Vented vs Unvented Hot Water Cylinders
The vented-versus-unvented question decides how your home stores and delivers hot water for the next fifteen years. Vented is the traditional loft-tank-fed system: simple, cheap, gentle pressure. Unvented connects the cylinder straight to the mains: powerful, balanced pressure everywhere, no loft tank — at higher cost and with legally regulated installation. Neither is universally right; your mains supply, house layout and budget pick the winner.
How each system works
A vented cylinder is fed by gravity from a cold water storage tank in the loft; an open vent pipe back to the tank makes the system inherently unpressurised and self-relieving. Hot water pressure at your taps equals the height of that tank above them — decent for a downstairs shower, modest for an upstairs one.
An unvented cylinder takes mains water directly, storing it hot under mains pressure. Every hot outlet then delivers at (near) mains pressure, matched to the cold — which is what makes showers feel dramatically better. The trade: stored pressurised hot water demands engineered safety (expansion capacity, temperature and pressure relief valves, tundish and discharge pipework), governed by Building Regulations G3.
Performance: where unvented earns its price
Unvented wins the shower experience decisively in homes with strong mains supply: balanced high pressure at every outlet, multiple outlets running well simultaneously, and no loft tank to freeze, overflow or contaminate. It also frees the loft and removes the tank-fed cold plumbing entirely.
Vented fights back on resilience and simplicity: it tolerates poor mains pressure (the tank buffers it), keeps working through mains interruptions until the tank empties, suits pumped showers (pumps are for tank-fed systems), and every component is cheap and universally understood. In a house with weak mains, an unvented cylinder is a mistake — it can only deliver what the main provides.
Cost, servicing and rules
Installed costs: vented like-for-like £600–£1,200; unvented £1,200–£2,500; conversion from vented to unvented £1,800–£3,000 including tank removal. Unvented adds an annual service (£80–£150) checking the safety valves and expansion — not optional in any meaningful sense, and often a warranty condition.
The legal line: unvented installation and maintenance is restricted to G3-qualified engineers and is notifiable to building control. Always verify the qualification; the paperwork protects your insurance and your resale conveyancing alike.
Choosing for your house
Choose unvented when: mains flow and pressure are verified strong (roughly 20 L/min and 1.5+ bar as working minimums), you want the best shower performance, you're renovating anyway, or the loft tank's space/risk bothers you. Choose (or keep) vented when: mains supply is weak or shared, budget is tight, you rely on a pumped shower, or the existing system is healthy and the upgrade solves no felt problem.
The measurement comes first: any plumber can gauge your flow and pressure in minutes, and that number — not the brochure — should drive the decision. And if mains-pressure hot water appeals but stored volume doesn't fit, note the adjacent options: combis (no cylinder, Gas Safe territory) and thermal stores each suit particular homes.
| Factor | Vented | Unvented |
|---|---|---|
| Hot water pressure | Gravity (modest) | Mains (strong) |
| Loft tank | Required | None |
| Installed cost | £600–£1,200 | £1,200–£2,500 |
| Annual service | Not required | Required (~£80–£150) |
| Installer | Any competent plumber | G3-qualified only |
| Weak mains suitability | Good | Poor |
| Pumped showers | Compatible | Not permitted |
The questions that settle it
Whichever way you're leaning, three questions stress-test the decision: What does my specific setup support (measured, not assumed)? What's the ten-year cost of each option, running and maintenance included, not just the install price? And what happens at failure — parts availability, engineer availability, disruption? The option that survives all three questions is usually the right one, even when it wasn't the cheapest on day one.
Frequently asked questions
Can I convert my vented system to unvented?
Usually, if your mains supply measures up — the conversion removes the loft tank, re-routes pipework and installs the full G3 safety package for £1,800–£3,000 typically. The flow/pressure check is the go/no-go gate.
Which lasts longer?
Comparable shells (10–15 years, hard water shortening both), but unvented adds serviceable components — expansion vessels and valves — that need periodic attention. A serviced unvented and a neglected vented age about the same; a neglected unvented doesn't.
Is stored pressurised hot water dangerous?
Properly installed and serviced, no — multiple independent safety devices back each other up. The regulations exist because a badly installed one genuinely can be; that's the whole case for verifying G3 credentials.
Do heat pumps change this decision?
Heat pump systems use cylinders (typically unvented, with larger coils) — so if a heat pump is in your five-year plan, a cylinder-based system keeps that door open and the cylinder choice should anticipate it. Worth raising with your installer.
Getting this fixed properly
If reading this has confirmed you need an engineer rather than a Saturday afternoon, that's a useful outcome — the expensive version of most plumbing problems is the one that was almost fixed. Our network's vetted local plumbers handle this work daily: they carry the common parts, price before starting, and are insured for the rare occasion something goes sideways. Emergencies get 24/7 response; planned work gets a proper quote. And if your problem turns out to be drainage rather than supply plumbing, our drainage partner picks it up seamlessly.