Boiler Expansion Vessel: Signs It's Failed, Recharging & Replacement Cost

Boiler expansion vessel replacement cost (UK 2026)

Replacing a boiler expansion vessel usually costs £180–£450 in 2026, parts and labour combined. The price swings on one thing: how hard it is to reach the vessel.

An external add-on vessel is a quick job; an internal one buried inside the boiler casing can be a half-day strip-down. Here is the at-a-glance picture. These are indicative ranges, last checked 2026 — always confirm with a written quote.

Job	Indicative cost (UK 2026)
External / add-on vessel fitted	£180–£350
Internal vessel swap	£250–£450+
Complex / strip-down job	£450–£600+
Vessel part only	£40–£160 (generic external tanks from ~£30)
Recharge-only service	£80–£180
Labour element	£120–£300+ (1–3 hrs)

Prices are indicative ranges for guidance only and vary by region, boiler model and accessibility. Get a written quote from a Gas Safe registered engineer before any work.

What an expansion vessel does (and why it fails)

Water expands when it heats up. In a sealed system boiler, the expansion vessel gives that extra volume somewhere to go, so pressure stays controlled instead of spiking.

Inside the steel vessel is a rubber diaphragm with a cushion of pressurised air (or nitrogen) behind it. As hot water expands, it pushes the diaphragm against the air cushion, absorbing the rise.

Two things go wrong. The air pre-charge slowly leaks away over the years (like a bike tyre left in a shed), or the diaphragm splits and lets water through. Either way the cushion is gone and pressure can no longer be absorbed.

Most vessels last around 5–15 years. A lost pre-charge can often be fixed by recharging; a split diaphragm means replacement.

7 signs your expansion vessel has failed

If you are also seeing the gauge behave oddly, our guide on what your boiler pressure should be sets the baseline. Watch for these symptoms of a flat or split vessel:

• Pressure climbs toward or over 3 bar when the boiler is hot.
• Water drips from the outside overflow pipe — the small copper pipe poking through an external wall.
• You top up the pressure every few days, but it keeps rising or falling again.
• The gauge needle swings wildly between cold and hot — a big difference points to a flat vessel.
• The boiler short-cycles or locks out on a pressure fault.
• Hot water and heating drop out when pressure trips the safety cut-off.
• A dull, waterlogged thud when the vessel is tapped, instead of a hollow ring.

A dripping discharge pipe is the classic giveaway — see our note on a pressure relief valve dripping from the overflow pipe. If pressure is the main issue, our pages on boiler pressure rising too high and boiler pressure that keeps dropping help you pin it down. The pressure-relief valve itself is a sealed-circuit safety component — diagnosing or replacing it is a job for a Gas Safe registered engineer.

The 30-second safe check (Schrader valve)

This is the single quickest check, and it tells you whether the vessel may be recoverable or needs replacing. The vessel has a Schrader valve — the same type as a car or bike tyre — usually capped at one end.

Press only briefly. A small puff of air is normal; a steady jet of water is a failed diaphragm. This is a read-only check on the air side of the vessel and does not touch the gas side. Recharging, replacing the vessel, or any work on the sealed water circuit or appliance internals should be done by a Gas Safe registered engineer — see the note below before attempting anything yourself.

Recharge vs replace: which do you need?

The valve test is the first pointer; an engineer will then confirm with a proper pressure check. As a rough guide you can also watch the gauge: note the reading cold, run the heating until hot, and note it again. A large swing points to a flat vessel.

Cold-to-hot pressure swing	What it suggests	Likely action
Under 0.5 bar	Healthy vessel	No action needed
0.5–1 bar	Suspect — pre-charge possibly low	Have it recharged and re-tested
1 bar or more	Likely flat or failed	Recharge; if it won't hold, replace

Replace if water came from the Schrader valve, if the vessel won't hold a recharge, or if it is old and showing corrosion. Recharge if air came out and the swing is large but the diaphragm is intact. Either way, the recharge and any replacement are best carried out by a Gas Safe registered engineer.

