Direct unvented cylinders from Flexiheat UKFlexiheat UK

Unvented direct cylinder / unvented direct cylinders

Direct unvented hot water cylinders duplex stainless steel from Flexiheat UK; hot water tanks; hot water system

Direct unvented hot water cylinders in duplex stainless steel from Flexiheat UK

An unvented direct cylinder or direct unvented hot water cylinders (they are both the same thing) vary from a vented cylinder in that the provide mains pressure hot water directly from the cold mains, rather than tank pressure water. The cold water is drawn straight from the main water supply and heated using electric immersion heaters for this direct unvented cylinder type. This direct form of water heating via electricity is 100% efficient.

These hot water storage heaters are also known as unvented direct hot water heaters or direct hot water cylinders.

Gravity fed systems

Unvented refers to the fact that the cylinder is directly fed from the mains water supply. In older times a cylinder used to be fed by a gravity feed system, with a feed and expansion tank in the loft or attic above the cylinder. The distance of the tank above the cylinder is what gave the hot water system the water pressure of the domestic hot water system and made it work correctly- This was called the “head” of the system unfortunately sometime this pressure was low due to the limited distance of the feed and expansion tank above the cylinder and the system wouldn’t work correctly . Then unvented cylinders came along with many benefits and changed everything as they used the mains water pressure to give the cylinder the pressure for the hot water heating system – please see this webpage for a more indepth explination of an unvented hot water system

The cylinder material also changed at this time – as a stainless steel cylinder has many benefits, they can tolerate exceptionally high pressure, paving the way for today’s unvented hot water cylinders. Stainless steel is an excellent product / material for a pressurised system.

The other benefit is that a stainless steel cylinder has a significantly higher resistance to all forms of corrosion, including cracks and stress, than a copper cylinder, making it more durable and guaranteeing a longer lifespan.

Indirect unvented hot water cylinder

An indirect unvented cylinder requires a heat source , which heats the water with gas or oil from a conventional boiler or even a heat pump, via a coil heat exchanger within the unvented water cylinders interior- as seen below in a cutaway picture of our unvented indirect cylinder / indirect unvented hot water heaters.

indirect unvented cylinder; domestic hot water; hot water cylinders; hot water system

An indirect unvented cylinder- which can be powered by a boiler or a heat pump.


The biggest advantage of unvented hot water cylinders is that they deliver superior pressure performance compared to a vented cylinder, since the hot water supply is delivered or fed directly from the cold mains at pressure, which ensures powerful showers at mains pressure. They are the ideal solution for properties with multiple bathrooms with fast delivery high flow rates.

Because an unvented cylinder is a ‘sealed’ system, it necessitates the installation of an expansion vessel to absorb the heated water’s extra volume as it expands within the cylinder. Vented cylinders, on the other hand require an expansion tank in the loft, to cope with the expansion of the hot water system.

Direct unvented cylinder / unvented cylinders

One of the most important advantages of an unvented hot water cylinders is the flexibility it provides when space is limited in the home, as there is no need for a feed and expansion tank that could freeze in the winter or be at risk of contamination.

Although direct unvented systems do require a system to account for the expansion of the hot water whilst water heating, there are various options to achieve this –

The most common is an external expansion vessel which is fitted onto the cylinder’s pipework -such as commonly used on the heatrae sadia megaflo eco range of cylinders.

Which is often bulky, and requires more space for installation, and also requires more fittings and pipework to be installed leading to longer site installation times, and thus more costs. Also, an expansion vessel, can require the replenishment of the air within it periodically

a diagram showing unvented hot water cylinders expansion vessel heating problems

We have solved these issues with a patented system that ensures no replenishment or external expansion vessel is required, with long product life as it has no moving parts, which reduces running costs and lifetime costs. Less space is also required, they are easy to install, with front access to the connections, factory fitted controls and immersion heater or heaters depending on the capacity of the unvented hot water cylinder.

Easy and compact installation for our unvented direct hot water heaters

As no expansion vessel is required – Less space is required for our direct cylinders

space saving installation of an unvented water heater or calorifier

Our unit installed on the left – space saving in comparison

The system uses fluid dynamics to create negative pressure known as the ‘Venturi effect’. With precision design this configuration draws enough air to recharge the air gap as the tank is filled and requires no future maintenance – a truly self-sustaining unvented system.

