Filling loop leaking —
is it the washer,
the O-ring or the valve?
Replacing the hose washer does not fix a valve body O-ring. Replacing the O-ring does not fix a valve that will not close fully. And sometimes the filling loop is not leaking at all — the boiler is losing pressure for a completely different reason. Identify the location first.
Safety scope: This article is for identifying likely water-side sealing and pressure-system symptoms. Do not open combustion chambers, gas-side components, flues or any appliance you suspect is unsafe. Gas work, boiler safety checks and unsafe appliance decisions belong with a competent Gas Safe registered engineer.
What a filling loop is and where it can leak
A filling loop is the temporary connection between the mains cold water supply and the central heating system used to top up system pressure. It typically consists of a flexible braided hose with a shut-off valve at each end — one connecting to the mains supply pipework and one connecting to the heating system return. When both valves are opened, mains pressure pushes water into the heating circuit until the system reaches the target pressure, then both valves are closed and the hose is removed or left in place closed.
Leaks can occur at four distinct locations, each with a different cause and a different sealing component involved:
The four leak locations — how to identify each one
The flexible hose has a flat washer at each swivel end. This washer sits inside the female end of the fitting and seals against the flat face of the male valve port when the nut is tightened. When the washer fails — through age, hardening, cracking, or incorrect size — water weeps from the threaded joint at the hose connector.
How to identify: the leak appears specifically at the nut-to-valve interface at either end of the hose. It may be a steady drip or weep when the system is under pressure. Tightening the nut may temporarily reduce it but does not fix a degraded washer.
What it is not: this is a flat washer in a compression-type swivel fitting — not a thread sealant application. PTFE tape does not belong here. The flat face must be sealed by the washer, not by thread engagement.
Fix: replace the hose washer — EPDM, matched to the fitting OD and ID → How to measure a flat washer before orderingThe quarter-turn ball valves commonly used on filling loops have an O-ring or stem seal inside the valve body that seals the rotating ball or spindle. This is a separate sealing component from the hose washer. When this seal degrades, water weeps from around the valve body or from the stem area — not from the hose connection nut.
How to identify: the leak is from the valve body itself — around the stem, the body joint, or from below the lever — rather than from the threaded fitting where the hose attaches. Replacing the hose washer will not stop this leak because the two sealing points are independent.
On many domestic filling loop valves, the valve body O-ring or stem seal is not a serviceable component — on many domestic types, valve replacement is usually more practical than attempting seal replacement. Confirm with the valve manufacturer or a replacement valve supplier before attempting to disassemble.
Fix: typically replace the valve body — not repairable on most domestic typesA filling loop valve that does not fully close allows a small amount of mains water to continue entering the heating circuit. This is not a joint leak in the usual sense — no water is visible externally — but it causes the boiler pressure to rise slowly over time and may eventually trigger the pressure relief valve to discharge.
How to identify: boiler pressure increases gradually after filling, despite both valves being in the closed position. The filling loop is left connected to the system between top-ups, which allows a partially closing valve to pass water continuously. Disconnecting the filling loop hose entirely — after closing both valves — and monitoring the boiler pressure confirms whether the filling loop is involved.
Causes include debris on the valve seat, a worn ball or seat, or a valve that was overtightened and deformed. Many domestic filling loop valves cannot be cleaned or adjusted — on many domestic types, valve replacement is usually more practical than attempting to service the seals.
Fix: replace the valve — or leave the hose disconnected when not in use → Boiler pressure too high — causes including filling loopIf the boiler loses pressure with the filling loop valves correctly closed — or with the hose physically disconnected — the cause is elsewhere in the system. A filling loop that is closed does not allow water to leave the heating circuit. Pressure loss under these conditions points to a leak somewhere else: a radiator valve, a compression fitting that has begun to weep, the heat exchanger, a leaking automatic air vent, or a pressure relief valve that is opening under normal pressure due to calibration drift or debris on the seat.
How to identify: disconnect the filling loop hose entirely. If pressure still drops, the filling loop is not the cause. Inspect the system for visible drips at radiator valves, compression fittings under floors or in service voids, and check whether the pressure relief valve discharge pipe shows signs of recent discharge.
