Cost to Replace a Hot Water Cylinder (2026)
Replacing a hot water cylinder is one of the widest price ranges in domestic plumbing — from a few hundred pounds for a like-for-like vented swap to well north of two thousand for an unvented conversion with new safety controls. The spread isn't padding; it reflects genuinely different jobs hiding behind one phrase. Here's how the pricing actually breaks down, and where the decision points sit.
Like-for-like vented replacement
A vented copper cylinder — the loft-tank-fed type in most airing cupboards — is the cheapest replacement. The cylinder itself typically costs £150–£400 depending on capacity and insulation grade, with installation adding £250–£500 for a straightforward swap: drain down, disconnect, fit, reconnect, refill and test. Total: commonly £400–£900. Complications that push it up: relocating the cylinder, replacing perished gate valves and immersion wiring while access is open (usually worth doing), and awkward airing cupboards where the old cylinder has to be cut out.
Unvented replacement and conversion
Unvented cylinders store hot water at mains pressure — stronger showers, no loft tank — and cost more at every stage. The cylinder runs £450–£1,100; installation is a bigger scope because the safety kit around it (pressure-reducing valve, expansion vessel, temperature/pressure relief, tundish and discharge pipework) must be installed and commissioned correctly. Like-for-like unvented swaps commonly land £900–£1,600 all-in; converting from a vented system adds pipework changes and discharge routing, typically £1,300–£2,400 total.
Unvented work is Building Regulations G3-controlled: installation must be carried out by a G3-qualified engineer and notified. This is not paperwork theatre — an unvented cylinder with faulty safety controls is storing pressurised hot water with no relief path. Any quote that doesn't mention G3 qualification is telling you to keep shopping.
Repair first: when replacement is premature
A cylinder under ten years old with a component fault — immersion element, thermostat, motorised valve, expansion vessel — is nearly always a repair, typically £90–£260. Replacement earns its cost when the shell itself leaks (unrepairable), when scale has strangled the coil in hard-water areas, or when repeated component failures signal a unit at end of life. A straight assessment prices both routes; be wary of any visit that reflexively lands on replacement without showing you why.
Choosing capacity and specification
Size to the household, not the cupboard: 120–150 litres suits 1–2 people, 180–210 litres a family bathroom household, 250+ litres for multiple bathrooms in simultaneous use. Insulation grade matters more than brand mystique — modern high-efficiency cylinders hold heat dramatically better, and the standing-loss difference compounds daily for a decade. In hard-water areas, budget for scale protection (an inline inhibitor or softener conversation) — it's the difference between a 10-year and a 20-year cylinder.
| Scenario | Typical total | Notes |
|---|---|---|
| Vented like-for-like swap | £400–£900 | Simplest case; loft tank retained |
| Unvented like-for-like swap | £900–£1,600 | G3-qualified engineer required |
| Vented → unvented conversion | £1,300–£2,400 | New safety kit + discharge pipework |
| Component repair instead | £90–£260 | Element, stat, valve, vessel |
Reading quotes against these numbers
National price bands earn their keep at comparison time: any quote inside the range with clear inclusions is negotiating in good faith; anything dramatically outside it — in either direction — is a prompt for questions rather than automatic rejection. Regional variation is real (London and the South East price higher throughout), as is access difficulty, which legitimately doubles some jobs. What never varies legitimately: surprise doorstep supplements and prices that grow after work has started.
Frequently asked questions
Who can legally replace an unvented cylinder?
A G3-qualified engineer — unvented installation and work on its safety controls are Building Regulations controlled. Jobs through this site involving unvented systems are tagged and routed to G3-qualified engineers automatically.
How long does a cylinder replacement take?
A like-for-like vented swap is typically half a day. Unvented swaps and conversions commonly run a full day, sometimes more where discharge pipework needs routing.
Is an unvented cylinder worth the extra cost?
If your incoming mains supply is strong, yes for most households: mains-pressure hot water at every outlet, no loft tank, better showers. If your mains flow is weak, no — survey first, spend second.
Why did my cylinder fail early?
In hard-water areas, scale is the usual culprit — it insulates elements and coils, forcing everything to run hotter and longer. Water treatment alongside a new cylinder addresses the cause rather than rebooking the symptom.
If you'd rather hand this over
Plenty of readers reach this point, weigh the afternoon involved, and sensibly hand the job to someone who does it weekly. No judgement — that trade-off is what our network exists for. Every plumber we work with is vetted, insured and locally based, quotes are agreed up front, and genuine emergencies are answered around the clock. Describe the symptoms you've identified from this guide and you'll get an accurate price rather than a guess — informed customers get better quotes.