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Boiler Losing Pressure Overnight? What a Slow Drop Really Means
Reviewed by the Devon Leak Detection team · Last updated June 2026
The short answer
If your boiler loses pressure slowly overnight while the heating is off, the cause is usually a small leak somewhere in the sealed system, not trapped air. Air does not lower the gauge on its own. A steady, gradual drop points to water escaping from a pipe, radiator valve or joint, and is worth tracing before it spreads.
A boiler that drips below its normal pressure overnight is one of the most common heating worries we hear about across Devon. You top it up before bed, the gauge reads a healthy 1.2 to 1.5 bar, and by morning it has sunk again. It is annoying, and it can feel like a mystery. The good news is that the pattern itself usually tells you a lot about what is going on, and a few simple checks at home will point you in the right direction before anyone needs to lift a single floorboard.
This guide walks through why a slow overnight drop almost always means a leak rather than air, how to run a simple monitoring test yourself, the warning signs that point to the expansion vessel instead, and when it makes sense to call for proper leak tracing.
Why a Slow Overnight Drop Usually Means a Leak, Not Air
Your central heating runs as a sealed system. It holds a fixed amount of water under gentle pressure, and that pressure is what the gauge on the front of your boiler shows. Trapped air can make radiators feel cold at the top or gurgle when the pump runs, but air alone does not push the needle down. For the pressure to fall, something has to physically leave the system, and the only thing that can leave is water.
That is why a slow, steady drop overnight is such a strong clue. While the heating is off, the system is not being topped up or drawn on, so a falling gauge means water is escaping somewhere. It can be a weeping radiator valve, a tired joint under a floor, a pinhole in a buried pipe, or a slow drip from the boiler itself. Many of these leaks are tiny, far too small to leave a puddle, and the water often evaporates or soaks away before you ever see it.
Leaks can also seem worse at night for a simple reason: as the system cools, the metal pipework contracts slightly, which can open up a hairline gap that stays closed when everything is warm. The house is quieter too, so the slow loss is easier to notice the next morning.
Is a Small Drop Ever Normal?
A very gradual fall over many months is not always a problem. It is fairly common for a healthy sealed system to shed a little pressure over time, and a slow slide from, say, 1.5 bar down to 1.0 bar across half a year is often nothing to lose sleep over. The thing to watch is the pace. If you only need to top up once or twice a year, the system is probably fine. If re-pressurising has become a weekly or monthly chore, you are treating the symptom of a leak rather than the cause, and it is worth getting to the bottom of it.
A Simple Monitoring Test You Can Do at Home
Before calling anyone out, you can gather useful evidence with nothing more than the boiler gauge and a notepad. The aim is to see how quickly the pressure falls and whether it falls at all when the system is left completely alone.
- Top the boiler up to its normal cold pressure, usually around 1.0 to 1.5 bar. Check your manual for the figure your boiler maker recommends.
- Write down the exact reading and the time. A photo of the gauge on your phone is the easiest record.
- Leave the heating switched off overnight, or for a full day, so the system stays cold and untouched.
- Check the gauge again at the same temperature and note any drop. A fall while the system is cold and off strongly suggests a leak.
- Repeat over a few days to see whether the loss is steady, speeding up, or only happening when the heating runs.
Comparing readings at the same temperature matters, because pressure naturally rises as the water heats and falls again as it cools. By only comparing cold-to-cold readings, you strip that swing out and see the real picture. While you are at it, have a quick look at the obvious suspects: the underside of radiator valves, any visible pipe joints, and the pressure relief valve discharge pipe, which often exits through an outside wall. A drip from that outside pipe is a useful clue in its own right, as we explain below.
When the Expansion Vessel Is the Culprit
Not every pressure problem is a leak. Inside most modern boilers sits an expansion vessel, a small sealed tank with an air cushion on one side and a rubber membrane separating it from the water. As the heating warms up, the water expands, and that air cushion soaks up the extra volume so the pressure stays sensible. When the membrane fails or the air charge leaks away, the vessel can no longer do its job, and the pressure starts behaving oddly.
