Why Is My Boiler Losing Pressure? Six Causes Ranked by How Worried to Be

A combi boiler that slowly loses pressure between top-ups is normal. A combi boiler that drops noticeably every week, or that needs the filling loop touched every couple of days, is trying to tell you something.

The trick is reading the rate of loss and matching it to the cause. Here’s how.

What “normal” looks like

A healthy combi boiler should sit at around 1.0 to 1.5 bar when cold, and climb to 1.5 to 2.0 bar when hot (because water expands when heated — that’s normal physics, not a fault).

Over a year of normal use, a small amount of pressure loss is expected — air being burned off by automatic vents, tiny seepage through fittings, the natural settling of a sealed system. Topping up once or twice a year is normal. Topping up monthly is borderline. Topping up weekly is a problem.

Look at the gauge over a few days while the heating’s been off. Note the cold reading. Then check it again after a few days. If it’s dropped:

• < 0.1 bar in a week → probably fine, monitor it
• 0.1-0.3 bar in a week → small leak somewhere, worth investigating
• > 0.3 bar in a week → real problem, get it looked at

The six causes, ranked by how worried to be

1. Recent top-up settled (no action needed)

If you only just topped up the boiler and it drops 0.1-0.2 bar over the first few days, that’s often air being released by automatic air vents on the system. The pressure stabilises after a week or so.

Fix: wait a week. Top up again only if it’s still dropping after that.

2. Radiator bleeding lets pressure out

Bleeding radiators releases trapped air, but it also drops the system pressure as the air leaves. If you’ve bled several radiators recently, that’s why the boiler’s reading low — not a leak.

Fix: top the boiler back up via the filling loop. It’ll be stable now.

3. Pressure relief valve (PRV) discharge

A combi boiler has a safety valve that opens if the pressure gets too high — usually because the expansion vessel inside the boiler has failed. The valve discharges water down a small overflow pipe outside the house.

Tell-tale: look for a small (~22mm) copper or white plastic pipe sticking out of the external wall, usually near the boiler’s wall position. After the boiler runs for a while, is there water dripping from this pipe? If yes, this is your problem.

Why it’s worrying: it means the expansion vessel inside the boiler is failing. As the system heats, water expands, the vessel can’t absorb the expansion, pressure spikes, the PRV opens. Then the system cools, pressure drops below normal, you top it up, repeat.

Fix: Gas Safe engineer. The expansion vessel can sometimes be re-pressurised on the boiler (a 30-minute job, £90 callout). If the bladder inside has split, it needs replacing (£150-£300 parts + labour).

4. Visible leak somewhere on the system

Check every radiator valve, every joint you can see in the airing cupboard, the boiler itself for any obvious wet patches, drips, or hard white/green mineral deposits (calcium / verdigris). Don’t forget under sinks where the heating pipework sometimes passes.

Tell-tale: any actual evidence of water. Even a small drip mark or staining counts.

Fix: plumber. Most visible joint leaks are 30-minute fixes (tightening, replacing a washer, re-doing a compression joint). A few are bigger — leaking radiator valves usually need replacement.

5. Leak inside the floor or wall (the bad one)

Sometimes there’s no visible leak because the pipe is running through a floor void or behind a wall. This is the most awkward category to diagnose because you can’t see it.

Tell-tale: consistent slow pressure loss (0.2-0.5 bar a week), no visible leaks anywhere, no PRV discharge. Sometimes a damp patch appears on a ceiling below where the pipe is leaking, but often not.

Fix: specialist plumber with leak-detection kit (thermal imaging, acoustic detection, or hydrogen tracer). £150-£300 for the detection visit, plus repair cost depending on what they find. Sometimes the right answer is running a new feed via a different route rather than chasing the leak.

6. Internal boiler leak (the worst)

Rare, but possible. If a heat exchanger inside the boiler develops a pinhole, it leaks water internally — often visible as a small puddle under the boiler, or staining inside the boiler casing.

Tell-tale: water under or behind the boiler. Sometimes the boiler smells like burnt copper. Performance often degrades alongside — slower hot water, less heating output.

Fix: Gas Safe engineer, possibly new boiler. A new heat exchanger is sometimes £500+ in parts; on a 10+ year old boiler, replacement often makes more sense than repair.

What you can safely do yourself

Top up the pressure. Find the filling loop — a flexible silver hose with two valves, somewhere on or near the boiler. Open both valves until the gauge climbs to 1.2 bar, then close them. Some boilers (like Worcester Bosch) have a built-in key system instead of a flexible hose; the principle’s the same.

Bleed radiators. Standard 30-second job with a bleed key.

Note the rate of loss. Photograph the gauge with the date once a week. A plumber will diagnose much faster with that data.

What to leave to a Gas Safe engineer

Anything to do with the boiler internals. Heat exchangers, PRVs, expansion vessels, diverter valves. These are sealed gas appliances and DIY-modifying them is illegal in the UK as well as dangerous.

Repressurising the expansion vessel. Even though it sounds simple, doing it wrong floods the vessel and damages the boiler.

Anything where you smell gas. Stop, ventilate, call the Gas Emergency Service on 0800 111 999.

Quick diagnostic

Symptom	Likely cause	Action
0.1 bar drop, then stable	Just settling	Wait, re-test in a week
Steady slow drop, water dripping from PRV pipe outside	Failing expansion vessel	Gas Safe engineer
Steady slow drop, no PRV discharge, no visible leak	Hidden pipe leak	Plumber with leak-detection
Drop + visible drip somewhere	Joint or valve	Plumber, often quick fix
Drop + water under boiler	Internal leak	Gas Safe engineer urgently

Find a tradesperson

For pressure top-ups, leak detection, valve replacements, and anything on the wet side: browse Scotland’s top-rated plumbers.

For boiler internals, expansion vessels, heat exchangers, and any gas-related diagnosis: browse Scotland’s top-rated gas engineers. Gas Safe registration is legally required for any work on gas appliances in the UK.

If you’re not sure who to call, plumbers can usually do top-ups, joint repairs, and leak detection; gas engineers cover anything inside the boiler itself. Many tradespeople are dual-registered and can handle both — worth asking when you book.