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Unvented Hot Water Storage System

Exam revision materials for Plumbing Level 2&3 and ACS Gas Safety test

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Unvented Cylinders

Types of Unvented Cylinders

Advantages and Disadvantages

Advantages Disadvantages
  • High-pressure operation
  • Dependance on incoming mains supply
  • Balanced pressure at outlets
  • No stored water
  • Reduction in pipe diameter
  • Discharge pipe required
  • Quicker to install
  • Space requirements
  • Easier siting (only restrictions for discharge route)
  • Purchase price
  • Reduced water-contamination risk
< Go Back2014-04-19


10% of all unvented cylinders on market come as a Unit

  • Unit has all the safety and functional devices fitted by the manufacture:
    • Line Strainer
    • Pressure Reducing or Limiting Valve
    • Single Check Valve
    • Tundish
    • Expansion Vessel
    • Pressure Relief Valve
    • Temperature Relief Valve
    • High Limit Thermostat and Cylinder Primary Motorised Valve (indirect)
    • Immersion Heater and built in Energy Cut Out
  • System Fit Unit - the same as Unit plus all the appropriate central heating system pump and controls
< Go Back2014-04-07


  • 90% of all unvented cylinders on market come as a Package
  • the difference is that not all of the items are fitted by manufacturer but only some of them
  • Items factory fitted:
    • Temperature Relief Valve
    • Immersion heater, thermostat and built in energy cut out
    • Thermostat and ECO for indirect (may be supplied separately)
  • Supplied to be fitted by installer:
    • Line strainer
    • Cylinder primary motorised valve (indirect unit)
    • Pressure reducing or Limiting valve
    • Single check valve
    • Tundish
    • Expansion vessel
    • Pressure relief valve
< Go Back2014-04-07

Copper Unvented Cylinders

  • grade 2 cylinders (1.6mm thick)
  • operating at 2 bar (less than the cylinders made of other materials)
  • should have a sacrificial anode installed to prevent corrosion
< Go Back2014-04-07

Mild Steel Lined Unvented Cylinders

  • inner coated with ceramic, enamel or glass lining material
  • have a sacrificial anode installed (must be inspected annually)
  • operates between 3 and 3.5 bar

Glass Lined Steel cylinders are made up of a mild steel cylinder that is coated on the inside with a layer of glass (similar to the enamel coating on a cast iron bath) to provide resistance to corrosion.   Cylinders typically have operating pressures in the region of 3.5 bar, but can often take far higher pressures.  These type of cylinders are cost effective, typically being mass produced on the continent and in the USA. Care must be taken not to crack the glass lining around bosses, and it is wise to check that primary coils are supported to avoid movement during transit that may stress and possible crack the glass lining. Cylinders should also be fitted with powered anodes to further protection from corrosion. 

< Go Back2014-04-07

Stainless Steel Unvented Cylinder

  • majority of new unvented cylinders
  • may or may not require a sacrificial anode
  • operates between 3 and 3.5 bar
< Go Back2014-04-07