Boiler Losing Pressure But No Sign of a Leak? What's Going On

Can a boiler really lose pressure with no leak?

Yes, and it's one of the most common heating worries. Your boiler is a sealed, pressurised system, so when the gauge drops the water usually has to be going somewhere, but that somewhere is often invisible.

A healthy combi or system boiler typically sits at roughly 1.0–1.5 bar when cold and rises to around 1.5–2.0 bar once the heating is hot (your manual will give the exact figures for your model). If your needle keeps sliding toward the bottom with no puddle to show for it, one of four invisible culprits is often to blame:

• A failed or flat expansion vessel losing its air cushion.
• The pressure relief valve (PRV) quietly weeping water out of a copper pipe on your outside wall.
• A hidden leak under floors, in screed, behind walls or inside the boiler itself.
• Air being released after you bleed radiators, or simply a faulty pressure gauge.

The good news: you can often narrow down which one you have before anyone visits. For the wider picture of why your boiler pressure keeps dropping, we've a dedicated guide, but the test below is the place to start.

The observation that helps most: when does the pressure drop?

Many guides hand you a long list of causes and leave you to guess. The single most useful thing you can do is note when the needle falls.

Watch your gauge cold first thing, then again once the heating has been running for 20–30 minutes, then again after it has cooled. The pattern often points toward the cause:

• Drops only when the heating heats up, then recovers when it cools → often a failed or flat expansion vessel. The system has nowhere to absorb the expanding hot water, so pressure spikes, vents, then falls back low once cold.
• Drops steadily whether hot or cold, and gets lower over days → more likely a genuine leak somewhere, or a PRV that's passing water continuously.
• Dropped suddenly right after you bled radiators or topped up → often normal. You released trapped air; just repressurise once.

Cause 1: Failed or flat expansion vessel

The expansion vessel is a sealed tank, usually inside the boiler, split by a rubber diaphragm. One side holds water from your heating; the other holds a cushion of nitrogen or air pre-charged to roughly 0.75–1.0 bar (check your boiler's spec for the exact figure).

When the heating fires and water expands, that air cushion squashes to absorb it. Over years the air can leak out or the diaphragm can split, the cushion goes flat, and there's nothing to soak up the expansion. Pressure then spikes hot, forces water out through the PRV, and reads low again once cold, with nothing ever visible in the room.

On most combi and system boilers the expansion vessel sits inside the sealed casing, so checking or re-charging it means opening the boiler — and that is a Gas Safe registered engineer job, not DIY. The simple behaviour test above (low cold, high hot, sags back) is the safe check a homeowner can do.

For context only, an engineer's diagnosis typically involves the Schrader valve check: with the system fully depressurised and the casing already off, they press the tyre-style Schrader valve on the vessel. If water sprays out instead of air, the diaphragm has ruptured and the vessel needs replacing; if only air comes out but it's flat, it can sometimes be re-charged to the pre-charge figure. Some properties have an externally mounted add-on vessel that's more accessible, but the gas and sealed-circuit rules still apply.

Re-charging the air typically costs roughly £80–£150 for an engineer visit. A full replacement runs around £150–£450, climbing toward £600 if the vessel is buried deep in the boiler and needs a long strip-down (often a 1–3 hour job). See our guide to expansion vessel signs, recharge and replacement cost.

Cause 2: Pressure relief valve (PRV) weeping out the discharge pipe

Every sealed boiler has a safety valve, the PRV, typically set to open at around 3 bar to dump excess pressure outside rather than risk the system. It connects to a copper discharge pipe that runs through your wall and ends outside.

If that valve gets weakened by limescale or a brief over-pressure event, it can stop sealing and let a steady trickle escape. You may see nothing indoors, the water simply running away down the pipe.

To check, find the copper pipe sticking out of your outside wall, usually low down near the boiler's position. Look and feel for drips, a constant damp patch, green/white limescale staining or a watermark below it. Any of those suggests the valve may be passing.

A PRV is a sealed-circuit safety component, so replacement is a Gas Safe registered engineer job, not a DIY one. As a rough guide, budget around £90–£200 for a swap, with industry survey data putting the average near £100. Our explainer covers pressure relief valve problems in full.

Cause 3: Hidden leaks you can't see

Sometimes there genuinely is a leak, just not where you'd ever spot it. Pipework can weep under floorboards, inside a concrete screed, or behind a wall, and inside the boiler a corroded heat exchanger or perished seal can lose water onto internal trays that drain away unseen.

Tell-tale signs of a hidden leak include:

• A warm or damp patch on a floor where a buried pipe runs.
• Staining, blistering paint or a musty smell on a wall.
• A tide-mark or bubbling on plaster or skirting.
• Pressure that keeps falling steadily, hot and cold alike.

Crucially, the boiler casing must only ever be removed by a Gas Safe registered engineer, because behind it sit the gas, burner, flue and sealed water circuit. A professional leak detection and pressure test typically costs from around £100–£300+, with the repair on top depending on what's found. Internal corrosion is often a sign of system sludge, which we cover under sludge and corrosion in your heating system, and our guide to tracing a boiler leak and 2026 repair costs goes deeper.

