indirect unvented cylinders - stainless steel from Flexiheat UKFlexiheat UK

Indirect unvented cylinders – Unvented hot water cylinders

indirect unvented cylinders - stainless steel from 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 hot 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.

Indirect unvented hot 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 hot water cylinder, heating the surrounding domestic water without mixing with it. Thus, the term “indirect” hot water heating.

unvented hot water cylinders indirect

Cut away picture of our 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 water directly, again at mains water supply pressure.

Thus, the term ” direct unvented cylinders”

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 mains pressure heated domestic hot water, 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. 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 hot 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, 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 an expansion 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.

commercial; delivered; collect; trading; England;

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

easy installation; access;


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 hot water cylinder.

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

A low-consumption household will require 20 to 30 ltrs 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 ltrs at 60 C with the other 40 ltrs being cold – to get the mixed water temperature of 40C

A standard UK shower head is around 8 ltrs of hot water per minute – will use around 5 ltrs 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 hot water usage. Two identically sized families can consume drastically different quantities of hot water, with one needing twice as much as the other, as people usage patterns can differ drastically.

Unvented indirect hot water cylinders range

All of our indirect unvented hot water cylinder models 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 domestic unvented cylinders, and 5 years when used as commercial unvented cylinders i.e., non-domestic applications.

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



indirect unvented cylinders dimensions



indirect water heaters technical data table

Immersion heaters

Our immersion heater is manufactured in Incoloy 825 – which offers great resistance to hard water.

We can also do pre plumbed units as well

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 unvented cylinders. All heating engineers / plumbers must be qualified in “G3” unvented cylinder system installs.


Should you require any more information or 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) , 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 for our latest offers.