A failing expansion vessel tends to produce a very different pattern from a plumbing leak. Look out for these signs:
- Pressure climbs sharply when the heating is on, often to 2.5 to 3 bar or more, then falls back when everything cools down.
- Water drips from the pressure relief valve pipe outside the house, usually only while the boiler is hot.
- The gauge swings up and down a lot more than it used to during normal running.
- You keep topping up, but the pressure never seems to settle and behave predictably.
The key difference is timing. A leak loses pressure steadily, even when the system is cold and switched off. A faulty vessel mostly causes trouble when the boiler is hot and the water is trying to expand. If your loss only shows up during heating cycles and the relief valve is dripping outside, the vessel or the relief valve is the more likely cause, and that is a job for a Gas Safe registered heating engineer to recharge or replace. If the gauge falls overnight with the heating off, a leak is the stronger candidate, and that is where leak tracing comes in.
When to Call for Proper Leak Tracing
If your monitoring test confirms a steady drop while the system is cold, and there is no visible drip at the valves or relief pipe, the leak is most likely hidden, often in pipework running under floors or buried in walls. This is the point to bring in a specialist rather than start lifting floors on a hunch. Our central heating leak detection service uses non-invasive methods such as thermal imaging, acoustic listening equipment and tracer gas to pinpoint the source before any flooring or plaster is disturbed.
It is worth getting a hidden leak traced sooner rather than later. Even a slow escape of warm water can damage joists, screed and insulation over time, and a system that is constantly topped up draws in fresh oxygenated water that speeds up internal corrosion. Acting early tends to mean a smaller, cleaner repair.
There may be help with the cost, too. Many home insurance policies include what is known as trace and access cover, which contributes towards finding the source of an escape of water and making good the area opened up to reach it. Cover and limits vary a good deal between insurers, and it usually does not pay for the repair of the leak itself, so it is always worth checking your own policy wording or speaking to your insurer before work begins. We are happy to provide a clear written report to support a claim. You can read more about how we work across the county on our Devon leak detection page.
If you have spotted damp patches, warm spots on the floor or the unmistakable hiss of water where there should not be one, it is also worth reading our guide on signs of a water leak under the floor, which covers the wider clues a hidden leak leaves behind.
Frequently Asked Questions
Not on its own. Air in the system can cause cold spots or gurgling radiators, but it does not push the pressure gauge down. For the pressure to fall, water has to physically leave the sealed system. So a steady overnight drop points to a leak rather than air, even if you also have air-related symptoms to bleed out separately.
A very slow fall over many months, such as 1.5 bar drifting to 1.0 bar over half a year, is often normal. The warning sign is pace. If you only top up once or twice a year, the system is likely healthy. If you are re-pressurising every week or month, that usually points to a leak worth tracing.
Timing is the giveaway. A leak loses pressure steadily even when the heating is off and the system is cold. A failing expansion vessel mostly misbehaves when the boiler is hot, with pressure spiking high then dropping back, and often water dripping from the relief pipe outside. A cold, off, overnight drop leans towards a leak.
Topping up is a fine short-term fix, but if you are doing it often you are masking a leak. Each top-up draws fresh oxygenated water into the system, which speeds up internal corrosion, and any hidden escape can damage floors and joists over time. It is better to trace and fix the cause than keep re-pressurising.
Many policies include trace and access cover, which helps towards locating an escape of water and making good the area opened up to reach it. Limits and terms vary widely between insurers, and it usually does not pay for repairing the leak itself. Always check your own policy or ask your insurer before work starts.
Usually not. We use non-invasive methods such as thermal imaging, acoustic listening equipment and tracer gas to pinpoint a leak before anything is disturbed. The aim is to find the exact spot first, so any access is as small and targeted as possible rather than lifting whole rooms of flooring.
Worried About a Leak in Devon?
If your boiler keeps losing pressure, we can trace the source without tearing your home apart. Get in touch for friendly, expert help.