Summary

Cylinder replacement is one of those jobs that prices simply on paper and badly in practice. The cylinder itself is the smallest cost line. The real money sits in: G3 notification and discharge pipework for unvented installations, balanced cold supply pipework, immersion circuits, the upgrade route from gravity-fed to mains-pressure, and the inevitable surprises (asbestos lagging, redundant feeder pipework, cold water tank decommissioning).

Three scenarios dominate UK domestic cylinder work. First, like-for-like vented copper swap — typical in older two-tank gravity systems, simplest pricing. Second, vented to unvented upgrade — much higher value, requires G3 qualification and £150-£350 in discharge pipework alone. Third, heat-pump compatible cylinder — typically stainless steel, oversized (210-300L) with a larger coil for low-temperature output, costing 40-90% more than a standard unvented.

This guide covers all three with labour, materials, regulatory cost, and risk premium for older properties. For boiler installs see boiler installation pricing guide; for heat pumps see air source heat pump pricing guide; for central heating overhauls see central heating installation pricing guide.

Key Facts

Quick Reference Table

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Job Type Cylinder Size Spec Labour Days Total Range (Regional) Total Range (London)
Vented copper like-for-like 120-210L Direct or indirect 0.5-1 plumber £600-£1,100 £750-£1,400
Vented to vented upgrade (larger) 250-300L Indirect indirect 1 plumber £1,000-£1,500 £1,300-£1,900
Unvented swap (existing G3) 150-210L Stainless 1 plumber + 0.5 electrician £1,600-£2,400 £2,000-£3,000
Vented to unvented upgrade 210L Stainless + discharge 1.5-2 plumber + 0.5 electrician £2,200-£3,500 £2,800-£4,200
Heat-pump cylinder install 210-250L HP coil + thermal store 2 plumber + 0.5 electrician £2,800-£4,800 £3,500-£5,800
Twin-coil solar/dual-fuel 250L Coil + control 2-2.5 plumber + 0.5 electrician £3,000-£5,200 £3,800-£6,400

Detailed Guidance

Vented like-for-like — the basic job

A like-for-like vented cylinder swap is the simplest cylinder job in UK plumbing. The existing cold feed and vent pipe stay, the cylinder is drained, disconnected (compression joints typically), lifted out via the airing cupboard, and replaced. Time is half a day for a competent plumber on accessible installations.

Critical pricing variable: access. Loft cylinders are 30-50% more expensive in labour because the lifting and dismantling adds 2-3 hours. Some 1970s installations have the cylinder boxed in with plasterboard and timber — opening, swapping, and reinstating adds half a day and a joiner cost. Always price-in this discovery cost as a provisional sum on older properties.

The most-missed cost is the immersion element. Old single-element 3 kW immersions are typically corroded and worth replacing during the cylinder swap (cheaper than recalling later). Always test the existing thermostat and replace if marginal. Pre-2002 cylinders are unlikely to meet Part L1B insulation; the new factory-foamed cylinders meet the standard automatically.

Vented to unvented upgrade — the value job

This is where pricing matters. Converting from a gravity-fed vented system to a mains-pressure unvented installation is one of the highest-margin cylinder jobs in UK plumbing and one of the most regulated. The installer must hold a G3 unvented qualification (BPEC, City & Guilds, LCL Awards) and the work is notifiable to Building Control unless self-certified via OFTEC, APHC, CIPHE or similar competent person scheme.

The job adds the following beyond a like-for-like swap:

  1. Decommissioning the cold water tank — drain, disconnect, cap off the vent pipe and cold feed, often remove the tank from the loft (£180-£350 labour add)
  2. Balanced cold supply — a 22mm cold mains feed from the rising main with a pressure reducing valve (3 bar), strainer, check valve, and isolation valve (£120-£250 in fittings + 2-3 hours plumbing)
  3. Discharge pipework — D1 (22mm copper, 500mm vertical) and D2 (22mm or 28mm, falling at minimum 1:200, terminating in a visible safe location) with tundish and air gap (£80-£200 materials + 1-2 hours)
  4. Expansion vessel — internal in modern cylinders, but external 18-24L for retrofits without (£80-£180)
  5. Thermal expansion — pressure reducing valve and expansion valve, balanced cold feed to all outlets serving the cylinder

Building Regulations Part G3 requires a discharge route that terminates "in a visible safe location" — typically outside the building, at low level, with a 300mm clearance. Discharge into a soil stack via a tundish is permitted via BS 7593 Annex A but local authority interpretation varies. Always check with Building Control on first-time conversions.

Commissioning includes pressure-testing to 1.5x working pressure, T&P valve operation, expansion vessel charge check, and Benchmark certificate. The Benchmark must be left with the homeowner — without it, manufacturer warranty is voided.

