A direct cylinder is the simplest, cleanest way to deliver mains-pressure hot water to every tap and shower in a UK home — without a boiler, without a flue, and without a loft tank. Cold mains water enters at the bottom, an electric immersion heater brings it up to temperature, and it's stored ready for use at full mains pressure. That's the whole system. If your property is all-electric, running off solar PV, or you simply want to decouple hot water from your heating system, a direct unvented cylinder is almost always the right answer.
This guide walks through exactly how direct cylinders work, why they don't need a boiler, the specific cases where they can sit alongside one, and the applications where they genuinely excel. We'll then look at our three featured ranges — OSO SuperXpress, Joule Cyclone and Mixergy X — side by side, so you can see which fits your household.
How a Direct Cylinder Works
Direct cylinders are mechanically simpler than indirect models because there's no coil, no pump, no boiler interface — just mains water, an electric element, and insulation.
Mains water enters the tank
Cold water feeds into the bottom of the cylinder through a dedicated supply. A pressure reducing valve (PRV) regulates incoming mains pressure to keep it within safe limits for the tank.
Immersion heater warms the water
One or two electric immersion elements — typically 3kW each, giving a 6kW total on twin-element models — heat the stored water directly. The thermostat holds it at a set temperature, usually 60–70 °C.
Safety valves handle pressure
An expansion vessel absorbs water-expansion pressure as it heats. A temperature and pressure relief valve (TPRV) vents to a tundish if something goes wrong — all mandatory under Part G of Building Regulations.
Hot water exits at mains pressure
Hot water leaves from the top of the cylinder and travels to your taps, showers and appliances at the same pressure as the incoming mains — no pump, no gravity, no pressure drop across multiple outlets.
A boiler's job is to burn fuel — gas, oil, LPG — or to use electric resistance to heat water that's then pumped around your home. An indirect cylinder includes a coil inside the tank, and hot water from the boiler passes through that coil to warm the cylinder's water. No coil, no boiler input. A direct cylinder has no coil. Instead, the water is heated directly by the immersion element sitting inside the tank itself.
That means your hot water system is completely independent of any central heating appliance. You don't need a gas supply, a flue, or a meter. You don't need a pump. You don't need a separate boiler servicing contract. The cylinder runs purely on electricity — from the grid, from rooftop solar, from an off-peak tariff, or from all three. For flats with no gas, for new-build all-electric homes, for off-grid properties and for anyone consciously moving away from combustion, a direct cylinder is the most self-contained hot water solution available.
When Direct Cylinders Can Work Alongside a Boiler
Direct cylinders don't physically connect to a boiler — they have no coil to accept hot water from one. But that doesn't mean boiler-and-cylinder pairings are off the table. The key distinction is between heating your cylinder (which needs an indirect cylinder with a coil) and heating your home (which a boiler can still do independently while a direct cylinder handles hot water).
The rule: if you want a boiler to heat your cylinder, you need an indirect unvented cylinder. If you want a boiler to heat your radiators only, and your cylinder to handle hot water independently, a direct cylinder works perfectly.
Scenario 1: Combi boiler for central heating, direct cylinder for hot water backup
Combi boilers produce hot water on demand but can struggle with simultaneous outlets (two showers, for instance) or very large homes. Pairing a combi with a direct cylinder gives you high-pressure stored hot water as a buffer, while the combi continues to drive the radiator circuit. Useful in family homes where the existing combi stays in place.
Scenario 2: Heat-only or system boiler feeding radiators only
If your boiler only runs a central heating circuit (not hot water), a direct cylinder can run completely independently on electricity. Common in older properties being partially upgraded, or in homes with a heat-only electric boiler for radiators.
Scenario 3: Transitional all-electric setup
Homeowners phasing out gas often fit a direct cylinder first (to eliminate gas for hot water) while keeping the gas boiler temporarily on radiators. Once radiators move to electric or a heat pump is added, the gas connection can be cut entirely. A direct cylinder is the foundation of that transition — and it can later be paired with solar PV or a heat pump for deeper savings.
Scenario 4: Apartments and flats with district heating
Many newer flats have communal heat networks for space heating but need their own hot water provision. A direct cylinder installed in a utility cupboard handles the hot water side entirely on electricity, fully metered to that flat — ideal where the district system doesn't cover domestic hot water.