Investigate system — leak is not from the filling loop → Boiler losing pressure — full diagnosis · → When the leak is not where it appearsThe washer — what material and what size
The flat washer in a filling loop hose connector is typically a simple disc of rubber material seated in the female swivel fitting. It seals against a flat machined face on the male port. The washer must be:
- The correct diameter — OD large enough to cover the sealing face, ID large enough to allow water flow without obstruction. Measure the existing washer OD, ID and thickness, or measure the recess in the fitting before ordering.
- The correct material — EPDM rubber is appropriate for filling loop applications as it is compatible with both mains cold water and central heating system water. A generic rubber washer is not automatically the right material — confirm EPDM before fitting.
- The correct thickness — a washer that is too thick will prevent the fitting from tightening fully; too thin and it will not produce adequate seating stress when the nut is tightened to its correct assembled condition.
Do not use PTFE tape on the filling loop hose swivel fitting. The hose swivel fitting seals by flat face compression — the washer is pressed against a flat face as the nut is tightened. PTFE tape on a flat face does not improve the seal and may prevent the washer from seating correctly. Clean the face, fit a correctly sized EPDM washer, and tighten the fitting to its normal assembled condition.
Pressure loss after filling — a common misdiagnosis
A very common scenario: the boiler has been topped up through the filling loop, the valves are closed, and an hour later the pressure gauge shows a drop. The natural assumption is that the filling loop is leaking. Often it is not.
When a cold heating system is filled and then heated, the air in the system compresses and the water expands. If the system was not fully bled of air after filling, the pressure may read higher than the actual water content and then drop as air vents or redistributes. This is normal behaviour and is not a leak. Similarly, if the system expansion vessel is undercharged, heating causes pressure to rise beyond the relief valve setting, which discharges water, and then the pressure drops as the system cools.
If the filling loop leaks when the system is at pressure but not when filling, suspect system pressure rather than the washer. A washer that is slightly degraded may hold at low pressure during filling but weep when the system reaches operating pressure. Washer replacement is often the correct response if the leak is confirmed at the hose connection — but confirming the location first avoids replacing the wrong component.
Locate the leak precisely before replacing anything.
The four leak locations on a filling loop require four different responses. Hose washer failure is fixed with a correctly sized EPDM washer. Valve body leaks typically require a valve replacement. A valve not closing fully causes pressure rise rather than a visible external leak. And if the pressure drops with the hose disconnected, the filling loop is not the cause at all. Identify which of these is actually happening before ordering parts or calling a plumber.
FAQ
Why is my filling loop leaking after I replaced the washer?
If the filling loop still leaks after the washer has been replaced, the leak may not be from the hose connection washer. Common alternative sources include the O-ring within the valve body — which seals the valve stem and is a separate component from the hose washer — a valve that is not closing fully due to debris on the seat or wear on the valve mechanism, or a system pressure that is too high and pushing water past a partially seated valve. Confirm exactly where the leak is presenting — at the hose fitting, at the valve body, from the valve stem, or from a separate threaded connection — before fitting another washer.
Why does my boiler lose pressure if the filling loop valves are closed?
A boiler that loses pressure with the filling loop valves closed is usually losing water from somewhere else in the system — a leaking radiator valve, a weeping compression fitting, a leaking heat exchanger, or a faulty pressure relief valve that is discharging small amounts. The filling loop itself is normally not the cause of pressure loss when the valves are correctly closed. If the filling loop valves are not closing fully — due to debris on the seat or a worn valve mechanism — a small amount of mains water may continue entering the system and then exit through the pressure relief valve, creating the appearance of pressure loss when the actual problem is valve integrity.
What size washer does a filling loop use?
Many domestic filling loops use a flat rubber washer at the hose connection point. The washer usually sits in the female end of the hose connector and seals against a flat face. Sizes vary by hose and fitting design — the existing washer should always be measured directly before replacement. OD, ID and thickness all matter. As a rough orientation, many domestic filling loop hose connectors use washers in the range of approximately 15mm to 20mm OD, but this varies between manufacturers and should not be used as a lookup shortcut.