Why Is My Boiler Losing Pressure? Causes, Fixes & When to Call an Engineer

If your combi or system boiler keeps losing pressure, the cause is almost always one of five things: a leak somewhere in the system, a failing expansion vessel inside the boiler, a stuck pressure relief valve, a recently bled radiator, or a faulty pressure gauge. Most of these you can diagnose yourself in 10 minutes. Some of them you should not try to fix yourself.

This guide walks through the 5 real causes in the order they typically show up, what to check first, what’s safe to fix yourself, and when to call an engineer. If you’re in Leicester or the surrounding LE postcodes and want a Gas Safe registered engineer to look at it, our boiler repair page covers what we do — or just call 0116 488 6858.

First — what is “normal” boiler pressure?

Most modern UK combi and system boilers want a cold pressure of 1.0 to 1.5 bar. Hot, with the heating running, it’ll rise to about 1.8 to 2.2 bar — that’s normal. If your gauge is sitting below 1.0 bar cold, the boiler will usually lock out (often with an F22, E119, or low-pressure error code on the display) and the heating will stop working until you top it back up.

“Losing pressure” means the cold pressure keeps drifting downward over days or weeks, so you keep having to top it back up via the filling loop. Once a year is normal. Once a week is not. Once a day means there’s a problem.

The 5 real causes — in order of likelihood

1. A leak somewhere in the system

By far the most common cause. Water has to go somewhere — if pressure is dropping, water is escaping the sealed system. The leak might be:

• A radiator valve dripping — check every TRV (thermostatic) and lockshield valve on every radiator. Even a slow weep over weeks adds up.
• A pinhole in old pipework — usually under floors or behind plasterboard, often near radiator tails. You won’t see this without lifting boards.
• Towel rail joints — bathroom towel rails are a classic slow-weep spot, especially if the joints were rushed when fitted.
• Pump or boiler joints — joints inside the boiler casing or on the pump can weep. Often a tell-tale white-crust or rust streak gives it away.
• An auto air vent — the small valve at the top of pumps or radiators that releases trapped air. If it’s stuck open it can leak constantly. Look for damp or staining underneath.

How to diagnose: walk every radiator and feel underneath each valve and joint. Run your finger along the joint — even a slight wetness counts. Check the floor or carpet under the boiler and pump for staining. If you find nothing visible and pressure is still dropping fast, the leak is almost certainly under floors or in walls and needs a plumber with a thermal camera or pressure-test kit.

2. A failing expansion vessel inside the boiler

Every combi has a small expansion vessel — a sealed metal cylinder with a rubber diaphragm and an air charge inside. Its job is to absorb pressure spikes when the system heats up. After 8-12 years, the air charge leaks out and the diaphragm fails. When that happens, hot pressure spikes have nowhere to go, the pressure relief valve dumps water outside, and the cold pressure drops the next morning. Cycle repeats.

How to diagnose: watch the gauge as the heating runs from cold for an hour. If pressure climbs above 2.5 bar when hot, the vessel is failing. Also check the condensate or pressure-relief overflow pipe outside — usually near where the boiler is mounted on the wall. Drips or water staining outside means the PRV is releasing because the vessel can’t absorb the spike.

Fix: the vessel can sometimes be re-charged with a bicycle pump (Schrader valve, depressurised system, 1 bar of air pressure) — but if it’s failed because the diaphragm is split, a re-charge won’t hold. Realistically, this is a job for a Gas Safe engineer, either to re-charge properly or to fit an external expansion vessel as a workaround. Internal vessel replacement is fiddly and usually pushes the case for a new boiler if the unit is over 10 years old.

3. A stuck pressure relief valve (PRV)

The PRV is a safety valve set to release water if the system pressure climbs above ~3 bar. After years of heat cycling, the brass seat can corrode or pick up scale, and the valve sticks slightly open — slowly bleeding water out via the overflow pipe outside.

How to diagnose: step outside and look at the small copper or plastic overflow pipe coming out of the wall near the boiler (usually pointing downward). If it’s dripping or shows green/white staining, the PRV is leaking. Sometimes a sharp click-tap-click on the valve dislodges the debris, but a properly stuck valve needs replacing.