Flexiheat UK Installing 3S Air Gap device information for unvented hot water cylinders

Full manual is here for download

Flexiheat manual for direct & indirect hot water cylinders

Direct unvented hot water cylinder sizing

Direct unvented cylinders should be sized based on the number of bedrooms, baths, and showers. This should give a hot water tank that is appropriate for the property, but if in any doubt move up to the next capacity in the range to provide some future proofing and flexibility, with more hot water delivery for your taps or showers.

Property                                Direct Hot Water Cylinders Size

1 Bedroom with Bath/Shower         120/150 Litre direct unvented hot water cylinders
2 Bedroom with Bath/Shower         180 Litre direct unvented hot water cylinders
3 Bedroom with 2 Baths/Shower    210 Litre direct unvented hot water cylinders
4 Bedroom with 2 Baths/Shower    250 Litre/300 Litre direct unvented cylinders
Larger Properties                               300 Litres Plus capacity direct unvented cylinders

If you require large units, or units fitted with more heating power, then you need our commercial range – please see our web page – commercial electric water heaters

Unvented hot water cylinders with direct water heating

Advantages of our direct cylinders


• Warranty –An industry leading 25 year’s for the high grade stainless steel tank – ancillary components 1 year

Our Duplex stainless steel cylinder / hot water tanks – ensures that the cylinder has a long and trouble free product life, with low maintenance

• Designed and manufactured in the UK – local support and spares and service back up

Approvals – BBA, CE, WRAS (Water Regulations Approval Scheme is an independent UK certification body for plumbing products and materials, helping business and consumers choose compliant products that keep water safe), KIWA, ISO, WRC.

• ErP Water Heating Efficiency- “B”

• 80 & 100 Litre fitted with 1 x 3kW immersion heater; 130 – 500 Litre fitted with 2 x 3 kW immersion heaters for rapid heat recovery

 Our immersion heater is a high quality unit – made from Incoloy 825 stainless steel

• Inlet diffuser – for optimal blending of cold & hot water and the ideal solution for high flow rates

Optimal blending of the hot and cold water with the inlet diffuser; megaflo eco; heatrae sadia; shop; deliver; cookies; delivery; price; news; page; next day delivery; product code; inc vat;

Optimal blending of the hot and cold water with the inlet diffuser

• Hot water cylinders pressure tested to 10 Bar – top performance from a top brand


Safety device – Factory fitted with 7 Bar/90ºC pressure & temperature relief valve for “G3 building regulation” compliance


Minimal heat loss – 60mm insulation – which is injected polyurethane foam with a outer casing resulting in lower energy use and standing heat loss


Balanced hot & cold-water mains pressure to all outlets ensuring high performance showering


Width – Standard or slimline models (allowing you to utilise smaller installation locations) are available.


• Price – we consider our price to be a great and fair market price for these stainless steel direct heaters.


• Delivery – we normally deliver the units from store to site on a 2/3-day pallet service, although a next day delivery service is also available.


Unvented cylinder range – dimensions and domestic hot water heating times

Unvented direct hot water cylinders in duplex stainless steel dimensions Flexiheat UK; hot water cylinders or hot water system with temperature and pressure relief valve; unvented direct cylinders

Unvented direct hot water cylinders – Dimensions


Dimensions of our electric domestic hot water heaters in duplex stainless steel from Flexiheat UK; hot water system

Heating and recovery times


Hot water heating time data for our hot water unvented direct cylinders Flexiheat UK

Hot water heating time data for our hot water unvented direct cylinders Flexiheat UK

Download the Hot water cylinders direct and indirect unvented brochure from Flexiheat UK

Solar PV domestic hot water

For off-grid homes or properties that want to utilize solar PV (photovoltaics (PV)- which are also know as solar electricity panels) which work by capturing the sun’s energy and converting it into electricity for your property

Thus, you can generate your own renewable electricity by installing solar panels and have a carbon free hot water system for your property or off-grid homes if you use the electric to power the immersion heaters on the domestic hot water tanks.

Unvented hot water cylinders

For more information, prices or lead times on the best direct unvented cylinder / Unvented direct cylinder range on the UK market, manufactured in high grade stainless steel – with a 25 year warranty, which are WRAS approved, and benefits from a built-in expansion vessel, or should you be interested in any other of our heating products on this website, then call us on 01202 822221, or use the contact form above on the website. For the latest offers and product updates follow us on Facebook or Twitter.