BoilerBuddy · symptom guide
Boiler losing pressure — the five things that drive it.
If the gauge sits just below 1.0 bar and the heating won't fire on a combi, the system is telling you one of five things — it just needs a single top-up via the filling loop, the pressure-relief valve is passing water instead of holding it, the sealed system has a small leak somewhere, the expansion vessel has lost its air charge (the cause of the passing PRV in nearly every case), or limescale has built up inside the heat exchanger and is producing both a noise and a steady pressure drop. The cards below rank them most-likely-first; the guided top-up walkthrough is the natural next stop if the gauge is below 1.0 bar and your boiler has just locked out.
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Why this happens
Five causes, ranked by how often we see them on a combi.
Pressure inside a sealed heating system is the column of water pushing against the PRV and the gauge. If a combi needs topping up more than once in a short period, the five most-likely culprits, in order, are a one-off static-pressure dip after a bleed (a re-pressurise via the filling loop fixes it), a pressure-relief valve that is passing (the symptom becomes a steady drip from the outside discharge pipe), a small system leak (a pin-hole drip on a radiator valve, a weeping pump seal, a slow drip on the filling-loop joints), a expansion vessel that has lost its air charge (the underlying reason the PRV starts to pass — treat the two together), or limescale building up inside the heat exchanger (the same scale family that produces the kettle-like rumble). Each has a different physical sign; the cards describe each.
Re-pressurising after a bleed or static dip
Bleeding a radiator lets air out, but it also lets fresh air in, and the air settles in the system as a small pressure dip. A combi that needs a single top-up once every few weeks is most often simply settling rather than leaking — read the pressure gauge, and if it reads between 1.0 and 1.5 bar with the system cold, the right move is to leave it alone. If it reads below 1.0 bar, top up once via the filling loop; the guided walkthrough at /guides/top-up-pressure walks you through it step by step. If it drops again within 24 hours, the cause is no longer a static dip — move to the next three causes.
Safe at home · re-pressurise via the filling loop
Run this in the triage →Failing pressure-relief valve
The pressure-relief valve is a safety device bolted to the side of the boiler, designed to open only if the system over-pressurises. On a healthy combi it should never drip. If it is passing — even slightly — the cause is almost always that the expansion vessel has lost its air charge, so the system pressure climbs on every heating cycle and the PRV starts relieving just to keep the pressure in range. You will see this from the outside as a steady drip from the small copper discharge pipe that exits through an outside wall; do not cap or block the pipe — it is there to keep the boiler safe.
Engineer territory · PRV + vessel
Run this in the triage →Small system leak
Even a pin-hole drip on a radiator valve, around the pump, on the filling-loop joints, or on a corroded pipe can drop the gauge by about 0.1 bar a day. Walk round the boiler and look for puddles or staining; walk round each radiator and look for weeping at the valves; look at the pipework in airing cupboards and under floorboards for slow drips. A leak on a joint the engineer can re-seat is a quick fix; a leak on a sealed component — a heat-exchanger body, a pump seal, a manifold body — is already engineer territory. Don't keep topping the system up — the water has to be going somewhere, and a leak only ever gets worse.
Engineer territory · find and fix the leak
Run this in the triage →Failed expansion vessel
The expansion vessel is the football-sized tank of pressurised air — usually red or white — that sits on the wall behind the boiler or in the airing cupboard. It absorbs the expansion of the water as it heats, so the static pressure on the pressure gauge stays the same. Once it has lost its air charge (the Schrader valve on the end is the same as a car-tyre valve), the system over-pressurises every cycle, the PRV starts relieving, and the gauge drops back below 1.0 bar within days of every top-up. Treat the failing PRV (#2) and the failed vessel (#4) in concert — engineering territory, usually a repressurise of the vessel or a new vessel.
Engineer territory · repressurise or replace the vessel
Run this in the triage →Limescale in the heat exchanger
On a combi in a hard-water area, limescale builds up inside the heat exchanger over time. The same scale narrows the copper fins, produces a kettle-like rumble or bang on first firing, and a slow steady pressure drop on the gauge as scale-driven micro-leaks develop in the exchanger body. The next page over is the kettling one — if the pressure drop sits alongside a kettle-like noise, the two are usually the same story. Engineer territory; the fix is a descale or a replacement exchanger depending on age, and the kettling page has the full cousin on the scale family.
Engineer territory · descale or replace the exchanger
Run this in the triage →Try this first
If the gauge is below 1.0 bar — open the guided top-up walkthrough.
A single top-up to bring the gauge back into the green band fixes nearly half of the cases you see on a combi — the ones caused by a recent bleed or an overnight static-pressure dip. Our guided walkthrough at /guides/top-up-pressure walks you through it step by step, with a clear handoff into calling a Gas Safe engineer the moment any step says it should. If that matches your symptom, the top-up walkthrough is the right next stop; if it doesn't, the five reasons above are the next port of call.
5 reasons ranked. A calm top-up first, the engineer line if the gauge falls again.
Know when to stop
Stop if you notice
- A smell of gas, or a yellow, orange or sooty flame.
- A carbon monoxide alarm is sounding, or someone has possible CO symptoms.
- Smoke, scorching, visible damage, overheating or a leak around the boiler.
- The boiler keeps locking out or will not stay lit.
Where the safe-at-home line stops
Stop and call a Gas Safe engineer if any of these apply.
These are the lines that mean the pressure drop is past the safe-at-home boundary. Any of them — leave the boiler in its current state, don't keep topping up the pressure to mask it, and book a registered engineer today, not next week:
- The gauge drops back below 1.0 bar within 24 hours of a successful top-up — that is a system leak, not a static dip, and is registered-engineer territory.
- The pressure-relief discharge pipe outside is dripping, even when the gauge reads under 1.5 bar — a failing PRV or a vessel that has lost its air charge; also engineer territory.
Next step
Run the full triage in about a minute.
Tell BoilerBuddy what your boiler is doing. We'll return a ranked list of likely causes, the safe checks to do first, and the exact line that means 'call a Gas Safe registered engineer'.
FAQ
The five questions this symptom raises.
Need a qualified engineer?
BoilerBuddy can pass what you've worked out to a local Gas Safe engineer — symptoms, checks, and the bits that point to the likely cause.
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Engineer handover
Send this sheet with you to the visit.
Print or email it to your engineer so they arrive already knowing what your boiler is doing and what you've already tried.