Boiler Losing Pressure? Why It Happens and How to Fix It

Boilers lose pressure because water is escaping the sealed central heating system somewhere, air is being released from a radiator, or a valve hasn't been closed properly after maintenance. The three most common causes we see on jobs across London are a weeping pipe joint, a failing pressure relief valve, and a filling loop that wasn't shut off tightly.

• A small leak at a radiator valve, pipe joint, or the boiler itself
• Trapped air in the system being released, especially after bleeding a radiator
• A filling loop valve left slightly open
• A faulty pressure relief valve letting water escape through the overflow pipe
• A failing expansion vessel that can no longer hold the system's pressure properly

Gradual pressure loss over several days or weeks almost always points to a small leak somewhere in the pipework or radiators, while a sudden drop to zero bar is more often a mechanical fault, such as a stuck pressure relief valve or a failed expansion vessel. Knowing which pattern you're seeing helps narrow down the fix before anyone even arrives.

If your boiler is dropping pressure slowly and you top it up every week or two, don't just keep re-pressurising it — that's the clearest sign of a leak that needs tracing rather than a one-off fault.

You can safely bring a combi boiler back up to pressure yourself using the filling loop in under ten minutes, provided the drop is a one-off rather than a recurring problem. Follow these steps in order:

1. Turn off the boiler and let it cool. Switch the boiler off and wait until the system has cooled down before touching the filling loop, so you're not working with hot pressurised water.
2. Find the filling loop. Locate the flexible braided filling loop underneath or beside the boiler — it connects the mains cold water supply to the central heating system and usually has a valve at each end.
3. Open both filling loop valves. Open both valves slowly, either by turning a lever or unscrewing a knob depending on your boiler, and listen for water moving into the system.
4. Watch the pressure gauge. Keep an eye on the boiler's pressure gauge as it rises and close the valves as soon as it reaches around 1 to 1.5 bar — don't walk away and leave it unattended.
5. Close the valves and reset. Shut both filling loop valves firmly, then switch the boiler back on. Most modern boilers will show a fault code briefly before resetting once they detect correct pressure.
6. Bleed any trapped air. If the pressure still looks low or radiators are cold at the top, bleed each radiator with a radiator key to release trapped air, then top up the pressure again if needed.

If the pressure won't hold above around 0.5 bar even after topping up, or it drops again within a day, stop repeating the process and get it looked at rather than continuing to top it up.

Needing to re-pressurise your boiler more than once every few weeks means water is escaping the system somewhere and isn't going to stop on its own. The leak can be almost invisible — a damp patch under a floorboard, a slightly corroded radiator valve, or moisture inside the boiler casing — which is why it often needs a plumber with the right diagnostic kit to trace it rather than guesswork.

If you can see actual water, damp patches on ceilings or walls, or staining near pipework anywhere in the property, that's a leak in progress and worth getting checked promptly — see our leak detection & repair page for how we trace hidden leaks across London properties.

Call a plumber if your boiler pressure keeps dropping after you've topped it up more than twice, if you can't find or reach the filling loop, or if the boiler is showing a fault code alongside the low pressure. Continuing to top up a leaking system without fixing the cause can lead to corrosion, sludge build-up, or damage to the boiler itself over time.

RenoPlumb is Gas Safe registered and covers boiler and heating call-outs across Tower Hamlets, Hackney, Islington, Newham, Southwark and the rest of East and Central London from our base in Whitechapel, E1. See our emergency boiler repair page, or call 07460 824073 for a same-day visit.

There's no single answer, because the cost depends entirely on what's causing the pressure loss — a stuck valve is usually a quick job, while tracing a hidden leak behind a wall or under a floor takes longer. We always diagnose the actual cause first and give a free, no-obligation quote before any work starts. See our pricing page for how we quote jobs.

Need this fixed by someone local? See our page for a plumber in Hackney for borough-specific coverage and response times.