indirect unvented cylinders stainless steel Flexiheat UKFlexiheat UK

Indirect unvented cylinders – Unvented hot water cylinders

indirect unvented cylinders stainless steel Flexiheat UK

Indirect unvented hot water cylinders, take their supply water directly from the mains for the domestic hot water requirements. Because they can produce warm water at mains water pressure, indirect unvented hot water cylinders are great for properties with multiple bathrooms, as they have the flow capacity to run multiple showers, baths or taps. Mains pressure has many benefits, the most important being is a quicker flow rate compared to gravity feed system hot water cylinder heating products.

Indirect unvented water cylinders require an external source of heating, normally a gas boiler, heat pump or an oil boiler, for example. This warm water then circulates via a stainless steel heat exchanger coil in the water cylinder, heating the surrounding domestic water without mixing with it. Thus, the term “indirect” water heating.

indirect unvented cylinders – a picture of the internals showing the heating coil which is stainless steel in which heating hot water flows in our indirect unvented cylinder – the domestic hot water resides in the hot water storage tank of the indirect cylinder fed directly from the mains water system.

Cut away picture of our indirect unvented cylinder / unvented hot water cylinders indirect from Flexiheat UK

A direct unvented cylinder on the other hand, uses electric immersion heaters, which are installed into water tank / cylinder to heat the hot water directly, again at mains water supply pressure. Whereby an indirect cylinder has a heating coil within the tank which is powered by a boiler or other heating source.

Thus, the term mains pressure ” direct unvented cylinders” / direct unvented cylinder

We do an extensive range of this type of direct unvented cylinders on this dedicated webpage – direct unvented cylinder

Indirect unvented hot water cylinders

So, what are the benefits of unvented hot water cylinders?

In one line – its a mains pressure domestic water hot water system, with all the benefits of higher pressure and flow for your hot water heating

The advantage of utilizing an unvented cylinder is that a cold-water feed tank in the property is not necessary,

As the water is fed directly which means that properties with limited space will not have to give up valuable storage space in the attic, which you require with gravity-fed systems. The eradication of this cold-water feed tank also eliminates the possibility of it freezing during prolonged cold spells.

In an unvented system, the pressure of the warm water flowing out of taps and showers is higher

This is due to the fact that water is supplied from the cold-water mains supply rather than a water tank . Higher pressure is especially significant when there are many bathrooms that may be running warm water from multiple showers or taps at the same time.

Unvented cylinders can benefit homeowners by reducing noise.

This is due to the feed tank not being filled with cold water, as none is required in an unvented system

Because an unvented system is a closed water supply system

It benefits from reduced danger of contamination, with fewer opportunities for foreign contaminants to enter the cylinder and consequently cause harm or damage to the domestic water heating system. This is a real possibility with vented cylinders in a property.

Unvented cylinders

Unvented cylinders require an internal air bubble or expansion vessel to be installed to work correctly, as any stored water in the cylinder expands when it heats up. This heated volume expands and needs to be absorbed, so that the pressure does not get excessive, and a vessel or air bubble takes this expansion.

Expansion vessels are installed external to the unvented cylinder, whilst an internal air bubble is obviously internal to the unvented cylinder, as the name suggests, these systems are used by the heatrae sadia megaflo  or sadia megaflo eco units.

expansion vessel problems with an unvented system; megaflo eco;

Our innovating and unique solution to this problem is a self-sustaining air gap system for our unvented cylinders.

Details on how the system works on unvented cylinders

The mixing of liquid with air or, more precisely, the induction of air bubbles into the liquid, is what distinguishes the patented self-sustaining system technology, with the system now capable of maintaining the internal air gap when hot water is drawn off farther upstream during hot water demand i.e., when taking a shower, bath or using the hot taps in the kitchen or sinks.

There is no requirement for expansion vessel refilling or regular servicing since it continually replaces the internal expansion mechanism in this high quality cylinder.