Heat-pump compatible cylinders — the future job

Heat pumps deliver hot water at 50-55°C flow temperature (versus 60-80°C for boilers). Standard unvented cylinders with a 0.8-1.2m² coil cannot transfer enough heat at the lower delta-T. Heat-pump compatible cylinders use a larger coil (2.5-3.5m²) and are sized larger (210-300L typical for a 3-4 bed home) to provide sufficient stored hot water at lower temperature.

Pricing-wise, the cylinder is 40-90% more expensive than standard unvented, the coil connections are typically 22mm or 28mm (vs 15-22mm), and a Legionella protection cycle is built into the heat pump controller (weekly 60°C boost) which uses the immersion element for the pasteurisation cycle. Specification check: MCS-compliant installations require a specific cylinder volume per heat pump output, typically 35-45L per kW of heat pump capacity. A 10 kW heat pump needs 350-450L of cylinder volume — frequently a 300L cylinder plus buffer tank.

The buffer tank requirement is the hidden cost. MCS-compliant heat pumps often need 25-50L of buffer to handle the defrost cycle and short cycling. Buffer tanks are £350-£900 supplied. See air source heat pump pricing guide for fuller pricing context.

Twin-coil solar / dual-fuel cylinders

Twin-coil cylinders have two heating coils — typically a lower coil for solar thermal or heat pump, an upper coil for boiler back-up. Used in dual-fuel installations and in solar thermal retrofits. The cylinder volume is sized for solar (60-80L per m² of collector area, typically 250-300L for a 4-collector array). Installation is more complex: two separate primary circuits, two pumps, two motorised valves, and a two-stage controller.

Pricing premium is 15-25% over single-coil equivalent for the cylinder, and a half-day labour premium for the additional pipework and commissioning. The hidden cost is the controller — a competent two-input solar controller is £180-£450, often missed by quoting only on the cylinder.

Hidden costs and risk premium on older properties

The five most-missed cost lines in cylinder quotes are: (1) asbestos lagging removal on pre-2000 cylinders (specialist surveyor £150-£280 + removal £180-£550); (2) lead service pipe replacement if the cold supply is lead (often £350-£900 from boundary box); (3) airing cupboard rebuild — old cylinders sit on a plinth that doesn't suit the new unit's dimensions, requiring £150-£400 of joinery; (4) electric showers and pumps fed from the old cylinder, which no longer have a vent and feed circuit and need re-routing; (5) Building Control inspection fee if not self-certified (£180-£350).

Risk premium of 10-15% is standard on pre-1980 properties: lead pipework, asbestos, redundant tanks in lofts, and undocumented modifications are likely.

Frequently Asked Questions

Do I need to remove the cold water tank when upgrading to unvented?

Not legally — but practically yes. The cold water tank loses its purpose once unvented is fitted (the unvented cylinder is fed directly from the mains), but a redundant cold tank in the loft becomes a Legionella risk if left stagnant. Best practice is full decommissioning: drain, disconnect, cap the cold feed and vent pipe, and either remove or thoroughly sterilise and leave dry. Discuss with the customer — some prefer to retain for water-supply interruption resilience.

What G3 qualification is needed to install an unvented cylinder?

G3 unvented hot water qualification is the minimum. The current recognised routes are BPEC G3 (most common), City & Guilds 6035, and LCL Awards Level 3 NVQ Diploma in Domestic Plumbing & Heating. The qualification must be held by the named installer, not the company. The installation must be notified to Building Control (self-certified via OFTEC, APHC, CIPHE if registered) and a Building Regulations Compliance Certificate issued.

Can I fit an unvented cylinder in a loft?

Yes, but Part G3 imposes specific requirements: the discharge pipework must terminate in a visible safe location (challenging from a loft), the cylinder must be on a load-bearing platform, and the cold mains feed must be 22mm minimum with adequate pressure (1.5 bar minimum, typically 3 bar working). Loft installations also need adequate access for service — Manufacturer instructions typically require 600mm clearance around the T&P valve and a clear route for cylinder replacement. Many installers prefer airing-cupboard installations for these reasons.

How long does a typical cylinder replacement take?

Like-for-like vented swap: 4-6 hours. Vented to unvented upgrade in a well-prepared airing cupboard: 1.5-2 days. Heat-pump cylinder install with new buffer and controls: 2-3 days. Add 0.25-0.5 day for an electrician for any new circuit work (S-plan or Y-plan wiring centre, thermostat circuits, immersion).

What is a "balanced cold supply" and why does it matter?

A balanced cold supply means the cold water feeding the cylinder and the cold water feeding all the other outlets in the property is at the same pressure. In a vented system, the cold tank feeds both at gravity pressure. In an unvented system, the mains feeds the cylinder at 3 bar; if the other cold outlets are still on a gravity tank feed, the hot taps will run at 3 bar and the cold at 0.2 bar — unusable mixing. The pressure reducing valve at the cylinder inlet must be teed off to feed all cold outlets at the same pressure, OR the cold outlets re-plumbed to the rising main.

Regulations & Standards