Cause 4: Air being released (and a faulty gauge)

Not every drop is a fault. If you've recently bled radiators or your boiler has an automatic air vent doing its job, you're releasing trapped air, and pressure legitimately falls as that air leaves the system. The fix is simply to top back up once, not to keep chasing it. Our walkthrough on how to bleed a radiator explains the order to do this in.

It's also worth ruling out a faulty gauge. Older analogue dials can stick or read low even when the system is fine. If the boiler runs normally, never locks out and you see no other symptoms, a misreading gauge is a possibility, and an engineer can confirm it against a known-good test gauge. For what the dial should actually show, see what your boiler pressure should be.

How fast is too fast? How often topping up is normal

A tiny, slow loss over a long period is normal on many systems. Needing to top up once or twice a year is usually nothing to worry about, especially around the start of the heating season.

What isn't normal is topping up every few days or weekly. That's usually a genuine fault rather than maintenance, and it means water is escaping faster than the system can tolerate. The more often you reach for the filling loop, the more worthwhile it is to find the cause rather than keep masking it.

How to repressurise safely (quick recap)

If pressure has dropped below about 1 bar, you can usually top it up yourself, following your boiler manufacturer's instructions:

• Switch the boiler off and let it cool.
• Find the filling loop (a braided silver hose or built-in key/lever under the boiler).
• Open the valve(s) slowly and watch the gauge climb.
• Close them once you reach 1.0–1.5 bar cold (or your model's stated figure), then switch the boiler back on.

Don't over-pressurise past about 1.5 bar cold, or you'll push the system toward the 3 bar PRV trip and waste water out the discharge pipe. If you're not confident, leave it to an engineer. Full step-by-step instructions are in our guide to how to repressurise a boiler that's too low.

What it costs to fix (2026)

Indicative UK ranges for 2026, last checked 2026. These are general guide figures only, not quotes — always confirm with your own Gas Safe registered engineer, as access and boiler make change the figure:

Fix	Indicative 2026 cost	Notes
Expansion vessel re-charge	~£80–£150	Engineer visit; vessel is usually inside the sealed casing, so not a DIY job
Expansion vessel replacement	£150–£450 (up to £600)	Higher end if buried in the boiler; often a 1–3 hr job
Pressure relief valve (PRV)	£90–£200	Gas Safe engineer only; survey average around £100
Leak detection + test	from £100–£300+	Repair extra, depends on location of leak
Power flush (sludge/corrosion)	£350–£850 (more for large homes)	Priced by number of radiators and contamination
New boiler (when repeated repairs add up)	~£2,000–£3,500+ fitted	Worth costing if the boiler is old and failing repeatedly

For a wider breakdown across common faults, see our typical boiler repair costs guide.

Will boiler cover pay for it?

This is where a symptom can turn into a saved bill, and it's worth knowing the general rules before you call anyone. Many breakdown plans treat a failed expansion vessel or a faulty PRV as a covered repair, because they're genuine component failures rather than wear-and-tear neglect — but cover varies by provider and plan, so always read the specific terms.

What's commonly excluded is just as important:

• Pre-existing faults the boiler had before the policy started.
• Sludge and corrosion damage, often treated as system condition rather than breakdown, so a power flush is frequently your cost.
• No service history, where some providers can decline if the boiler hasn't been serviced.
• Older boilers over a certain age, depending on the plan's terms.

To be clear on the labels: FCA-regulated boiler insurance and unregulated service/care plans are different products with different protections, and you should check which one a quote actually is. You can compare boiler cover that includes breakdown repairs across a selected panel of providers (not the whole market) to find a plan whose terms cover the faults above.

Disclosure: Boiler Cover UK is an affiliate. We compare a selected panel of providers, not the whole market, and we may earn a commission if you take out cover through our links. It never changes what you pay. Prices shown are indicative "from" figures, last checked 2026 — always confirm the latest terms and price on the provider's own page before buying.

When to call a Gas Safe engineer

Topping up and watching the gauge is fine for a confident homeowner. Beyond that, the sealed circuit and anything gas-related must be left to a Gas Safe registered engineer. Call one if you notice:

• Pressure dropping to zero repeatedly, however often you top up.
• The boiler locking out or showing a low-pressure fault code and refusing to fire.
• Any visible damp, staining or warm patches suggesting a hidden leak.
• Water weeping from the outside discharge pipe (a possible passing PRV).
• Any need to open or remove the boiler casing — never do this yourself.

If you ever smell gas or suspect a gas leak, leave it to the professionals and call the National Gas Emergency line on 0800 111 999.

This article is general information only, not financial, insurance or gas-safety advice for your specific boiler or circumstances. For anything involving gas, the burner, flue, sealed circuit, gas valve, PCB or pressure relief valve, use a Gas Safe registered engineer.

This article is general information, not financial or gas-safety advice. We compare a selected panel of providers, not the whole market, and may earn a commission if you buy through our links. Always have gas appliances checked and repaired by a Gas Safe registered engineer; in a gas emergency call 0800 111 999. Prices are indicative UK guides for 2026 — confirm current prices on the provider's own site.