Where Direct Cylinders Genuinely Excel
Direct cylinders aren't just for properties without gas. Across dozens of UK property types, the simplicity of electric-only hot water is a clear win.
All-electric new builds
The Future Homes Standard is steering new UK homes away from gas. Direct cylinders are a compliant, cost-effective way to deliver hot water without a combustion appliance on site.
Flats and apartments
No flue, no gas meter, no communal combustion. Direct cylinders fit cleanly into utility cupboards and share a footprint with a washing machine on compact OSO models.
Solar PV households
Pair with a solar immersion controller and the cylinder soaks up surplus PV generation, turning daytime solar into evening hot water — saving up to 40% on bills with smart models.
Economy 7 & time-of-use tariffs
Twin-element cylinders with dual-tariff wiring can heat overnight on cheap rates, then boost on demand. Ideal for households on Octopus Agile, Economy 7, or Intelligent Octopus Go.
Retrofit on old vented systems
Replacing an old vented cylinder and loft tank with a direct unvented unit removes the header tank entirely — cleaner loft, better pressure, higher flow rate at every outlet.
Off-grid and rural properties
Combined with solar PV and a battery storage system, a direct cylinder can deliver fully off-grid hot water without any fuel deliveries.
Holiday lets & second homes
No gas safety certificate, no annual boiler service, and the immersion can be turned off remotely between stays. Low-maintenance hot water for properties that sit empty.
Loft conversions & small spaces
Slimline direct cylinders (485–580mm diameter) fit where a full-size cylinder won't — under eaves, in airing cupboards, or beside a washing machine.
At-a-glance application list
OSO SuperXpress — The Pre-Plumbed Benchmark
OSO Hotwater is Europe's largest manufacturer of stainless steel cylinders, based in Norway since 1932. The SuperXpress (S2X) is their flagship direct range: a pre-plumbed, pre-wired cylinder that arrives with the expansion vessel, multifunction valve and blending valve already fitted under a hidden insulated lid. Installation time is cut by up to 70% compared to conventional cylinders, and every tank comes with a 25-year warranty on the duplex stainless steel body. For new-build and retrofit alike, it's the installer's benchmark — and it produces up to 30% more usable hot water than competing cylinders of the same volume.
OSO S2X 150L Pre-Plumbed Direct
Compact 150-litre pre-plumbed direct cylinder with 6kW twin immersion, duplex stainless steel body and 25-year warranty. Shares a footprint with a washing machine — ideal for flats and small utility cupboards.
OSO S2X 210L Pre-Plumbed Direct
The popular family-size 210L SuperXpress with 6kW output and pre-plumbed expansion kit. Handles 3–4 person households with a bath and two showers. Duplex stainless steel, 25-year warranty.
OSO Saga Charge 200 Smart Direct
The smart successor to the SuperXpress. Wi-Fi connected, ESA time-of-use tariff ready, and supercharged recovery — charges the tank on cheap off-peak electricity and delivers full hot water through the day.
See the full OSO Hotwater range
Joule Cyclone — UK's Fastest-Growing Cylinder Brand
Joule is an Irish-designed cylinder range that has become the UK's fastest-growing hot water cylinder brand — largely because it combines duplex stainless steel construction with prices that undercut most competitors. The Cyclone range spans budget (Invacyl, white finish) through to premium (Cyclone Air, grey metallic finish), all using 316L stainless fittings and offering a 25-year tank warranty. Two factory-fitted immersion elements come as standard on every direct model, with a full G3 unvented kit included in the box.
Joule Invacyl Direct 90–300L
Budget-friendly direct cylinder in a clean white finish. Duplex stainless steel body, two immersion heaters as standard, full G3 kit included. Available from 90L up to 300L — the widest range on the market.
Joule Cyclone Plus Direct 120–300L
The signature Cyclone Plus range with attractive grey casing, duplex stainless steel body and comprehensive controls pack. A step up from the Invacyl with improved insulation and superior recovery rates.
Joule Cyclone Air Direct 100–300L
Joule's premium Cyclone Air range — the highest-spec direct cylinder in the lineup. Improved thermal performance, robust construction and polished grey exterior. Range from compact 100L flat dwellings up to 300L family properties.
