G3 certified · Twin immersion · Smart tariff ready
Direct Unvented Cylinder London
Electric-heated unvented cylinders for all-electric flats, retrofit heat-pump properties, loft conversions and HMO backup DHW. Megaflo Eco, Telford Tempest, Gledhill Stainless Lite and OSO Super S. Building Notice and G3 certificate included on every install.
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Direct unvented cylinders — when they are the right choice
A direct unvented cylinder heats the stored water electrically via one or two immersion elements immersed directly in the water. There is no coil, no boiler primary connection, no gas involvement — the electricity does all the work. This is the correct cylinder specification in four scenarios that are common in modern London property: all-electric new-builds where there is no gas supply to the property, retrofits where a gas boiler has been removed in favour of an air-source heat pump, small loft-conversion bathrooms where the existing boiler has no spare capacity for a secondary coil, and HMO backup DHW where landlords want a second cylinder online in case the primary boiler-fed indirect fails.
The reason direct cylinders have historically been considered expensive to run in the UK is straightforward — electricity per kWh costs 3–4 times more than gas per kWh on standard variable-rate tariffs. What has changed the economics is the arrival of smart half-hourly tariffs (Octopus Agile, Octopus Go, Economy 7 and Economy 10) and cylinder-side smart controllers (Wattmaster, Mixergy, Sunamp) that time the immersion heating cycle to run during the cheapest half-hour slots of the day. Combined with a large-format storage cylinder (250L+) holding 2–3 days of hot water at cheap-rate temperature, actual hot water running cost on a modern direct cylinder can be 40–60% below the flat-rate electric benchmark.
The four brands we consistently install for London customers: Megaflo Eco (Heatrae Sadia — the market standard for like-for-like replacement, widely stocked, spare-parts backing), Telford Tempest (good-value duplex stainless with strong warranty), Gledhill Stainless Lite (compact form-factor for tight spaces — often the only cylinder that will fit in a period-property airing cupboard), and OSO Super S (premium Norwegian construction, 316L stainless, expensive but exceptional lifespan). All four carry a 25-year cylinder warranty provided the install is done by a G3 competent engineer with the Building Notice submitted to Building Control.
Every direct cylinder we install is by a G3 competent engineer (UK Certification Ltd certificate 136359 issued 8 September 2025, expiry 18 August 2030) with Water Regulations 1999 competency (WaterSafe registration, certificate 136356 same period). Public liability £5,000,000 via SiriusPoint through Eaton Gate MGU, policy BE26ACTT000000018221, period 07/05/2026 to 06/05/2027.
When a direct unvented cylinder is the right specification
All-electric property (no gas supply)
Modern London new-builds — particularly high-rise apartments and social-housing conversions — are frequently all-electric. No gas boiler feed to the cylinder means the water is heated electrically at the cylinder itself. A direct unvented cylinder with one or two Incoloy immersion elements is the standard specification.
Retrofit where gas boiler removed for heat pump
Domestic heat pumps typically heat only the low-temperature space-heating circuit, not the hot water at storage temperature. In retrofit installations the DHW cylinder is often converted to direct (electric) operation using the immersion, with the heat pump handling only the wet radiator or UFH loop. AK handles the cylinder-side conversion.
Loft conversion with new bathroom, no coil available
Where a new loft bathroom is added and there is no capacity in the existing boiler to feed a secondary coil (or the coil-to-flow distance is too long), a small direct unvented cylinder feeding the loft bathroom only is a common compact solution. 50–100 litre size, twin immersion for redundancy.
Backup DHW for high-availability requirements
HMOs and small serviced-apartment blocks sometimes specify an electric direct cylinder as backup DHW in addition to the primary boiler-fed indirect cylinder. If the boiler fails, the direct cylinder maintains hot water availability for tenants until the boiler is repaired.
Sizing guide — how many litres for how many people
Rule of thumb: 50L per occupant for baths, 30L for showers only. Adjust up if there is a spa bath or if the property has multiple simultaneous demand (e.g. two showers running).
| Occupants / property type | Cylinder size | Immersion spec |
|---|---|---|
| 1 person studio | 80–100 litres | Single 3kW |
| 1–2 people 1-bed flat | 120–150 litres | Twin 3kW (redundancy) |
| 2–3 people 2-bed flat | 180–210 litres | Twin 3kW |
| 3–4 people 3-bed flat | 210–250 litres | Twin 3kW |
| 4–5 people 4-bed home | 250–300 litres | Twin 3kW + off-peak timer |
The six specification highlights
Duplex stainless steel construction
All recommended cylinders (Megaflo Eco, Telford Tempest, Gledhill Stainless Lite, OSO Super S) use duplex or high-grade 316 stainless steel construction with a Building Regs G3 25-year cylinder warranty typically included.
