tank in tank indirect water heater;unvented hot water cylinders ; tank in tank hot water cylinder; high recovery unvented cylinderFlexiheat UK

Tank in tank cylinder

tank in tank indirect water heater;unvented hot water cylinders ; tank in tank hot water cylinder; high recovery unvented cylinder


The tank in tank hot water cylinder or hot water system is essentially made up of two tanks, one within the other. The transmission of heat from the exterior or primary tank to the interior or secondary tank (Domestic Hot Water) flows throughout the whole surface of the hot water cylinder.

They are an energy efficient indirect cylinder with fast hot water recovery times making them a great solution for homes with a high demand for hot water, and commercial and industrial water heating applications, as these hot water tanks perform well.

They are powered or heated directly by central heating systems, that may be a system boiler that is doing the central heating system as well. They are suited to applications where a combi boiler / combi boilers simply can’t give you the required hot water flow rate, and you need to store hot water for larger homes or commercial and industrial applications to provide hot water for showers, baths or taps.

What are tank in tank water heaters ?

working principle of tank in tank water heaters Flexiheat UK

The water in the outer tank, also known as the primary tank, is heated by an external heat source such as a gas boiler, heat pump, solar panels, (that is often doing the central heating requirements) this heated water travels through this hot water cylinder and transfers thermal energy to the water in the inner tank, generally referred to as the domestic hot water storage tank.

This has many advantages over an older conventional or older technology hot water system, that uses a coil within the indirect hot water cylinder.

These units are not a direct hot water cylinder – the water in a direct hot water cylinder is heated by an immersion heater located within the cylinder. As a result, the cylinder is not linked to a boiler.

Double walled hot water systems

Sometimes these hot water cylinders, are also called “double walled hot water cylinders ” or “double walled calorifiers” which again unlike a traditional indirect hot water cylinder or calorifiers, which utilise a spiral tube to heat the water, these calorifiers use a double wall concept, which is far more efficient. The hot water is heated in the same way as a “coil type” indirect hot water cylinder. In that an external source of heat is used to indirectly – via a heat exchanger – to provide hot water.

These double wall calorifiers feature a substantially larger heating area than a standard heating spiral tube due to the double wall concept. This explains why double-walled calorifiers heat water much quicker than traditional calorifiers / indirect cylinders.

Tank in tank hot water tank

tank in tank cylinder installation schematic for domestic hot water heating with a boiler as the heating source

The hot water cylinders tank warms a huge volume of domestic hot water in a short period of time due to the large heat exchanger surface (1.5 to 2.5 times greater than a typical coil type hot water cylinder). This minimises the amount of water that must be stored while also reducing energy loss, but with high hot water flow rates.

Often these hot water cylinders are known as high recovery unvented cylinders (when used on an unvented system) or high recovery vented hot water cylinders (when used on vented systems) depending on the domestic hot water heating system they are installed in.

Benefits of these hot water cylinders

tank in tank hot water ; new boiler or thermal store on the ground floor no cold water tank required


Internal stainless steel hot water tank

Nickel-chromium-molybdenum Stainless steel (AISI 316 L stainless steel) DHW hot water tank that is extremely resistant to pitting produced by halogen elements in potable water such as chlorine.

Hot water cylinder working pressures and temperatures

  • DHW storage tank maximum operating pressure: 8 Bar pressure (10 bar optional)
  • Maximum operating temperature of DHW storage tank: 90 C
  • Maximum operating pressure of the surrounding tank (primary circuit): 3 Bar pressure
  • Maximum operating temperature of the surrounding tank (primary circuit): 110 C

The primary circuit is either fed by a central heating system or a dedicated heat source and must be a sealed system.

Anti-Legionella Design

The extensive heat exchanger surface of the hot water tank, which is totally immersed in the primary water, permits the hot water to be kept at a consistent temperature of at least 60°C, eliminating legionella development within the hot water cylinder, with no cold zones inside the hot water tank.

Maintenance free hot water systems

This DHW tank requires no maintenance as it has no internal heat exchange components. In typical potable water conditions, cathodic protection is not required. The optional electric immersion heater for the electric heating models is in the primary circuit, thus there is no chance of lime scale or corrosion to the immersion heater.


The inner tank of the hot water cylinder is completely corrugated, also inner hot water storage cylinder is suspended and hence able to flex. This design provides significant pressure resistance and prevents lime scale adhesion by allowing the cylinder to expand and contract under hot water pressure. The corrugated walls vibrate in reaction to pressure fluctuations, preventing lime scale deposits from adhering and ensuring long-term efficiency.

Immersion heaters

An immersion heater can also be installed as a backup for the primary heating source. With the option of either AISI 321 flanged electric immersion heater elements or threaded immersion electric immersion heater elements range from 1.5kW all the way up to 12 kW depending on the hot water cylinder size.

immersion heaters for our hot water heaters from Flexiheat UK

Product approvals

These are WRAS approved hot water tanks as well as Directive 2014/68/UE: European Pressure Equipment Directive approved and are ErP compliant.

