shows moisture at the flat contact ring between the union nut and the pump body or pipe stub. If moisture is appearing elsewhere — at the pump shaft, at the body of the pump, higher on the pipework — the union may not be the source at all.
If it is confirmed at the union face, the next question is why the gasket face has not sealed. The usual causes are face contamination, a reused gasket, wrong dimensions, excessive compression or alignment stress.
A pump union that has been in service for years has scale, corrosion products and compressed gasket residue on its sealing face. When you open the union and remove the old gasket, the face looks clean to the eye — but run a fingernail across it and you feel the ridges.
Scale builds up as a hard thin ring where the gasket face contacts the metal. Old gasket material bonds to both surfaces. Corrosion leaves a rough texture. Any of these prevent the new gasket from compressing evenly against a flat surface. One section of the gasket is over-compressed. The rest is not compressed enough. Water finds the gap.
The new gasket was fine. The face it was sitting against was not.
Signal
Leaked immediately on refitting. Leak is at the same point around the union face each time. Visible lime staining or dark residue ring on the union face.
Fix
Inspect the face after the joint is opened. The sealing zone should be clean and even, with no raised ring that the gasket has to bridge. Avoid scratching the machined face while cleaning.
You remove the union. The old gasket comes out in one piece, no visible cracks, not obviously damaged. It goes back in. The union is reassembled. It holds for a day, a week, a month — then starts to weep.
A gasket that has been in service under compressive load has permanently deformed. The gasket material, whether fibre composite or elastomer has compressed and not recovered. The gasket is thinner at the contact ring than at its outer edge — sometimes visibly, sometimes only measurable with a calliper. When you put it back, it no longer provides the same compression force against the union faces. It seals initially under the assembly load, then loses contact pressure as the system cycles through temperature changes. The leak develops slowly.
Removed intact does not automatically mean reusable. It means it did not fail catastrophically. After service compression, replacement is the normal repair route unless the equipment procedure says otherwise.
Signal
Union held initially but started weeping after the first few heat cycles. Recovered gasket is noticeably thinner at the contact zone than at the outer edge when measured with a calliper.
Fix
Do not treat an intact removed gasket as new. If the joint has been opened, replace the gasket unless the equipment procedure specifically says otherwise.
The gasket must suit the recess in three ways: outside diameter, inside diameter and thickness. A gasket that is too large can catch or fold as the union closes. A gasket that is too small can leave part of the recess unsupported. A gasket that is too thick or too thin can put the compression outside the useful range.
The nominal pipe or thread description is only a clue. The union recess tells you the gasket size. That recess can vary between pump models and union suppliers, even where the connection description looks similar.
Signal
Leak appears immediately on pressurisation. Inspection after opening shows the gasket folded, off-centre, pinched at one side, or not sitting flat in the recess.
Fix
Measure the union recess OD, ID and depth with a calliper where possible. Order by measurement, not by assumption from pipe size alone.
When a union leaks after reassembly, the instinct is to tighten it further. This is wrong. A flat face gasket has a compression range — within that range it seals. Below it, insufficient contact. Above it, the gasket is crushed flat and loses its ability to recover between heat cycles. The seal that held on initial assembly starts to fail as the gasket creeps under sustained load and thermal cycling.
The other version of this problem is sealant on the face. Jointing compound or PTFE tape applied to a flat face union connection creates an uneven surface under the gasket. The gasket compresses against the sealant rather than against the metal face. The sealant moves under load. The seal is unreliable.
Flat face gaskets seal by metal-to-metal compression with a gasket between. No sealant. No tape. The gasket goes in dry onto a clean face.
Signal
Union held on first assembly but started leaking within days or weeks despite feeling solid. Recovered gasket is uniformly flat and hard — no compression recovery. Sealant visible on the face or gasket surface.
Fix
Remove sealant from the sealing faces. Tighten evenly to the manufacturer's specification where available. If a sensible assembly still weeps, re-check the face, gasket size and alignment instead of simply adding more force.
The reassembly checklist
Every pump union reassembly, in sequence:
The two-spanner rule matters more than it looks. A pump union tightened with a single spanner on the nut puts rotational force on whatever the other end is connected to — the pump body, the pipe stub, the valve. That rotational force can slightly misalign the faces as you tighten, creating an uneven compression ring. Use two spanners. One holds. One turns.
What gasket size for a pump union?
The correct gasket size comes from the union face recess. Measure the outside diameter, inside diameter and recess depth, or measure the old gasket if it can be removed intact. Pipe size and thread size help identify the family of fitting, but they do not replace direct measurement.
If the gasket was destroyed during removal, measure the recess itself. If the pump or union has been replaced, do not assume the old gasket dimensions still apply to the new union.
FAQ
How do I know if the pump union gasket is actually leaking?
Dry the area completely and watch where water first appears. A gasket-face leak starts at the flat joint line around the union. Water from the pump body, valve, shaft area or pipe above the union is a different source.
What causes a pump union gasket face to leak?
Common causes include old gasket residue, scale on the union face, corrosion, wrong gasket dimensions, wrong thickness, uneven alignment, overtightening or reuse of a gasket that has already lost compression recovery.
What gasket size do I need for a pump union?
Measure the union face recess or the old gasket. The useful specification is OD, ID and thickness, not pipe size alone.
Should I use sealant on a pump union gasket?
Normally no. A flat gasket seals by compression between clean faces. Sealant or PTFE tape on the flat face can stop the gasket from seating correctly.
Start with the source, then diagnose the gasket face.
A wet pump union is not automatically a failed gasket. First confirm where water starts. If it is the union face, check the face condition, gasket recovery, dimensions, thickness, alignment and assembly load. Those checks separate a real gasket problem from a pump, valve, pipework or preparation problem.