Because there are no internal moving components in the cylinder, and once the Venturi valve is installed, no more parts are required, this automated replenishment of the air gap saves time and money. This pinnacle of fit and forget technology will provide landlords, developers, self-builders and heating installers confidence that they do not have to inspect for wear and tear or be called out to repair problems on a regular basis.

commercial; delivered; collect; trading; England;

This means no external expansion vessel, thus less space is required great for limited space or access locations

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

Because there is no external expansion vessel, the cylinder may be installed in more compact locations, providing builders and heating installers considerably more freedom in where to install the cylinders, whether for a new build or restoration project.

easy installation; access;


Cylinder product accessories and valves

Each cylinder is factory fitted with temperature and pressure relief valves. These valves are pre-calibrated to open at 7 bar or 90°C
Prior to the cylinder, the supplied combined cold water valve can be installed anywhere on the cold water mains. If it is more convenient, it can be positioned near where the mains supply enters the property.

Water Pressure and Flow Rates

The minimal criteria for satisfactory operation of our cylinder are 1.5 bar pressure and a flow rate of 20 litres per minute. The cylinder will continue to function below this level, but two or more hot water outlets will not be able to be used at the same time.

Cylinder hot water supply 

Any type of boilers can feed the cylinder , be it gas boilers, oil boilers, electric boilers or even wood fired boilers.

Sizing an indirect cylinder- How many litres do I need?

Here’s what product manager recommends with the Hot Water Association of the United Kingdom provides a solid rule of thumb that might help you determine the size of your water cylinder.

In a typical home, 30 to 50 litres of warm water per person per day is required

A low-consumption household will require 20 to 30 litres of warm water per person per day.

In a high-consumption residence, 50 to 70 litres of warm water per person per day is recommended.

Please bearing in mind the following –

A standard UK bath requires 60 litres at 60 C with the other 40 litres being cold – to get the mixed water temperature of 40C

A standard UK shower head is around 8 litres of water per minute – will use around 5 litres of 60C water- a drench type shower can use an awful lot more!

However, it should be emphasised that personal behaviours have a significant role in overall warm water usage. Two identically sized families can consume drastically different quantities of warm water, with one needing twice as much as the other, as people usage patterns can differ drastically.

Should your require larger commerical units – then please look at our webpage Commercial hot water cylinders

Unvented indirect hot water cylinders range

All of our indirect unvented hot water cylinder products have a 22mm diameter stainless steel high performance coil & outlet with 1 x 3kw immersion heater, as a back-up direct unit, should you lose your external source of heat indirectly feeding the water cylinder / store.

Our unvented hot water cylinder range is manufactured from duplex stainless steel, which for your information is the best available – information product warranty is 25 years for our unvented cylinders when used in a domestic property, and 5 years when used as commercial indirect unvented cylinder i.e., non-domestic hot water heating applications.

Product ErP information / ErP information product / library ErP information – The 80 to 130 capacity litres cylinders are “A” rated , the 150 to 280 capacity litres cylinders are “A” minus or “B” plus , and the 305 to 500 cylinders are better than “C” label rated for this hot water unvented cylinder range of products.



indirect unvented cylinders dimensions



indirect water heaters technical data table

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

Installation manual is here

Flexiheat manual for direct & indirect hot water cylinders

We supply larger indirect cylinders – the please look at this webpage – Commercial hot water cylinders

Immersion heaters

Our immersion heater is manufactured in Incoloy 825 – which offers great resistance to hard water and are the standard market size of 44.45 mm / 1  3/4″  connection size.

We can also do pre plumbed units as well from 175 Litres to 300 Litres

pre plumbed unvented hot water cylinders from Fexiheat UK;

Unvented indirect hot water cylinder installation

Unvented water cylinders require the installation and maintenance by suitably qualified plumbers / heating engineers since they run at a greater pressure than a vented system and have numerous additional safety precautions incorporated into the hot water unvented cylinders. All heating engineers / plumbers installing the cylinder must be qualified in “G3” unvented cylinder system installs.


Should you require any more information or discount prices on the best indirect unvented cylinder / unvented hot water cylinders on the UK market with a warranty of 25 years (hi-flow commercial units are 5 years) as they are stainless steel, then give our sales team a call on 01202 822221 or use our contact us page, as you can direct shop with us. Follow us on Facebook or Twitter online for our latest offers . We produce full vat invoices for all sales.