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Copper & stainless · Fortic · Loft tank · Upgrade path

Vented Cylinder London

Vented (open-vent) hot water cylinder replacement, immersion element repair, header tank replacement, and full conversion to mains-pressure unvented. Traditional loft-fed and Fortic compact-format cylinders across every London borough.

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Vented cylinders — the traditional London hot water topology

A vented cylinder is fed from a loft header tank rather than directly from the mains. The cold water reaches the cylinder by gravity from the tank, so the working pressure at every outlet is set by the tank’s height above the cylinder — about 0.1 bar per metre head, giving a 3m loft tank a shower pressure of 0.3 bar. That is a fraction of the 3–4 bar mains pressure a modern unvented cylinder delivers. The vented topology has one advantage — the cylinder is never pressurised above the tank head, which means it does not need the G3-notifiable safety kit an unvented cylinder requires. The trade-off is weak shower pressure and the ongoing loft-tank flood risk.

Four variants are common in London homes. The classic copper indirect vented cylinder heated by an internal coil connected to a heat-only gas boiler — the standard 1980s–1990s specification, still working reliably in tens of thousands of London properties. The copper direct vented cylinder heated by an immersion element only, no coil — traditionally paired with Economy 7 tariff, still common in ex-council flats and post-war social housing. The Fortic cylinder with a small header tank built into the top of the cylinder itself, removing the need for a separate loft tank — compact, easy install, ideal for flats without loft access. And modern duplex stainless variants of any of the above, chosen for better hard-water resistance than copper.

The two questions that determine whether the answer is replace-like-for-like or upgrade-to-unvented: does the shower pressure matter enough to justify the £1,200–£2,000 cost difference, and is there any near-term project (loft conversion, property sale) that makes the upgrade self-funding through knock-on benefit. On most jobs we do the answer is one of the two — like-for-like where the customer priority is minimum cost and functional hot water, or a full unvented conversion where mains-pressure delivery, loft-tank removal, and property-sale positioning combine to justify the higher spend.

Every cylinder replacement — vented or unvented — is delivered by a Water Regulations 1999 competent engineer (WaterSafe registered, UK Certification Ltd certificate 136356 issued 8 September 2025, expiry 18 August 2030). Vented-to-unvented conversions additionally carry the HWSS G3 competency certificate (certificate 136359, same period) required by law for the unvented topology. Public liability £5,000,000 via SiriusPoint through Eaton Gate MGU, policy BE26ACTT000000018221, period 07/05/2026 to 06/05/2027.

Four common London vented cylinder types

Copper indirect vented

Traditional London specification — a copper cylinder fed from a loft header tank, heated by an internal coil connected to a heat-only (regular) gas boiler. Common in 1930s–1990s installs. Reliable but weak shower pressure (0.1–0.3 bar from a 2m loft-tank head).

Copper direct vented (immersion only)

Copper cylinder with no coil, heated electrically by an immersion element. Traditionally paired with Economy 7 tariff — the "Fortic" configuration is a direct vented copper cylinder with a small integrated header tank on top for compact installs. Very common in ex-council flats and post-war London social housing.

Stainless steel vented

Modern vented cylinder in duplex or 316L stainless steel — better hard-water resistance than copper. Fitted where the customer wants to keep the loft-fed topology (avoiding G3 notifiable work) but upgrade cylinder material quality. Rarer than unvented as a modern install.

Fortic cylinder (integrated header)

Compact vented cylinder with a small header tank built into the top of the cylinder itself. Removes the need for a separate loft header tank — ideal for flats and small properties without loft space. Direct (immersion-only) is most common; indirect Fortic exists but is unusual.

When to replace like-for-like

Vented is often the right answer. Four scenarios where like-for-like replacement is the sensible choice:

You need to keep the boiler you have

A working heat-only boiler that is still under warranty or recently serviced is worth keeping. Replacing the cylinder like-for-like (indirect vented) keeps the existing boiler configuration and heating controls without a full system change.

Loft space unavailable for header tank relocation

A vented cylinder needs the header tank in the loft above it. If the loft is a converted bedroom or the tank position is under a hip roof section that a builder cannot get to, keeping the existing loft-tank topology is simpler than the alternative reconfigurations.

Budget-conscious replacement

A like-for-like vented cylinder replacement costs £900–£1,450 fitted. An upgrade to unvented costs £2,650–£3,500 including cold-feed re-plumb, G3 kit, Building Notice. If the customer's priority is a working cylinder in the same footprint at minimum cost, vented is the right answer.

