Dual immersion cylinder range

A dual immersion cylinder or twin immersion direct cylinder are hot water cylinders that have two immersion heaters fitted to the cylinder.

Thus these direct hot water cylinders are not powered by a boiler, but with heating elements powered by a suitable electricity source.

Electricty is 100% energy-efficient, unlike an oil or natural gas boiler for example.

Hot water cylinders powered by twin immersions

A twin immersion hot water direct cylinder offers several practical and energy efficient benefits, particularly in UK homes, by using two separate electric heating elements (immersion heaters) positioned at different heights within the cylinder. Here’s a breakdown of the key advantages:

Flexible & Efficient Heating

• Top immersion: Heats only the top portion of the cylinder (typically enough for 1–2 showers or a bath).
• Bottom immersion: Heats the entire tank when more hot water is needed.
• You avoid heating a full tank unnecessarily, saving energy and reducing electricity bills— suitable for households or off-peak use.

Ideal for Economy 7 or Time-of-Use Tariffs

• Many UK households use Economy 7 or smart tariffs with cheaper off-peak electricity (usually overnight).
• The bottom immersion can be set on a timer to heat the full tank overnight using low-cost electricity.
• The top immersion can provide a quick top-up during the day if needed (e.g., for an unexpected bath for example).
• Maximise money savings by leveraging cheaper economy 7 rates.

Better Control Over Hot Water Usage

• Heat only what you need, when you need it.
• Great for holiday homes or properties with variable occupancy—heat a small amount for minimal use, or the full cylinder for guests.
• Reduces energy waste and aligns with modern energy-conscious lifestyles.

Compatibility with Renewable Energy

• Twin immersion cylinders work well with solar PV systems.
• Excess solar electricity generated during the day can power the top immersion to heat water “for free.”
• Some systems use solar PV diverters to automatically route surplus solar power to the cylinder.
• Lowers carbon footprint and running costs—aligned with UK net-zero goals.

Twin immersion cylinder range

Our stainless steel tank twin immersion direct cylinder range consists of units from 100 Litres to 500 Litres as detailed below.

 

The above chart shows specification details of the below cylinder.

Brocure is here for download

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

Full installation manual is here for download

Flexiheat manual for direct & indirect hot water cylinders

Special specification tanks are also available in many sizes.

Note: Customised products are non-returnable

Unvented cylinder

Most modern twin immersion cylinders are unvented, providing consistent, high-pressure hot water from the mains supply.

Unvented cylinders are sealed and filled directly from the mains water supply, designed to provide hot water at high pressure.

Power Requirement for immersion heaters

These immersion heaters can be powered by single or three-phase supplies.

Example 1: Three 6kW heaters installed in a 500 litre tank can output 18 kW from a single phase at 230 volts using 27 amps per heater, Or three phases 400 volts using 9 amps per phase. This setup is capable of heating 500 litres of water from 10°C to 65°C in 107 minutes.

Example 2: Two 3kW heaters connected to a 300 litre tank can output 6kW from a 230 volts single phase supply using 14 amps per heater, Or three phases 400 volts using 8 amps per phase. This setup is capable of heating 300 litres of water from 10°C to 65°C in 192 minutes.

Patented unvented hot water expansion system

The innovative internal hot water expansion mechanism, which employs a venturi valve to keep an air pocket within the stainless steel cylinder, eliminates the need for an external expansion vessel.

Our direct unvented hot water cylinders have an internal air gap and venturi valve technology that requires no expansion vessel and little maintenance.

Without moving parts, this unvented system provides a long product life and minimal operating and lifetime costs.

Internal air gap system for our direct unvented hot water cylinders: no external expansion vessel is needed because this self-sustaining air gap requires little to no maintenance and ensures mains pressure hot water flow throughout the property with our double immersion cylinder range.

Immersion heater

The immersion heating elements are screwed into the female tappings installed on the cylinder.

The 100 to 500 litre electric water heater / unvented cylinders are twin immersion cylinders with two 3 kW incoloy immersion heaters for rapid heat recovery as standard.

Additionally, we offer three 3 kW incoloy immersion heaters that may be installed on our 215 litre to 500 litre direct unvented cylinder versions or three 6 kW dual phase immersion heaters that can be wired either single or three phase, for a total power input of 18 kW.

Incoloy immersion elements

Our immersion heaters are high-quality units manufactured of Incoloy 825 stainless steel and equipped with a temperature control thermostat and an energy cutout thermostat.

Power Requirement for immersion heaters

These immersion heaters can be powered by single or three-phase supplies.

For the 3kW heating elements – these are 240 Volt / 1Phase powered and require 13 Amps of power per element.

If you upgrade to the 6kW heating elements – these can be either 1 phase / 240 volt or 3 phase / 415 Volt wired – if on 1 phase these require a supply of 26 Amps per heater, on three phase they require 9 Amps per phase for each element.

Domestic hot water heat up times

 

Tank Capacity (Litres)	Time to heat from 10C to 60C with 2 x 3kW immersion heater working (Minutes)
100	58.33
200	116.67
300	175.00
400	233.33
500	291.67

If you wish to contact us with a question or to find out further information

Should you require further assistance,information,questions on the product or to purchase / order one of our Dual or Twin immersion cylinder range,which are guaranteed for 25 year domestic use against faulty materials of the stainless steel tank ( 5 years guaranteed for commercial use) then please contact our customer sales team on 01202 822221 or click here for our email contact form to send an email