Twin Incoloy immersion elements
Direct cylinders in domestic use should specify twin 3kW immersion elements as standard — Incoloy 825 alloy for scale and corrosion resistance in London's hard-water regions. Twin elements allow the cylinder to remain in service if one immersion fails.
Factory-fitted G3 safety kit
Every unvented cylinder ships with the required G3 safety kit factory-installed on the cold feed: pressure-reducing valve, expansion vessel, single-check valve, tundish, temperature and pressure relief valve, expansion relief valve. AK verifies and re-torques all connections on install.
Off-peak timer compatibility
For customers on Economy 7, Economy 10, or the newer Agile Octopus half-hourly tariff, we install with a smart timer that uses the cheaper night-rate slot for the main heat cycle plus a booster during the day if needed. Cuts DHW energy cost by 30–45%.
BS EN 12897 pressure rating
Standard unvented cylinders are rated to 6–10 bar working pressure. A modern London mains typically delivers 3–4 bar at the internal stop-tap. A pressure-reducing valve on the cold feed sets the operating pressure to 3.5 bar (matched to the expansion vessel pre-charge) — protects the cylinder and downstream fittings.
G3 competency certificate
Every direct unvented cylinder install includes a G3 competency certificate from a certified engineer (UK Certification Ltd certificate 136359 issued 8 September 2025, expiry 18 August 2030) plus a Building Notice submitted to the local authority Building Control team.
Cost — direct unvented cylinder installation
| Scope | Price (inc. VAT) | Includes |
|---|---|---|
| Direct unvented cylinder — supply and installation (150L) | £1,650–£1,950 | Cylinder (Megaflo Eco 150L or equivalent), twin 3kW Incoloy immersions, factory G3 kit, install, Building Notice, G3 certificate |
| Direct unvented cylinder — supply and installation (210L) | £1,950–£2,350 | Cylinder 210L, twin 3kW immersions, install, Building Notice, G3 certificate |
| Direct unvented cylinder — supply and installation (250L) | £2,250–£2,650 | Cylinder 250L, twin 3kW immersions, install, Building Notice, G3 certificate |
| Replacement direct unvented (like-for-like same location) | £1,850–£2,450 | Cylinder replacement, immersion elements, safety kit refresh, waste haulage of old unit |
| New install with off-peak smart timer | +£200–£350 | Smart tariff-aware timer supply and integration with cylinder control |
| Annual G3 service (both immersions inspected, kit tested) | £150–£195 | Immersion element check, T&P relief valve test, expansion vessel pre-charge, tundish clear, service report |
Real London direct cylinder installs
Peckham 1-bed flat — replacement of 20-year-old direct cylinder
160L direct unvented cylinder from 2005 failing on the primary immersion, corroded flange gasket weeping. Full swap to Megaflo Eco 150L with twin 3kW Incoloys, Wattmaster smart timer for Economy 7. Full G3 kit refreshed. Total £1,850 including Building Notice.
Islington new-build studio — first-fit install after developer pull-out
Studio flat sold with cylinder space stubbed for install but no cylinder specified. Fitted 80L Telford Tempest with single 3kW immersion, factory G3 kit. Building Notice, G3 certificate. Total £1,650.
Camden loft conversion — small dedicated cylinder for new bathroom
Loft conversion with new en-suite bathroom, existing gas boiler at capacity for the main house. Fitted 50L Gledhill Stainless Lite direct unvented in the loft void feeding the new bathroom only. Twin immersion for redundancy given loft access. Total £1,750 including cabinet and Building Notice.
Direct unvented cylinder installation across every London borough
Frequently asked questions
What is the difference between a direct and indirect unvented cylinder?
Do I need a Building Notice for a direct unvented cylinder installation?
What certification do you hold for unvented cylinder work?
How long does the install take?
What is the annual service on a direct unvented cylinder?
Can I use a smart tariff like Octopus Agile to heat the water cheaply?
What cylinder brand do you recommend?
Do you carry public liability insurance?
Can I convert my old vented cylinder to unvented?
How quickly can you attend for an emergency direct cylinder failure?
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