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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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Types of Unvented Systems

Instantaneous Heaters (under 15L)

Multipoint Water Heaters

  • Outlet-controlled 
  • SHOULD have appropriate safety device for temperature and pressure to satisfy G3(3)
  • Gas Valve opens when pressure drops in a Differential Valve
  • NO check valves, pressure-reducing valves or stop valves (with loose jumpers) are fitted upstream of the water heater
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Continuous-flow Heaters (Single Point)

  • heated by Electrical Element
  • inlet or outlet controlled
  • 1 bar - MIN pressure
  • 55°C – on a MIN flow temperature
  • 45°C - MIN temperature on a full flow
  • Combination Boiler
    • 9 L/min – typical flow
    • 35°C - temperature rise
    • Diverter Valve directs the water as demand arises
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Displacement Heaters

  • Over-sink Storage Heater 
    • The water is heated to 60°C
  • Under-sink Storage Heater 
    • Special Mixer Tap is on the outlet
  • popular in commercial premises
  • Outlet MUST always be opened to the atmosphere
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Water-jacketed Tube Heaters

  • they heated indirectly via an internal coil
  • Examples: thermal storage and combination boilers
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Installation Type A

Using feed pipe to accommodate expansion

2.8m (10l) or 4.2m (15l) of 15mm pipe is required for expansion
Length of pipe EP 10 litre EP 15 litre
15mm pipe 2.8m 4.2m
22mm pipe 1.5m 2.2m
  • The pipe between the nearest cold water draw off and the water heater must be of sufficient volume to accommodate the expansion
  • The inlet water pressure is below 4 bar
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Installation Type B

Using an expansin kit

  • if its not possible to accommodate the expansion in the system pipework it is necessray to use an expansion vessel and non-return valve
  • the discharge should be made using a pressure relief valve and a tundish


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Installation Type C

Using an expansion kit and pressure reducing valve

  • if its not possible to accommodate the expansion in the system pipework it is necessary to use an expansion vessel and non-return valve
  • if the mains water pressure may exceed 3.5 bar a pressure reducing valve should be installed
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