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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

Components and Safety Devices


  • Maximum distance from a Temperature and Pressure Relief (T&PRV) valve is 600mm 
  • AUK3 type air gap (twice the diameter of the inlet)
  • Minimum straight drop below the tundish is 300mm 
  • 1:200 – continuous fall towards the drain
  • Pipe support requirement: minimum 300mm – between pipe clips for HEPvO method
  • Tundish outlet pipe must be a size bigger than an inlet pipe (Inlet pipe 15mm to outlet 22mm)

Discharge pipes sizing

  • D1 – between the TRV and the tundish (or by way of a manifold)
  • D2 (MAX 9m)– between the tundish and the termination point
    • at least one size greater than D1
    • If a single common discharge pipe serves more than 1 system (MAX 6 systems) - 1 size larger than the largest individual D2
    • MIN 1:200 - continuous fall
    • MIN 300mm straight drop after the Tundish
    • Tundish Air Gap - Type AUK3​
    Valve outlet size MIN D1 size MIN D2 size MAX resistance allowed Resistance of elbow or bend
    15mm 22mm up to 9m 0.8m
    28mm up to 18m 1.0m
    35mm up to 27m 1.4m
    22mm 28mm up to 9m 1.0m
    35mm up to 18m 1.4m
    42mm up to 27m 1.7m
    G1 28mm 35mm up to 9m 1.4m
    42mm up to 18m 1.7m
    54mm up to 27m 2.3m
< Go Back2014-04-12

Vacuum Relief Valve

  • Anti-vacuum Valve
  • Incorporated into some early Temperature Relief Valves
  • Unnecessary at present
  • For light gauge copper cylinders (old installations)
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In-line Blending Valve

  • Also known as Thermostatic mixing valve
  • Installed to prevent a risk of scalding
  • Type TMV2 for residential homes
  • Type TMV3 - for NHS installations
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Expansion Vessel

  • Should comply to BS 6144
  • To accommodate an expanded heated water internal or external expansion vessel MUST be fitted
  • External expansion vessel:
    • must be minimum of 4% size of the system
    • must be located downstream to the single check valve on a cold feed pipe
    • it should be charged with air to 3bar and to be checked and recharged every year when serviced
    • No valve MUST be fitted between an expansion vessel and a cylinder
  • Internal or Air gap (Bubble top)
    • Its located in the top part of the cylinder
    • Bubble check:
      1. Run the hot water
      2. Shut the cold supply
      3. Run the hot water
      4. Measure amount of water
        1. No water - no Bubble
        2. 10% of water - there is a Bubble
    • Bubble recharge:
      1. Isolate water supply to a cylinder
      2. Connect a drain hose and start draining water
      3. While draining open the Temperature and Pressure Relief Valve
      4. Wait until air stop entering the cylinder
  • Volume - MIN 4% of the system
  • 'Dead leg' overcoming:
    • use a through flow expansion vessel
    • provide an "Anti-legionella Valve"
      • uses venturi effect
      • incorporated isolating and drain-down valves
< Go Back2014-04-09

Expansion Valve

  • Also known as Pressure Relief Valve (PRV)
  • Should comply to BS 6283: Part 2
  • There are 2 types: Lever-top and Twist-top patterns
  • The meaning is to relief an excessive pressure from a system in case of malfunction of Pressure Reducing Valve, High limit thermostat, Expansion vessel or Bubble Top)
  • Operates at 6 to 8 bar (0.5bar higher then working pressure on old installations, 3.5bar for copper cylinders)
  • The discharge pipe should terminate in a safe location outside of a building or through a Tundish to a waste pipe
  • Fitted if capacity of the system > 15L
  • No valve must be fitted between this device and storage vessel
  • Adjusted by manufacture
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Three-tier Level of Safety Protection

  • Control Thermostat
    • Cuts off source of heat at 65oC
  • High Limit Thermostat 
    • Non-resetting Energy Cut Out Device (ECO) 
    • Should comply to BS 3955: Part 3
    • Cuts off source of heat at 80-85oC and requires a manual reset
    • Has a reset button
    • In indirectly heated systems its wired to a primary heat source control (motorized valve)
    • Indirect heated:
      • turns off the heat source to a cylinder by closing 2 port motorised valve (spring loaded)
      • Y-Plan configuration needs both a 3-port and the 2-port valves and Control Relay(optional) to wire both valves
      • ECO combined with Control Thermostat (wired in series; 1/3 from the bottom)
    • Directly heated:
      • special immersion heater is required (with built-in overheat thermostat)
    • Fitted by the manufacture at 1/3 from the bottom (immersion heater + control thermostat + ECO)
    • Separate ECO MUST be fitted at 100°C in case of Solar System
  • Temperature & Pressure Relief Valve
    • Opens up to release water at 85oC
    • Should comply to BS 6283-2:1991
    • Operates at 90-95°C
    • Protects the system in case of failure of both the Control Thermostat and Energy Cut-out Device
    • located directly on top of a storage vessel
    • sized to give a discharge rating according to BS 6283
    • Temperature & Pressure Relief Valve operates at a higher pressure (7-9 bar) 
    • Adjusted by manufacture
    • Anti-vacuum valve often built into TRVs for copper cylinders (unnecessary for steel cylinders)
    • 147.7°C – boiling point of water at 3.5 bar
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Pressure Control Valve

  • Should comply with BS 6283: Part 4 
  • Pressure Reducing Valve (Water Governor 22mm~£30)
    • Operates by action of back-pressure of a full diaphragm
    • Ideal for copper cylinders
    • New installations (3-3.5 bar)
    • Provides close control
    • It starts leaking when a diaphragm inside has perished
  • Pressure Limiting Valve
    • Spring-loaded device
    • Old installations (2 bar)
    • Could provide a higher then required pressure!
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Single Check Valve

  • Should comply to BS 6282: Part 1
  • Located on the cold water inlet upstream to the Expansion Vessel
  • Stops stored expanded hot water traveling back to the mains cold water supply pipe
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Composite Valve

There are 2 types of combination (composite) valves available

  • Composit Valve
    1. Pressure reducing valve at 3 - 3.5 bar
    2. Line Strainer
    3. Expansion valve at 6 bar
    4. Check valve
  • Balanced Composite Valve - provides an additional cold water connection downstream to the Pressure reducing valve to achieve an equal pressure for cold and hot water
    • Pressure reducing valve at 3.5 bar
    • Line Strainer
    • Expansion valve at 6 bar
    • Balanced cold water take-off
    • Check valve
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Isolating/Stop Valve

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In-line Strainer

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