How to Price a Hot Water Cylinder Replacement: Unvented vs Vented Labour and Materials

Quick Answer: A like-for-like vented copper cylinder swap in 2026 prices £450–£950 fitted, an indirect vented cylinder swap with new immersion £750–£1,400, and an unvented cylinder install £1,400–£2,800 fitted including all G3-mandated safety devices and discharge pipework. Building Regulations Part G3 makes unvented cylinder installation notifiable — only an engineer holding a current G3 ticket (most commonly via WRAS, BPEC, or Logic) may install or commission an unvented cylinder, and the work must be notified to Building Control via a Competent Person Scheme.

Summary

Hot water cylinder pricing splits along two main axes — vented vs unvented, and direct vs indirect — and a third axis of material (copper vs stainless steel). The headline price difference between vented and unvented is misleading. A £350 vented copper cylinder and a £700 unvented stainless steel cylinder look like a £350 spread on supply, but the unvented install adds T&P relief valve, expansion vessel, balanced cold combination valve, tundish, D1 and D2 discharge pipework runs, system pressure-test and commissioning certificate, and Building Control notification. The full delta on a fitted job is £900–£1,500, not £350.

The single biggest mistake on cylinder pricing is sizing. Undersized cylinders generate the most service calls — running out of hot water mid-shower for a family of four on a 120 L cylinder is a near-guaranteed callback. Oversized cylinders waste energy on standing losses and take longer to recover. The correct size is set by occupancy and bathroom count: 1–2 people 120 L, 3 people 150 L, 4 people 180 L, 5 people 210 L, 6+ people or two bathrooms 250 L. A site survey of household routine — peak simultaneous demand, bath versus shower preference, when hot water is used — refines this for borderline cases.

Vented cylinders (gravity-fed, with a header tank in the loft) remain on millions of UK properties but are no longer specified for new installations. Unvented cylinders (mains-pressurised, no header tank) deliver mains pressure to all hot outlets, work in flats and properties without loft space, and pair with high-output thermostatic showers without a pump. The crossover for retrofit is the upgrade cost — a vented-to-unvented conversion adds the G3 install premium, the discharge pipework run to outside, and often a mains pressure check and supply-pipe upgrade.

Key Facts

Quick Reference Table

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Cylinder type Capacity (L) Unit cost Total fitted 2026 Notes
Vented copper, direct 120 £140–£260 £450–£800 Immersion only — no boiler coil
Vented copper, direct 150 £160–£300 £500–£900 Most common direct swap
Vented copper, indirect 150 £180–£320 £550–£1,000 With boiler coil
Vented copper, indirect 180 £220–£380 £650–£1,200 4-person family
Vented copper, indirect 210 £280–£480 £750–£1,400 5-person family
Unvented stainless, indirect 150 £400–£780 £1,400–£2,200 Single-bathroom flat
Unvented stainless, indirect 180 £450–£900 £1,500–£2,500 4-person family
Unvented stainless, indirect 210 £550–£1,100 £1,800–£2,800 5-person, 2 bathrooms
Unvented stainless, indirect 250 £700–£1,400 £2,000–£3,200 6+ person or large household
Vented to unvented conversion (180 L) 180 £450–£900 £1,800–£3,500 Includes new pipework, discharge run
Heat-pump-ready cylinder (high-coil-area) 200–300 £900–£1,800 £2,200–£3,800 For ASHP / GSHP — see heat pump cylinder specification
Solar-thermal twin-coil cylinder 200–300 £700–£1,400 £2,000–£3,500 Two coils — boiler upper, solar lower

Detailed Guidance

Vented vs unvented — when each is the right choice

A vented system uses a cold-water header tank (in the loft) feeding the cylinder by gravity. Hot water leaves the cylinder at the same low gravity pressure — typically 0.1–0.5 bar at upstairs taps. The system is open to atmosphere through a vent pipe rising above the header tank, so there is no possibility of dangerous over-pressure. Vented cylinders are simple, cheap, and easy to service. They suffer in modern homes because gravity hot water cannot run a thermostatic shower at full flow, will not feed a multi-spray shower head, and is incompatible with mains-fed mixer taps without a pump.

An unvented system removes the header tank entirely. The cylinder is fed direct from the mains and pressurised to mains pressure (typically 2–4 bar). Hot water leaves at the same pressure as cold — both feeds at the tap balance perfectly, thermostatic mixers behave correctly, and showers run on mains pressure without a pump. Because the system is sealed and pressurised, it requires safety devices to prevent thermal over-pressure: a temperature-and-pressure relief valve (T&P), an expansion vessel sized to the cylinder volume, a balanced cold combination valve, and a tundish-fed discharge to outside.

Choice criteria:

Direct vs indirect

A direct cylinder is heated only by an immersion heater inside the cylinder body. It has no boiler coil. Direct cylinders are used when there is no boiler — typically all-electric properties, off-gas-grid, or where the boiler is heating-only with separate hot water provision.

An indirect cylinder has an internal heat-exchange coil through which boiler-heated water flows. The coil heats the stored water indirectly. Indirect cylinders are the standard for any boiler-driven hot water system. Most indirect cylinders also include a backup immersion (a "twin entry" or "dual" cylinder) that can heat the water when the boiler is offline.

Pricing implications:

See combi vs system vs heat-only system selection for the wider system context.

