TL;DR
If your boiler’s pressure keeps falling but you can’t find a wet joint, radiator or pipe, the water is still escaping somewhere. You just can’t see it. It could be inside the expansion vessel, through a weeping air vent, or via pipework hidden under a floor. A proper pressure test finds these concealed faults. Checking a few basics yourself is fine. However, tracking down a hidden leak is a job for a Gas Safe registered engineer — check the Gas Safe Register to find one near you.
Why “no visible leak” doesn’t mean no leak
This is a narrower question than the one covered in our guide to why your boiler keeps losing pressure. Here, we’re focused on cases where a visual check finds nothing at all.
A boiler’s pressure gauge measures how much water sits in the sealed heating circuit. When the needle drops, water is leaving the circuit somewhere. But that somewhere doesn’t have to be a dripping pipe you can see. Plenty of Slough homes have pipework buried under solid floors, or hidden inside walls, especially in older estates. A tiny seep in any of those spots can lower the pressure over hours or days. It won’t leave a damp patch you’d notice. The loss can also be entirely internal to the boiler, such as water escaping into the expansion vessel’s air side. Because the circuit is pressurised, even a near-invisible leak keeps bleeding off pressure. If you keep topping up via the filling loop, the gauge can look deceptively stable right up until the next slide.
The three most common hidden culprits
1. A failed or waterlogged expansion vessel. The expansion vessel absorbs extra water volume as it heats, using a rubber bladder pressurised with air. If that bladder ruptures, water floods the air side instead. The vessel can no longer do its job. You might notice the pressure spike when the boiler fires, then fall again as the excess is expelled. A faint gurgle from the vessel is another sign.
2. A weeping automatic air vent. These small spring-loaded valves sit at high points in the circuit, to release trapped air. Since the valve seat wears over time, a slow trickle escapes each time it opens. Tucked away in a loft or a cupboard, that water often evaporates before it pools. There’s a faint hiss to listen for, but nothing to see.
3. Hidden or under-floor pipework losing water slowly. Heating pipes embedded in screed, or run under floorboards, can develop a pinhole or a hairline crack. These leak only a few millilitres an hour. The water gets absorbed by the surrounding material, or evaporates, long before it’s noticeable. The only symptom is the gauge creeping down.
What a pressure test finds that a visual check can’t
A walk-round inspection only catches leaks that produce visible moisture. However, to locate a concealed fault, an engineer runs a pressure or isolation test instead. The system is drained, then pressurised slightly above normal. Sections of the circuit are isolated one at a time, using their valves. The gauge is then watched for a drop, typically over ten to thirty minutes. A steady decline in one section points straight at the fault. A drop that shows up everywhere usually means the expansion vessel itself is the problem, not the pipework. This is the only reliable way to tell the two apart. It’s also how an engineer confirms the repair holds afterwards.
Checks you can do yourself before booking anyone
A handful of basic checks don’t require opening the pressurised circuit at all.
- Check the filling loop is fully closed. A loop left slightly open constantly tops the system up, which masks a leak by keeping the gauge artificially steady.
- Listen near the automatic air vent while the boiler is running. A steady hiss or occasional drip suggests a weeping vent.
- Press the Schrader valve on the expansion vessel (the small valve on top, similar to a car tyre valve). If water comes out instead of air, the bladder has failed.
- Look over any accessible pipework and radiator valves for rust, white mineral deposits, or damp staining.
If any of these turn something up, that still isn’t a DIY repair, since it needs specialist parts and tools. A failed expansion vessel or a weeping vent needs replacing by a Gas Safe registered engineer — check the Gas Safe Register to find one. Repeatedly re-pressurising a system with a known fault can push it into over-pressure. If everything checks out clean and the gauge is still falling, the fault is hidden — either in the boiler internals or the buried pipework, and a professional pressure test is the only way forward from there. If the buried pipework itself turns out to be the culprit, get in touch online and our leak detection & repair work can help with that side of it. We also cover the wider area, including:
Frequently Asked Questions
Can a boiler lose pressure without a leak? Yes. Water can escape somewhere you can’t see. It might be inside the boiler via a failed expansion vessel, through a weeping air vent, or along pipework buried under a floor. Dampness on a visible surface can take a long time to follow.
My combi boiler keeps losing pressure but there’s no leak I can find — is this a particular brand’s problem? No. This happens across every major boiler brand, including Worcester Bosch, Ideal and Baxi. The cause sits in the expansion vessel, an air vent, or hidden pipework — not in which brand made the boiler.
How do I test if the expansion vessel is at fault? With the boiler off, press the Schrader valve on top of the expansion vessel. If water comes out instead of air, the internal bladder has failed. The vessel needs replacing — that’s not a DIY repair.
What happens if I keep topping up a boiler that’s losing pressure but I can’t find the leak? The boiler will keep dropping pressure until it locks out on a fault, cutting off heating and hot water. Left long enough, an unfound leak can also cause corrosion. That’s more expensive to put right than an early diagnosis, so a Gas Safe registered engineer is worth calling sooner rather than later.