How an expansion vessel is recharged (what's involved)

A recharge restores the air pre-charge behind the diaphragm. It is straightforward for a competent heating engineer, but it does involve draining and refilling the sealed system, so it is not a casual DIY job — and on a combi or system boiler the vessel sits inside a gas appliance.

So you know what the job involves, the steps an engineer follows are broadly:

1. Isolate the boiler — switch off, turn off the electrical supply and let it cool.
2. Drain to zero — bring system pressure to 0 bar, because the vessel can only be measured accurately with no system pressure on it.
3. Pump the Schrader valve to the maker's figure — typically around 0.75 bar (≈11 psi) for many combis, but the boiler manual is the authority and some models specify 1.0 bar.
4. Hold and check — if the pre-charge holds, the vessel is serviceable; if it drops, the diaphragm has likely failed and replacement is needed.
5. Refill via the filling loop to about 1.0–1.5 bar cold.
6. Bleed the radiators, then re-check and top up to the correct pressure.

Repressurising from low pressure (the final refill step) is something many homeowners do themselves — see how to repressurise a low-pressure boiler. Stop and call an engineer if you smell gas, the boiler won't restart, or the vessel won't hold its charge.

What affects the replacement cost

• Internal vs external vessel — internal vessels mean dismantling the boiler; external add-on tanks plumb in beside it and are far quicker.
• Accessibility — a tight cupboard, a strip-down to reach the part, or draining a large system all add labour.
• Brand part price — genuine manufacturer vessels cost more than generic equivalents (£40–£160 vs from around £30 for some external tanks).
• Emergency call-out — out-of-hours or same-day visits carry a premium.
• Region — London and the South East typically sit at the top of labour ranges.

How long does the job take?

• Recharge: 30–60 minutes.
• External add-on vessel fit: 1–2 hours.
• Internal vessel swap: up to half a day if the boiler needs a near-full strip-down.

The labour time, not the part, is what drives most of the cost difference between jobs.

Fitting an external add-on vessel instead

When the internal vessel is buried deep inside the boiler, many engineers fit an external add-on vessel onto the pipework instead of dismantling the appliance. It does the same job.

Pros: cheaper labour, faster, and the new vessel is easy to check or recharge in future. Cons: it takes up a little space near the boiler and isn't the tidiest look. For a buried internal vessel, it is often the sensible, lower-cost route. Your engineer will advise what's right for your specific boiler.

Replace the vessel or the whole boiler?

On a fairly modern boiler, replacing the vessel is usually worth it against the cost of a new appliance. The maths shifts as the boiler ages.

If your boiler is 12–15+ years old and stacking up faults, a £400 vessel job on top of other repairs may be money better put toward replacement. Weigh it against typical boiler repair costs and our guide on whether to repair or replace your boiler.

Does boiler cover pay for an expansion vessel?

This is where it pays to read the small print. The expansion vessel is a core boiler component, and on many plans replacing a failed vessel is covered — provided it is on the plan's component list and the fault is not pre-existing or excluded as wear and tear or sludge damage. Cover varies a lot between providers, so treat the table below as a general guide only.

Recharging, by contrast, is often treated as a maintenance or servicing task and may not be covered on its own.

Often covered	Often NOT covered
Replacement of a failed vessel (listed part)	Recharging as a maintenance task
A genuine breakdown of the part	Pre-existing faults at sign-up
Labour to fit the new vessel	Damage from sludge / no inhibitor
	Wear-and-tear exclusions on older boilers

Deciding if it's worth paying for? See whether boiler cover is worth it and what boiler cover doesn't cover, then compare boiler cover for 2026 across our selected panel of providers.

How to avoid expansion vessel failure

• Book an annual service — a good engineer checks the vessel pre-charge and tops it up before it causes problems.
• Keep the pre-charge correct — the right cold charge (around 0.75 bar for many combis, per the maker's manual) takes the strain off the diaphragm.
• Use a corrosion inhibitor and a magnetic system filter — clean system water reduces sludge stress on the vessel and the rest of the boiler.
• Don't over-pressurise — refilling to 1.0–1.5 bar cold, not higher, keeps the system within range.

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.