Boiler pressure rising with
the filling loop closed —
is a valve passing?
Either the valves were not fully closed after the last top-up, or the internal valve seats have worn and are passing water even when the handle says closed. Both push mains pressure water into the sealed heating circuit and show up as boiler pressure that rises without anyone touching the system. A visible external drip at the connection fitting is a separate, neighbouring fault — covered at the end of this article.
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 actually is
A sealed central heating system is a closed loop of water. It does not connect to the mains supply during normal operation. The filling loop is a temporary connection — a short hose or rigid link between the cold mains supply and the heating circuit — used to pressurise the system when water has been lost.
When not in use, the filling loop should normally be isolated from the heating circuit. It has two valves — one on the mains side, one on the heating side — that must both be fully closed after filling. Many heating engineers remove disconnectable filling loops entirely after use and reconnect them only when the system needs topping up.
The problem is simple: mains water pressure in the UK is typically 2 to 4 bar. Your heating system runs at 1.0 to 1.5 bar cold. Any path between the mains and the heating circuit — however small — will push water in until the pressure equalises or the PRV releases it.
The two causes
The filling loop has two isolation valves — one on the mains supply side, one connecting to the heating circuit. Both must be fully closed. Not mostly closed. Fully closed.
After topping up the system, it is easy to close the valves to the point where the handle feels firm and stop there. On many filling loop valves, a quarter turn of residual opening is enough to pass water continuously at mains pressure. The handle looks closed. The valve is not.
The tell-tale sign is timing. If pressure started rising shortly after the system was last topped up — within days or a week — and was stable before that, valve position is the first thing to check. It costs nothing to verify and takes thirty seconds.
How to confirm it
Locate both filling loop valves. Turn each one fully to the closed position — feel for firm mechanical stop, not just resistance. If pressure stabilises over the following 24 hours with no heating running, the valves were the cause.
What to do
Close both valves fully and verify. If the system has gained significant pressure from the ingress, some water may need to be removed to bring cold pressure back to the specified level. Check the boiler data plate for the correct cold fill pressure.
Filling loop valves have internal rubber or composite seats that form the seal when the valve is closed. Over time — particularly with hard water, with heat cycling and with the small abrasive particles that circulate in mains water — these seats wear. The surface that should form a watertight seal develops micro-imperfections. Water passes through even when the handle is in the closed position.
This is the more insidious cause because it is invisible. The handle is closed. You cannot see or hear the water passing. The only evidence is boiler pressure that keeps rising, with both filling loop valves confirmed fully closed.
This can develop gradually over years. A filling loop that has worked correctly for a decade starts to pass slowly. Boiler pressure starts climbing a little higher than usual. The homeowner tops it up. It climbs again. They top it up again. The filling loop seat is the cause and nobody has checked it.
How to confirm it
Close both valves fully. Switch the boiler off completely. Check the pressure reading. Return after 12 to 24 hours without running any heating. If pressure has risen with both valves confirmed closed and no other water source identified, the valve seats are passing.
What to do
Replace the filling loop. Individual valve seat replacement is possible but typically not cost-effective — a complete loop replacement is more reliable. If the filling loop is the disconnectable type, disconnect it entirely after the next fill and reconnect only when the system genuinely needs topping up.
The consequence — why it matters
Both causes produce the same downstream effect: mains water continuously entering the sealed heating system. This matters for more than just pressure readings.
Pressure above the PRV set point. The PRV opens to release the overpressure. The loop continues adding water. The PRV continues dripping. This is often mistaken for a faulty PRV. The PRV is working correctly — the filling loop is the problem.
Dilution of corrosion inhibitor. The heating system water contains corrosion inhibitor that protects radiators, the pump and the boiler heat exchanger from internal rust. Every litre of mains water entering the system dilutes the inhibitor concentration. Over months, the inhibitor level drops below effective protection levels. Corrosion begins — invisibly, on the inside of every metal component in the circuit.
Oxygen ingress. Mains water contains dissolved oxygen. A sealed heating system is designed to have its oxygen content consumed early in its life, after which the remaining water is essentially deoxygenated. Continuous fresh mains water introduction continuously reintroduces oxygen. This accelerates corrosion of radiators and system components.
Accumulated overfill. Each time pressure climbs and the homeowner tops up — not realising the filling loop is the cause — more water enters. Eventually the system has significantly more water than the expansion vessel was sized for. Hot pressure spikes higher than expected. The expansion vessel gets blamed. The filling loop was the actual cause all along.
The test that costs nothing: switch the boiler off completely. Note the cold pressure. Come back 12 to 24 hours later without running any heating. If pressure has risen — with no one touching the filling loop valves — water is entering the system from outside. The filling loop is a common source worth eliminating first. If pressure is still rising after confirming the loop is closed and not passing, other causes — expansion vessel, plate heat exchanger — should be investigated.
A separate but related fault — the connection gasket
This is a different problem from internal valve passing — but it looks similar at first glance. The filling loop connects to the heating circuit through a 1/2 inch BSP threaded flat face fitting. At that connection sits a flat gasket.
If this gasket has been compressed flat over years of service, the connection point weeps visibly at the fitting face — moisture appears at the joint itself, not from inside the valve body. This does not cause rising system pressure the way internal passing does. It is an external drip at a threaded joint, not water entering the sealed circuit.
The two faults can usually be distinguished: internal valve passing shows up as rising system pressure with no visible external leak. External gasket weep shows as moisture at the flat face fitting with no unexplained pressure rise. The connection uses a standard 1/2 inch BSP flat gasket — approximately 18.5 mm OD — fitted dry to a clean flat face.
Pressure keeps rising with no one touching the system. Start with the filling loop — it is one of the first and most useful checks.
Close both valves fully. Switch the boiler off. Wait 24 hours. If pressure rises — the loop is passing. Either the valves need a final firm close, or the seats have worn and the loop needs replacing. If the loop has a disconnectable section, disconnect it. Remove the path for mains water to enter the system and the pressure problem stops.
FAQ
Why does my boiler keep gaining pressure?
Boiler pressure that rises when the system is cold and switched off — not just when heating — can point to external water entering the circuit. The filling loop is one early thing to check: confirm both valves are fully closed, then test whether pressure still rises over 12 to 24 hours. Other causes include a passing plate heat exchanger. Either the filling loop valves are not fully closed after the last top-up, or the internal valve seats have worn and are allowing mains pressure water to enter continuously. Mains pressure in the UK is commonly around 2 to 4 bar — significantly higher than the 1.0 to 1.5 bar the heating system runs at — so even a small passing causes steady pressure gain.
What is a filling loop and what does it do?
A filling loop is a short flexible hose or rigid connection between the mains cold water supply and the sealed heating circuit. It is used to pressurise the system when water has been lost — typically after bleeding radiators or draining the system for repair. When not in use, the filling loop should normally be isolated from the heating circuit by closing its valves. Many installers remove the filling loop entirely after filling and reconnect it only when needed.
Should I remove the filling loop after filling the boiler?
Some heating engineers recommend removing a disconnectable filling loop after filling the system, rather than leaving it permanently connected. A loop left connected can introduce a potential path for mains water to enter the sealed system if the valves are not perfectly closed or if the internal seats wear over time. Some modern boilers have built-in filling mechanisms that do not use a separate external loop.