Listed building or Conservation Area constraints

Some listed buildings have restrictions on external features that the tundish discharge pipe from an unvented cylinder would need. In those cases keeping the existing vented topology avoids the planning consent risk.

When to upgrade to unvented

The four scenarios where the extra £1,200–£2,000 for an unvented conversion is self-funding through knock-on benefit:

You want mains-pressure hot water at every outlet

The main reason to upgrade — a vented system delivers 0.1–0.3 bar shower pressure from the loft head. An unvented delivers 3–4 bar from the mains. Every shower, every tap, dramatically improved.

You want the loft tank gone

Loft tanks are the single most common cause of catastrophic residential flooding — a failed ball-valve, a split copper connector, a burst tank in a cold snap floods the property from above. Removing the tank as part of an unvented conversion eliminates the risk permanently.

You are converting the loft to habitable space

A loft conversion cannot proceed with a header tank still in the loft. Converting to unvented as part of the loft-conversion works is standard practice — the tank comes out, cold feed moves to the mains, cylinder relocates if necessary.

You want to be able to sell the property to modern buyers

Post-2010 London buyers increasingly ask their surveyors specifically about hot water pressure and cylinder type. A vented system is a negative point at survey. An unvented cylinder is a positive point. On a property already scheduled for sale within 2 years, the upgrade is often self-funding through the sale price.

Cost — vented cylinder replacement

ScopePrice (inc. VAT)Includes
Like-for-like vented copper indirect cylinder replacement£900–£1,250Copper cylinder (British standard sizes), immersion element, connections re-made, safety valve, waste haulage of old unit
Like-for-like vented stainless steel indirect replacement£1,150–£1,450Duplex stainless cylinder (better hard-water resistance), immersion, connections re-made
Vented direct (immersion only) replacement — Economy 7 configuration£850–£1,150Copper direct cylinder, dual immersion, Economy 7 timer (if not already fitted)
Fortic (integrated header) cylinder replacement£950–£1,350Compact Fortic cylinder, no separate header tank required, install and connections
Full vented-to-unvented conversion£2,650–£3,500Unvented cylinder, G3 safety kit, cold-feed re-plumb from mains, discharge pipework to external tundish outlet, decommission of loft tank, Building Notice, G3 certificate. See /indirect-unvented-cylinder-london for full spec.
Header tank replacement (loft)£350–£550New byelaw-standard tank (with lid, insulation and screened overflow), refit ball-valve, isolate and refill
Emergency vented cylinder repair (immersion element failure)£195–£320Diagnosis, immersion element replacement, cylinder tested and refilled

Real London vented cylinder jobs

Peckham ex-council flat — Fortic replacement Economy 7

Existing 1988 Fortic direct cylinder failing on the primary immersion element with intermittent hot water. Customer on Economy 7 tariff wanted to keep the compact Fortic footprint (no loft, no separate header). Replaced like-for-like with a modern Fortic 115L direct twin-immersion cylinder. Total £1,150 including waste haulage and new isolation valve.

Balham 3-bed terrace — vented indirect like-for-like

Existing 1995 copper indirect cylinder failing on the coil primary — pinhole leak on the coil connector at the airing cupboard. Customer wanted to keep existing 24kW heat-only boiler (recently serviced, still under warranty). Replaced with copper indirect 210L vented cylinder. Kept existing loft tank (recently replaced). Total £1,250.

Chiswick 4-bed home — full vented-to-unvented conversion

Family upgrading before loft conversion project. Existing 250L vented copper cylinder plus 250L loft tank both removed. Fitted 250L Megaflo Eco Indirect unvented cylinder in same airing cupboard, cold feed re-plumbed to mains, discharge pipework to external wall tundish outlet, loft tank decommissioned and boxed. Total conversion £3,150 including Building Notice and G3 certificate. Loft ready for conversion works.

Vented cylinder work across every London borough

Barking and DagenhamBarnetBexleyBrentBromleyCamdenCroydonEalingEnfieldGreenwichHackneyHammersmith and FulhamHaringeyHarrowHaveringHillingdonHounslowIslingtonKensington and ChelseaKingston upon ThamesLambethLewishamMertonNewhamRedbridgeRichmond upon ThamesSouthwarkSuttonTower HamletsWaltham ForestWandsworthWestminster

