Venturi Indirect VXI Unvented Hi-flow

400 | 500 LITRES

An indirect unvented cylinder for use with a conventional boiler with electric back-up

The Venturi Indirect VXI Unvented Cylinder – Hi-flow unvented water heater is part of The Plumbing Group’s Venturi range of water heaters, developed at The Plumbing Group’s UK manufacturing facility in Ellesmere, Shropshire, to cater for higher throughput water volumes, using The Plumbing Group’s patented 3S Technology.

“The world’s first true self-sustaining system”, it is based on an internal expansion system and offers major advantages.

This 3S Technology was pioneered by The Plumbing Group and is a self-sustaining internal expansion system, adopting the Venturi principle in which atmospheric air is drawn into the system as water is demanded upstream. Because this occurs in accordance with hot water demand, it does not cause any deterioration in performance.

This process means the internal air gap is permanently replenished, eradicating the need for recharging or regular servicing.

There are no internal, moving parts in the cylinder and, once in place, there are no additional parts required. The absence of an external expansion vessel means the tank can fit into more compact installation spaces.

This specific product has been modified to meet the demand for higher flows rates within high-end or multiple occupancy domestic residences. It is also suitable for light commercial applications, for example catering or guest houses. To achieve this, the VXI has a new 28mm Venturi valve on the inlet and 28mm outlet fitting.

The absence of an external expansion vessel means the tank can fit into compact installation spaces, and The Plumbing Group also offers a pre-plumb version.

These days, you are lucky if you can persuade a homeowner to get their boiler serviced. So, to convince them to get their hot water cylinder serviced, you have got no chance. because the VXI is self-sustaining and, if it’s fitted and forgotten about, it’s going to have a much better chance than a standard unvented cylinder [which needs regular servicing].


Both models are pressure tested to 10 bar and come complete with a temperature and pressure relief valve which is factory set at 7 bar and 90 degrees Celsius. Both the 400L and 5000L have a secondary return.

Venturi Indirect Unvented Cylinder

Venturi Indirect Unvented Cylinder Heat Loss Performance

Features of a ThermaCoil Venturi Indirect Unvented Cylinder

  • 25-Year cylinder and 3-Year parts warranty
  • No risk of legionella from the expansion pipe “dead leg”.
  • Sizes 400L & 500L capacity
  • Self-sustaining air-gap system
  • Cylinder manufactured from high grade Duplex stainless steel
  • 60mm fire retardant insulation minimising heat loss
  • 100% CFC/HCFC free
  • Long life with no internal moving parts
  • Preinstalled with no additional parts required
  • No external expansion vessel, quicker installation
  • 3 Bar pressure at outlets
  • All sizes WRAS approved
  • Designed & manufactured in the UK


Venturi Indirect Unvented Cylinder Schematic

Venturi VDE Indirect Schematic

How the Venturi VXI Works

The 3S technology 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.

Venturi Indirect

Problems with expansion in existing unvented systems

Click here to find out the benefits of using Venturi 3S Technology


Venturi VXI Product Construction – Typical

Immersion heater ☞ 3kw, 240V, 1ph, Incoloy 825, with thermostat incl. cut-out
Thermal insulation ☞ CFC/HCFC free, (ODP=0, GWP=1), 50mm fire retardant insulation
Outer casing ☞ P200 Plastisol
Inner cylinder ☞ Duplex stainless steel
Heating coil ☞ High performance, finned
Indirect thermostat ☞ Twin thermostat with independent reset, 30-70ºC
Safety device ☞ Factory fitted with 7bar/90ºC pressure & temperature relief valve
Connections ☞ Secondary return on 400L & 500L

Venturi VXI Product Specification

Inlet diffuser ☞ For optimal blending of cold & hot water, see fig. 1
Expansion Technology☞ VXI – 3 bar internal expansion with 3S, All other ranges – 3 bar external expansion
Pressure test☞ Tested to 10 bar
Secondary return☞ Standard fit on models 175L & above
Approvals☞ BBA, CE, WRAS, KIWA, ISO, WRC
Guarantee☞ Internal cylinder 25 years* Ancillary components 1 year
Direct thermostat☞ Element thermostat, nominal setting 60ºC
Indirect model ☞ 28mm diameter stainless steel high performance coil & outlet with 1 immersion heater
Manufacturing ☞ All UK using state-of-the-art robot technology

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