Fix: PRV replacement is a Gas Safe engineer job — system has to be drained, the brass body cut out, the new valve fitted and tested. Roughly 1-2 hours of labour plus the part.

4. A radiator was recently bled

The simplest cause and the easiest to miss. Bleeding a radiator releases trapped air, which lowers system pressure. Bleed several radiators and the pressure can drop below 1 bar. The fix is just to top it back up via the filling loop and stop worrying.

How to diagnose: did you (or someone in the house) bleed any radiators in the last week? If yes, that’s almost certainly your answer. Top up to 1.2-1.5 bar cold via the filling loop and watch the gauge for a few days. If it stays put, you’re fine. See our guide to bleeding a radiator properly and our how to top up boiler pressure walkthrough.

5. A faulty pressure gauge

Rare but worth knowing. The pressure gauge itself can fail — usually reading lower than the actual system pressure. So you “see” pressure dropping, top it up, the boiler over-pressurises, the PRV dumps water, and you chase your tail.

How to diagnose: if you’ve ruled out 1-4 and the gauge needle behaves erratically (jumping, sticking, not responding to top-up), suspect the gauge. A Gas Safe engineer can confirm with a test gauge and replace it if needed.

Quick decision tree

• Pressure dropped after bleeding a radiator? → Top up. Done.
• Visible drip under a valve, joint, or towel rail? → Tighten the joint a quarter-turn (no more). If it stops, problem solved. If not, get a plumber out.
• Outside overflow pipe dripping or stained? → Either expansion vessel (cause 2) or stuck PRV (cause 3). Engineer call.
• No visible leak, no overflow drip, but pressure keeps falling? → Hidden leak under floor or in wall. Engineer with diagnostic kit.
• Gauge behaves erratically? → Gauge fault. Engineer to confirm.

When to stop and call an engineer

Top-up is fine to do yourself — the filling loop is designed for it. But anything inside the boiler casing (the expansion vessel, the PRV, the diverter valve, the gauge) is Gas Safe engineer territory only. It’s a sealed-system gas appliance and DIY work voids both your warranty and (potentially) your house insurance.

Specific moments to stop and call:

• You’re topping up more than once a week.
• The outside overflow is actively dripping, not just stained.
• You can see water staining or a wet patch on a ceiling, wall, or floor near the boiler or pipework.
• The boiler keeps locking out with low-pressure errors within hours of a top-up.
• You’ve tried to identify a leak and cannot find one — that means it’s hidden, and a thermal camera or pressure test is needed.

Frequently asked questions

Is it safe to keep using my boiler if it’s losing pressure?

Short-term, yes — modern boilers will lock out automatically if pressure drops below safe levels, so they won’t run dry and damage the heat exchanger. Long-term, no — you need to find and fix the cause, because chasing top-ups masks a leak that’s slowly damaging your house or your boiler.

How often is “normal” to top up boiler pressure?

Once or twice a year is normal — pressure naturally drifts down a touch over time. Once a week or more is a problem. Once a day means there’s an active leak or a failing component, and you need an engineer.

Could the cold weather be causing it?

Indirectly, yes. In a cold snap, your heating runs harder and longer — heat-cycling stresses the system and tiny leaks open up that were dormant in summer. It’s a common pattern: pressure issues appear in November, get fixed, then come back next November.

Can I just keep topping it up forever?

You can, but you shouldn’t. Every top-up adds fresh, oxygenated mains water to your sealed system. Over months, this accelerates rust inside radiators and the boiler heat exchanger. So even if “topping up every week” feels like a small inconvenience, it’s silently wrecking the system. Find the cause, fix it.

How much does fixing low pressure typically cost?

It depends on the cause. A radiator valve tightening is a £75-£120 callout. A stuck PRV replacement is £150-£280 with parts. An expansion vessel re-charge or external vessel fit is £200-£400. A hidden under-floor leak — diagnosis and repair — is £250-£600 depending on access. Always get a written quote first, no upfront deposit.

Need a plumber to look at it?

If you’re in Leicester or the surrounding LE postcodes — Hinckley, Loughborough, Oadby, Wigston and the 33 towns we cover — call 0116 488 6858. We’ll diagnose the cause, give you a written quote before any work starts, and explain what’s happening in plain English. No upfront deposit, no pressure-sales, no fake “today only” offers.