Direct unvented cylinders from Flexiheat UKFlexiheat UK

Unvented direct cylinder

Direct unvented cylinders from Flexiheat UK

Direct unvented cylinder 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.

An indirect unvented cylinder, on the other hand, heats the water with gas or oil from a boiler, via a coil heat exchanger within the unvented hot water cylinders interior- as below picture

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The biggest advantage of unvented hot water cylinders is that they deliver superior pressure performance than a vented cylinder since the hot water supply is delivered or fed directly from the cold mains at pressure.

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.

Unvented Cylinders

One of the most important advantages of an unvented cylinder 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 cylinders do require a system to account for the expansion of the hot water, there are various options to achieve this –

The most common is an external expansion vessel which is fitted onto the direct cylinder’s pipework –

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

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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.

Easy and compact installation for our units ( no vessel required )

easy installation; site installation

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 system.

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 years for the tank – ancillary components 1 year


• 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


• Inlet diffuser – for optimal blending of cold & hot water – 

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

• Pressure tested to 10 Bar – top perfomance 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 with outer casing resulting in lower energy use


• 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 heating times

unvented direct cylinders -dimensions standard - from Flexiheat UK; deliver;inc vat; store; litres; inc vat; price


domestic hot water heaters; inc vat; dhw electric heater; inc vat

Heating and recovery times

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Unvented hot water cylinders

For more information on the best direct unvented cylinder / Unvented direct cylinder range on the UK market, manufactured in high quality stainless steel, 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 please give us our sales team a call on 01202 822221, or use the contact form above on the website.