Frequently asked questions

What is a vented cylinder and how is it different from unvented?
A vented cylinder is fed by a header tank sitting above the cylinder — typically in the loft. The cold water reaches the cylinder by gravity from the tank, so the working pressure is set by the tank's height above the cylinder (about 0.1 bar per metre head — a 3m head gives 0.3 bar shower pressure). The tank has an open vent to the atmosphere, meaning the cylinder itself is never pressurised above the tank head. An unvented cylinder is fed directly from the mains supply, so it operates at 3–4 bar mains pressure and delivers strong shower flow from every outlet without any loft tank required.
Should I replace like-for-like or upgrade to unvented?
Depends on four things. (1) If you have a working heat-only boiler you want to keep, like-for-like vented replacement is simpler. (2) If you have loft space available for the tank, either option works. (3) If your priority is minimum cost and functional hot water, vented is the answer. (4) If you want mains-pressure showers, want to remove the loft tank flood risk, are planning a loft conversion, or expect to sell within 2 years, upgrade to unvented is the answer. On balance most upgrades we do are unvented conversions — the mains-pressure benefit is the most-cited driver.
What is a Fortic cylinder?
A Fortic cylinder is a compact vented cylinder with a small header tank built into the top of the cylinder itself rather than mounted separately in the loft. This removes the need for a separate loft tank — the entire hot-water storage footprint fits in an airing cupboard or a kitchen tall cabinet. Fortic cylinders are extremely common in ex-council flats, post-war London social housing, and small studios or 1-bed conversions where there is no loft. Most Fortic cylinders are direct (immersion only) on Economy 7 tariff, though indirect Fortic variants exist.
Can I keep my Economy 7 tariff with a modern replacement cylinder?
Yes — a modern direct (immersion-only) cylinder including Fortic is Economy 7 compatible. We fit a smart Economy 7 timer with the replacement as standard practice. On newer smart tariffs (Octopus Go, Agile) the same cylinder plus a Wattmaster or Mixergy smart controller can deliver even lower running costs than legacy Economy 7 by timing the immersion cycle to the half-hourly cheapest slots.
How long does a like-for-like replacement take?
A like-for-like vented cylinder replacement typically completes in 3–5 hours. A Fortic replacement in 2–4 hours (no separate tank work). A vented-to-unvented conversion 1 day (includes tank decommission, cold-feed re-plumb, discharge pipework, and G3 certification). All installs include a written report of the work done, replacement warranty documentation, and waste haulage of the old cylinder.
What certification do you hold?
For vented cylinder work, Water Regulations 1999 competency (WaterSafe registration, UK Certification Ltd certificate 136356 issued 8 September 2025, expiry 18 August 2030). For vented-to-unvented conversions we also carry the HWSS G3 competency certificate (certificate 136359, same period) — required by law for any unvented cylinder install. Both certificates are supplied with the install quote.
Is my loft header tank likely to fail?
Header tanks fail more often than customers realise. The typical failure modes are: (1) ball-valve wear producing a slow overflow that goes unnoticed until the loft insulation is saturated, (2) plastic tank material becoming brittle after 20–25 years and splitting in a cold snap, (3) tank collapse under insulation weight if the support platform was under-specified. Tanks over 20 years old should either be replaced (£350–£550) or removed entirely as part of an unvented conversion (removes the risk permanently).
Do you offer emergency same-day cylinder work?
Yes — same-day dispatch on the emergency line for cylinder failures with no hot water. Immersion element failures can often be replaced same visit from stock. Cylinder leaks are isolated and drained immediately and a replacement quote issued on the visit. Fortic cylinders sometimes require a next-day return because compact-format sizes are less commonly stocked on the van.
Do you carry public liability insurance?
Yes — £5,000,000 public liability via SiriusPoint International Insurance Corporation (UK Branch) acting through Eaton Gate MGU Ltd, policy number BE26ACTT000000018221, current period 07/05/2026 to 06/05/2027.
Do I need Building Regulations approval for a vented cylinder replacement?
No — a like-for-like vented cylinder replacement is not notifiable work under Building Regulations. No Building Notice or Building Control involvement required. The G3 notification framework only applies to unvented cylinders. Vented cylinder installs do still require Water Regulations 1999 competency for the water-supply connections — WaterSafe registration is what covers this.

Related services

Hot water cylinder replacement — main hub

Full cylinder replacement process and type comparison.

Indirect unvented cylinder

The upgrade path from vented indirect to mains-pressure unvented.

Unvented cylinders — overview

Type comparison and G3 safety framework.

Direct unvented cylinder

Electric-only unvented for all-electric properties.

Book a vented cylinder replacement

Copper, stainless, Fortic. Like-for-like or upgrade to unvented — quoted both ways so you can choose.

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