Boiler Losing Pressure Overnight? How to Tell if It’s a Leak, the Expansion Vessel or the PRV

Quick answer: A sealed heating system should sit at around 1–1.5 bar and barely move overnight. If yours drops noticeably by morning, the cause is almost always one of three things: a small leak somewhere in the system, a failed expansion vessel, or a pressure relief valve that won’t reseal. The pattern of the loss tells you which.

Most modern combi and system boilers run on a sealed circuit of water — no header tank in the loft, just a fixed volume of water under pressure (our guide to central heating system types explains the difference). Topping the pressure up now and then is normal; doing it every morning is not. If you just need a refresher on reading the gauge and using the filling loop, start with our low pressure fix guide — then come back here to work out why it keeps dropping.

The three usual suspects

1. A small leak in the system

A leak loses water — and therefore pressure — at a steady rate whether the heating is on or off. The awkward part is that heating leaks are often invisible: a weep of a few drops an hour on a hot pipe evaporates before it can form a puddle. Look for tell-tale crusty white or green staining around radiator valves and compression joints, lifted or stained flooring near pipe runs, and damp patches on ceilings below bathrooms. Pipes buried in floors or walls can leak with no visible sign at all.

2. A flat or failed expansion vessel

The expansion vessel is a small tank with a rubber diaphragm and a cushion of air on one side. Water expands as it heats, and the cushion absorbs the increase. When the cushion has leaked away — or the diaphragm has split — there’s nowhere for the expansion to go, so pressure spikes when the heating runs (watch the needle swing towards 2.5–3 bar), the safety valve dumps the excess water outside, and by morning the system is low again. The classic signature: you top up, everything looks fine, the heating cycles, and next day the pressure is back on the floor.

There’s a simple test. The vessel has a car-tyre-style Schrader valve under a cap. Depress the pin briefly: if water comes out, the diaphragm has failed and the vessel (or on many combis, an add-on external vessel) needs replacing. If air hisses out but the boiler keeps losing pressure after heating cycles, the cushion probably just needs re-charging to around 1 bar with a pump — a routine job an engineer can do during an annual service.

3. A pressure relief valve (PRV) that’s letting by

The PRV is the safety valve that dumps water out of a copper discharge pipe on your outside wall if system pressure hits 3 bar. Once it has lifted — often because of the expansion vessel problem above — a speck of grit can stop it resealing, leaving a permanent slow dribble. Go outside and check the small copper pipe: dripping, green staining or a limescale trail down the wall means the PRV is passing and needs replacing. It’s an engineer job, and a good one will always ask why the valve lifted in the first place.

Read the pattern

The overnight test

1. With the system cold, top up to about 1.2 bar and note the exact reading.
2. Leave the heating off overnight (pick a mild night, or do this in summer).
3. Check in the morning. If pressure has dropped with no heating cycle at all, the water physically left the system — that’s a leak or a passing PRV, not the vessel.
4. If it held steady, run the heating hard and watch the gauge. A climb towards 3 bar points squarely at the expansion vessel.

Why you shouldn’t just keep topping up

Every top-up pulls fresh mains water into the circuit, and fresh water brings dissolved oxygen and dilutes the corrosion inhibitor. The result over months is rusting radiators and black sludge — which brings kettling noises, cold spots and, eventually, a bill for a power flush on top of the original fault. Repressurising weekly is a symptom to investigate, not a maintenance routine.

If the pressure problem is one of several — lockouts, noises, radiators that never quite warm — weigh up whether the money is better spent on replacement: see our guides to the signs a boiler needs replacing and new boiler costs in 2026.

FAQs

How much pressure loss overnight is normal?

Effectively none. The gauge reads slightly lower when the system is stone cold than when warm — that’s physics, not a fault — but a genuine drop of 0.3 bar or more overnight, repeatedly, means water is going somewhere and is worth investigating.

Is it safe to keep repressurising the boiler?

Topping up via the filling loop is safe and there’s no risk in doing it while you arrange a repair. As a rule of thumb, needing to do it more than once or twice a year means something is wrong.

Can I fix any of this myself?

You can safely check the gauge, bleed radiators, use the filling loop and do the Schrader pin test. Re-charging a vessel involves partially draining the system — possible for a confident DIYer on an external vessel, but sensibly left to an engineer — and any work inside the boiler casing or on the gas side must be done by a Gas Safe registered engineer.