G3 ticket — who can install an unvented cylinder

Building Regulations Part G3 makes unvented cylinder installation a notifiable activity. Only an engineer holding a current G3 qualification can lawfully install, modify, or commission an unvented system. The qualification is achieved through one of:

The G3 ticket is renewed every 5 years. Building Control notification is automatic for engineers registered with NAPIT or CIPHE. For non-scheme engineers, the homeowner notifies Building Control before work starts (£200–£400 fee). See G3 qualification and discharge pipe sizing for the full notification routes.

Discharge pipework — D1 and D2

Every unvented cylinder has a tundish (an air-gap funnel) immediately downstream of the T&P relief valve. The pipework before the tundish is D1 and after the tundish is D2.

D1 (T&P valve to tundish):

D2 (tundish to discharge point outside):

Common discharge points: through the external wall to a hopper above a yard gully, into a soakaway with at least 50 mm air gap, into a pumping chamber sized for the discharge volume. Discharge into a soil stack is permitted only when fitted with a tundish-rated air admittance valve (rare in domestic).

The discharge pipework is the single most complex pricing element. A short straight run (4 m, no bends, through external wall) is £40–£90 in materials and 1 hour's labour. A long run from a top-floor airing cupboard down to a back garden discharge point can be 8–14 m of 28 mm copper, two or three bends, possibly a chase through a wall, and 4–6 hours' labour.

Capacity sizing for occupancy

The British standard sizing matrix (BS EN 12897, supported by industry rule of thumb):

Occupancy Cylinder size Notes
1 person 120 L Studio or small flat
2 people 120–150 L Couple, single bathroom
3 people 150 L Family of three
4 people 180 L Standard 3-bed family
5 people 210 L Large family or HMO
6+ people 250 L+ Multi-bathroom or bath-heavy household
Two bathrooms upgrade by 30 L Account for simultaneous use
Bath-only family (vs shower) upgrade by 30 L Bath uses 80–150 L per fill
Heat-pump cylinder 200–300 L ASHP needs larger buffer for low flow temps

A bath holds 80–150 L; a shower at 8 L/min for 8 minutes uses 64 L; a basin and kitchen sink rarely contribute meaningfully. The cylinder needs to handle the simultaneous peak — typically morning shower + breakfast washing-up — without recovery during use. Recovery time is set by the boiler output and coil rating: 18 kW output and 24 kW-rated coil give roughly 18 minutes from cold to 60 °C on a 180 L cylinder.

Location options and access

The historic location is the airing cupboard — adjacent to the bathroom, accessible, vented for the cylinder's standing losses to warm linen. Alternative locations:

Access matters at swap time. A 250 L unvented cylinder is 1.95 m tall and weighs 50 kg empty. Threading it up a staircase, through a 60 cm doorway, into a 65 cm-wide airing cupboard requires planning. Some installs need cylinder removal in pieces (cut and remove the old one, deliver new one in two parts where the model permits) or even temporary stair removal — adding £200–£800 to the labour line.

BS 7593 system commissioning

After installation (and on every cylinder change to an existing heating system), BS 7593 requires:

Skipping BS 7593 voids the cylinder manufacturer's warranty and most boiler manufacturer warranties on the wider system. See when to power flush versus chemical flush and stand-alone flush pricing.

Annual maintenance

Unvented cylinder warranties typically require annual servicing. The service covers:

Cost £75–£140 for a service; £150–£250 if the expansion vessel needs re-pressurisation or the anode needs replacement.

Frequently Asked Questions

How much does it cost to replace a hot water cylinder UK 2026?

Vented copper swap £450–£950, indirect vented swap £650–£1,400, unvented stainless install £1,400–£2,800, vented-to-unvented conversion £1,800–£3,500. For a like-for-like swap with no system upgrade, expect the lower end of those ranges. A change of cylinder type (vented to unvented) adds significant labour for new pipework, expansion vessel mount, and discharge pipework run.

Can I install an unvented cylinder myself?

No — Building Regulations Part G3 makes unvented installation notifiable, and only a G3-ticketed engineer may carry out the work lawfully. A non-G3 install is not building-regs compliant, the homeowner has no commissioning certificate, the cylinder manufacturer's warranty is void, and on resale a buyer's surveyor will flag it. Vented cylinders are not notifiable but still need competent installation for warranty.

How long does a hot water cylinder last?

Vented copper cylinders typically last 15–25 years before pinhole failure; unvented stainless steel cylinders 20–30 years. The sacrificial anode in unvented cylinders is the wear part — replace at year 5–8 in soft-water areas, year 3–5 in hard-water. Cylinder failure is usually preceded by visible weeping at the access cover or a slow pressure drop on the heating side.

What size cylinder do I need for a family of four?

180 L for a single-bathroom 4-person home with a shower-mostly routine. 210 L if there are two bathrooms or the family takes baths frequently. 150 L is enough for two adults and two young children. Going one size up from the table matrix is rarely a mistake; going one size down often is.

Why is the discharge pipework so expensive?

The D2 discharge run from cylinder to outside is heavy-gauge copper, must fall continuously, must terminate in a visible location, and on a top-floor cylinder may run 10–14 m through stud walls and floor voids. Fittings include the tundish, swept bends, copper soldered joints, and external termination with a downturn. The labour can take 3–6 hours plus chasing for concealment. £200–£600 of materials and labour for a complex run is typical.

Regulations & Standards