WRAS approved hot water cylinders product

WRAS Approved hot water cylinders Product

Vented hot water cylinder

These hot water cylinders can be used as a vented hot water cylinder whereby they are supplied cold water from a cold water storage tank, feeder tank or header tank (often positioned in the attic or loft space) and utilise gravity to circulate the hot water supply throughout the building.

The issue is that the cold water storage tank height above the vented hot water cylinder dictates the water pressure you get out of hot water system. They also require a vent pipe, as this allows the hot water to expand up the vent pipe from the hot water storage cylinder, as the water heats up in vented systems.

Vented cylinders were often copper tanks, as they didn’t have to deal with the higher pressure of mains water pressure, and this is the reason that unvented hot water cylinders are made of stainless steel these days so they cope with the cold water mains pressure that enters the indirect hot water cylinder heating system.

As open vent systems (which shoud be used on a wood burner or solid fuel boiler etc), introduce air into the central heating water system, and the outer wall of our tank in tank is manufactured in carbon steel, which when subjected to oxygen and water will corrode – thus you cannot use these twin wall hot water cylinders as vented hot water cylinders. Please use our high gain stainless steel indirect hot water cylinders with a high-performance coil, our GX – HL range as detailed on this brochure – page 26 of the following brochure.

Stainless Steel cylinders for domestic hot water production and storage brochure Flexiheat UK

These units don’t requireas any additional safety features that are required for an unvented hot water cylinder.

Unvented hot water cylinders

Unvented hot water cylinders came onto the UK markets in the mid 1980’s, since then unvented cylinders have gained in popularity to become the most popular hot water cylinder used these days due to the higher water pressure an unvented cylinder can handle

There is no separate cold water tank required with an unvented hot water cylinder system; instead, the sealed hot water cylinder is supplied directly from the cold water mains. Because they operate at mains pressure, they normally provide substantially larger flow rates, ensuring that your mixer showers and bath performance will be enhanced compared to a vented cylinder.

Unvented cylinders require an expansion vessel or internal air bubble (also known as a bubble top unit) to allow heated water to expand, and this is where they differ from vented hot water cylinders, as the cold water heats up, this expansion must be accommodated in unvented hot water cylinders.

Product Range

With a full range of tank in tank indirect water heaters from 60 Litres all the way up to 712 Litres of DHW storage

a picture showing unvented hot water cylinders from 60 to 500 litre dimensions Flexiheat UK


Technical details

unvented hot water cylinders from 60 to 500 litre data table Flexiheat UK

And the larger units up to 712 Litres 
unvented hot water cylinders from 140 to 1000 litre dimensions from Flexiheat UK

Technical details

unvented hot water cylinders from 140 to 1000 litre data table Flexiheat UK

The brochure can be download here –

Flexiheat Tank in Tank Hot Water Cylinder Brochure 

Domestic hot water performancehot water storage cylinders pressure and ERP ratings Flexiheat UK

Based on a Primary at 85°C and cold water Inlet at 10°C

water heating output figures for our tank in tank indirect water heater range - Flexiheat UK

Based on primary water at 85C up to the 600 model for the larger sizes units – please see the brochure

hot water heating performance of our tank in tank water heater range up to 600 litres Flexiheat UK

At other various primary temperatures with domestic hot water risen from 10C to 60C

tank in tank water heater outputs 10 to 60 Degrees C performance data 90 to 600 Litre from Flexiheat UK


and with domestic hot water risen from 10C to 45C

tank in tank hot water outputs 10 to 45 Degrees C performance data Flexiheat UK

Should you require ant help on commercial water heating sizing – then please see our webpage here – https://www.flexiheatuk.com/how-to-size-a-commercial-water-heater/

Why our tank in tank indirect water heater is the best on the market

We have a top opening of 200mm, which is manufactured by a bending process of the stainless steel sheet (we avoid any welding, which could make the surface susceptible to corrosion), used to install a flanged inspection/ cleaning and pipe connection plate to the inner hot water storage cylinder. If a threaded pipe connection gets damaged, you just have to replace this inspection connection plate and fit a new one.

This makes cleaning and inspection of the internal tank easysomething not possible with other manufacturers 

This is not possible with our competitors’ tanks, where you must replace the whole indirect cylinder, who also weld the pipe connections directly to the upper dished end from the outside, this being precisely the weakest point in its design and with the greatest risk of corrosion.

tank in tank water heater advantages over other manufacturers - Flexiheat UK

As leading manufacturers of hot water cylinders, good product engineering is important to us for the longevity of our tank cylinders. The flanged top opening plate (200mm) is used first to protect welds directly with an inert atmosphere during the welding process, achieving the best possible internal weld quality result, and second, to pickle and passivate the internal welding zone between the dished ends and the walls of the inner stainless steel tank. This is important to achieve a perfect finish on the welded zone and all the surfaces in contact with the domestic hot water.

If you require any further assistance with our tank in tank hot water cylinder / tank in tank water heater range which are high recovery hot water cylinders, please click here to contact us via email or call us direct on 01202 822221.