External filling loop
connection leak —
washer or O-ring?
A dripping filling loop is not always a flat washer problem. The connection may use an O-ring rather than a flat washer. The valve may not be fully closing. The drip may be water tracking along the hose from a connection somewhere else. Or the washer face may be scaled or damaged — in which case a new washer seats on the same problem and fails for the same reason.
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.
Scope: This article is a water-side filling-loop diagnostic guide. It is not a gas appliance repair instruction. Any work involving gas-side components, combustion safety, boiler casing access or regulated appliance servicing must be carried out by a Gas Safe registered engineer.
This article covers filling loop connections on central heating systems — the flexible hose assembly used to pressurise the system from the mains water supply. It is a diagnostic reference. Any work involving gas appliances or regulated gas-side components must be carried out by a Gas Safe registered engineer.
Where a filling loop can actually leak
A typical filling loop assembly has several points where water can escape. Treating them all as the same problem — "the washer needs replacing" — misses the diagnostic step that determines which one is actually leaking.
Flat washer versus O-ring — how to tell which your connection uses
Flat washer connection
The hose end fitting has a union nut that screws onto the valve connection. Inside the nut, or sitting on the valve face, is a flat disc — the washer. The face it seats against is flat and machined. Removing the union nut reveals the flat washer sitting in or against the connection face.
Replacement: flat washer of the correct OD, ID and thickness to match the connection.
O-ring connection
The hose end fitting inserts into or over the valve connection with a push or screw engagement. An O-ring sits in a groove in the male or female fitting and is compressed as the connection is made. The sealing element is round in cross-section, not flat.
Replacement: O-ring of the correct cross-section diameter and inside diameter to match the groove geometry.
When you remove the connection and find a flat disc, it is a flat washer. When you find a round-section ring in a groove, it is an O-ring. If the connection uses an O-ring and a flat washer is fitted, the flat disc sits against the wrong geometry and the connection will leak regardless of washer condition. If it uses a flat washer and an O-ring is fitted, the O-ring may seat partially but will not produce the same contact pattern as a correctly fitted flat washer.
Why a new washer still leaks — the common causes
- Scale or debris on the valve face. In hard water areas, mineral scale can build up on the flat seating face of the filling valve. A new washer contacts the scale rather than the clean metal face. The result is an uneven seating contact that leaks despite the new washer. Clean the valve face before fitting the replacement.
- Wrong washer thickness. A washer that is too thick prevents the union from fully closing — the nut reaches its thread limit before the washer is fully compressed. A washer that is too thin allows the union to close without adequately compressing the washer. Both produce a leak. Confirm the thickness matches the original or the connection specification.
- Reused washer. A washer removed from the connection has already permanently deformed to the face condition it was seating against. Refitting the same washer — particularly after cleaning the face — may not produce the same seating geometry. Always fit a new washer at each reassembly.
- Damaged valve seat. If the flat face of the valve connection is scored, pitted or out-of-flat, a new washer seats on the same damage. The leak path is in the face, not the washer. A new washer of any specification will not reliably seal a significantly damaged valve face. The valve requires replacement.
- Union overtightened on previous attempt. Overtightening can damage the washer and distort the seating geometry. If the previous washer was overtightened, inspect the valve face for damage before fitting the replacement.
A filling loop that leaks at the union after repeated washer replacement may have a damaged valve face, not a washer problem. At that point, the valve — not the washer — requires attention. Continued re-gasketing of a failed valve seat produces repeated failures. A qualified heating engineer should assess the valve condition and advise on replacement where required.
Reading the location of the drip
- Drip from the union face joint — around the nut circumference: flat washer or O-ring at that connection. Confirm which sealing element type is used, then check face condition and washer/O-ring specification before fitting a replacement.
- Drip from the valve body or stem when valves are fully closed: external valve leak. Internal seat passing is a separate fault that may not be externally visible. The valve requires inspection — a new washer at the union connection does not address an internal valve fault.
- Drip appears on the hose surface or runs down from above the union: possible tracking from a higher connection or condensation. Dry and re-pressurise to confirm origin before disassembling the union.
- Leak appears only when the filling valves are open: confirms water is entering the loop from mains — look at both union connections and the valve seats under flow conditions.
Identify the source before ordering the replacement.
A dripping filling loop has five possible sources — flat washer, O-ring, valve seat, threaded connection, or water tracking from elsewhere. Each requires a different response. Replacing the flat washer addresses one of them. Confirm which component is actually leaking, confirm the sealing element type, inspect the valve face, and fit the correct replacement. That sequence avoids the repeat failure that comes from fitting the right part in the wrong diagnosis.
FAQ
Why is my filling loop still dripping after I replaced the washer?
If the filling loop continues to drip after a new washer has been fitted, the most common reasons are: the leak is not from the flat washer connection — it may be from an O-ring in a different part of the filling assembly, from the valve not fully closing, from a threaded connection, or from water tracking along the hose from a different point; the replacement washer is the wrong thickness or diameter for the specific connection; the face the washer seats against has scale, residue or damage that prevents flat contact; or the valve seat itself is damaged and a new washer on a damaged valve seat does not produce a reliable seal. Before fitting a third washer, confirm that the water is actually coming from the flat face connection and not from somewhere else in the filling assembly.
How do I know if the filling loop leak is from the washer or the valve?
A flat washer leak at a filling loop connection typically appears at the union face — the joint between the hose end fitting and the system connection. Water appears around the circumference of the joint or drips from the connection point. An external stem or body leak is visible at the valve assembly. Internal seat passing is different and may show no external drip; it is inferred from pressure behaviour or flow through the closed filling path after the connection itself has been ruled out. Dry the area completely, make sure the filling valves are closed, and observe where water first reappears without opening the boiler casing or disturbing regulated components. If water appears at the union face joint, a washer problem is more likely. If the valve body or stem becomes wet, treat it as an external valve leak. If the assembly stays dry but pressure rises or flow continues through the closed filling path, investigate internal valve passing.
What type of sealing element does a filling loop use?
Filling loop connections use different sealing elements depending on the connection type. Older-style filling loops with threaded braided hoses typically use a flat rubber washer at each end union connection — the same principle as a tap connector. Flexible filling loops with push-fit or compression-type ends may use O-rings rather than flat washers. Some filling link assemblies use a combination. The type of sealing element at a specific connection determines which replacement is needed — a flat washer and an O-ring are not interchangeable. If the sealing element type is not obvious, inspect the removed element or the fitting recess before ordering.