Mixergy X — The Smart Cylinder
Mixergy takes direct cylinder technology into genuinely smart territory. The patented volumetric heating system heats only the portion of water you actually need (top-down rather than the whole tank), delivering hot water up to 10× faster than a conventional cylinder and using less electricity to do it. The Mixergy app gives you real-time hot water levels, smart scheduling, voice control via Alexa or Google Home, and integration with dynamic tariffs like Octopus Agile. Energy Savings Trust–verified bill reductions, a 25-year warranty, and full machine-learning optimisation built in.
Mixergy X Direct 180L Smart
Compact smart direct cylinder with Wi-Fi, app control, volumetric heating and smart tariff integration. Delivers hot water 10× faster than conventional tanks and saves up to 40% on bills with solar PV.
Mixergy X Direct 210L Smart
Family-size smart direct cylinder with full Mixergy iO integration. Machine-learning adapts to your usage pattern automatically. Octopus Agile and dynamic tariff ready for maximum off-peak savings.
Mixergy X Direct 250L Smart
Large-household smart direct cylinder for 4–5 person homes. Full volumetric heating, Mixergy app, voice control and smart tariff integration. 25-year warranty and proven compliance uplift on SAP 10.2.
Comparison at a Glance
The three brands solve different problems. OSO leads on pre-plumbed installation speed and Norwegian build quality; Joule leads on price-to-spec value; Mixergy leads on smart controls and demand-side energy savings. Here's how they stack up on the specs that matter most.
| Feature | OSO SuperXpress S2X | Joule Cyclone Plus | Mixergy X Direct |
|---|---|---|---|
| Tank material | Duplex stainless steel | Duplex stainless steel | Duplex stainless steel |
| Immersion heaters | 2 × 3kW (6kW total) | 2 × 3kW (6kW total) | Electric immersion + optional heat exchanger |
| Pre-plumbed | Yes — factory fitted | No — G3 kit supplied | No — G3 kit supplied |
| Smart / app control | Saga Charge model only | No | Yes — full app & API |
| Sizes available | 150 / 210 / 300L | 90 / 120 / 150 / 180 / 210 / 250 / 300L | 120 / 150 / 180 / 210 / 250 / 300L+ |
| Tank warranty | 25 years | 25 years | 25 years |
| Starting price (inc. VAT) | £990 (150L) | £495 (Invacyl 90L) | £1,700 (180L) |
| Best for | Speed of install, trade fits | Value-driven homeowners | Smart homes, solar PV, dynamic tariffs |
Useful Add-Ons for a Direct Cylinder Install
A direct cylinder install is rarely just the tank. Depending on your setup, you may also need any of the following — all available at Aizo Quality Heating.
Potable expansion vessels
If your cylinder isn't pre-plumbed (i.e. not an OSO SuperXpress), you'll need a suitably sized potable expansion vessel to absorb water expansion on heating.
Shop expansion vessels →Solar immersion controllers
Diverters like the myenergi Eddi and Marlec Solar iBoost send surplus PV generation to the immersion element — free hot water from sunshine.
Shop immersion controllers →Water pressure booster systems
If your mains pressure is below 1.5 bar, an accumulator vessel or booster pump will be needed to get the most from an unvented cylinder.
Shop pressure boosters →Water softeners & scale inhibitors
In hard-water areas, a softener or scale inhibitor protects the immersion element and internal surfaces, extending the cylinder's working life.
Shop water softeners →The Bottom Line
A direct unvented cylinder is the right choice when your home is, or is becoming, all-electric. It's the right choice when you want to separate your hot water from your heating, when you're on a smart tariff or have solar PV, or when gas simply isn't on the table. It delivers mains-pressure hot water to every outlet from a single, self-contained unit — no boiler, no flue, no loft tank. Direct cylinders can work alongside a boiler handling the radiators, but they don't need one, and that independence is their single biggest strength.
If you want the fastest installation, the strongest pre-plumbed trade reputation and Norwegian engineering, go OSO SuperXpress. If you want duplex stainless steel quality at the best price per litre, go Joule Cyclone. If you want genuine smart control, Energy Savings Trust–verified savings and solar/tariff integration, go Mixergy X. All three are stocked at Aizo Quality Heating with free UK delivery over £150 and expert advice a phone call away.
Installation note
All unvented cylinders must be installed by a G3-qualified engineer under Part G of the Building Regulations. This isn't a DIY job. If you're unsure about sizing or electrical capacity, our team can specify the right cylinder for your household — request a quote and we'll come back with a full proposal.
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