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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

Installation Requirements

Secondary Circulation

  • MAX length of un-insulated hot water pipes [Water Regulations]
    • 12mm - 20m
    • ≤ 22mm - 12m
    • ≤ 28mm - 8m
    • > 28mm - 3m
  • Returns about 25% of hot water back to cylinder
  • Trace Heating - alternative to Secondary Circulation
  • The Pump (WRAS approved bronze) is normally operated by a time clock
  • Secondary return pipe (usually 15mm) includes a Single Check Valve near the Pump
  • Additional Expansion Vessel to Secondary Return:
    • if secondary circuit > 10L
    • 8mm - 0.036L/m
    • 10mm - 0.058L/m
    • 15mm - 0.13L/m (10L - 77m)
    • 22mm - 0.38L/m (10L - 26m)
    • 28mm - 0.55L/m (10L - 18m)
< Go Back2014-04-18


  1. Visual Inspection
    • all fittings/joints are completed
    • the installation complied with all relevant specifications/regulations
    • inspection carried aut before any pipework is concealed
    • CWSC fitted with a cover and adequately supported
  2. Testing
    • fill with water and inspect for leaks
      • manually set 2-port valve to the manual open position until the primary circuit is full
    • pressure test [BS 6700] (different for plastic and non-plastic)
    • Expansion Valve - open for 20-30 sec
    • Temperature&Pressure Relief Valve - open for 20-30 sec
  3. Cleansing the system
    • flush the system before being brought into operation
      • dismantle the Line Strainer
    • disinfection for non-dwellings installation
  4. Performance Testing
    • all valves and fittings operate correctly
    • the water in CWSC is at correct level
    • Thermostat (boiler, cylinder, thermal cut outs) operate correctly and set at 60-65°C
    • the water temperature at outlets is as per regulations
  5. Final checks for leaks and adequate support of the CWSC and Storage Vessel
  6. Issue a "Commissioning Certificate" if installed on Oil Fired Boiler
< Go Back2014-04-18

Cylinder Labelling

  • The Manufacturer's name and contact details
  • A model reference
  • The rated storage capacity of the storage water heater
  • The operating pressure of the system and operating pressure of the expansion valve
  • Relevant operating data on each of the safety devices fitted
  • The MAX primary circuit pressure and flow temperature of indirect hot water storage system units or packages
  • The Warning should be marked so that it is visible after installation 
< Go Back2014-04-17

Cylinder Sizing

Domestic Cylinder sizing Guidance

Property Size Indirect Cylinder Direct Cylinder
Bedsit 70-90 L 70-90 L (3 kW)
1 bed, 1 bath, shower 100-125 L 125-145 L (3 or 6 kW)
2 beds, 1 bath, shower 125-140 L 145-170 L (6 kW)
3 beds, 1 bath, shower 145-160 L 210 L (6-9 kW)
4 beds, 2 baths, shower 170-200 L 210 L (6-12 kW)
2-3 beds. 2 bath, shower 170-200 L 210 L (6-12 kW)
4-5 beds. 2 bath, shower 210-240 L 250 L (6-12 kW)
Large homes 250-300 L 300 L (up to 12 kW)

Commercial and industrial cylinder

Application Hot Water Storage
Hand washing 1 L per person per use
Hand washing (spray tap) 0.5 L per person per use
Showering (general) 10 L per minute per shower
Showering (sports) 6 L per person per 5 min shower
Bath 55 L per bath
Kitchen 3-5 L per meal served
Dishwasher 2.5 L per meal served
Washing machine 20 L per wash
Cleaner's sink 10 L per per sink
Hair dressing 10 L per shampoo
< Go Back2014-04-17

Pressure Discharge Arrangement

Low level discharges

  • Termination into a gully
    • exposed discharge pipes should be thermally insulated
    • the end of discharge pipe has to terminate below the lid
  • Low level termination
    • MAX 100mm above ground level
    • MIN 70mm above ground level
    • Risk Assessment to see if requires caging

High level termination

  • Hopper termination
    • hopper MUST be metal
    • Down Pipe MUST be at least 1 size greater than D2
  • Discharge onto a roof
    • MIN 3m away from a plastic gutter
    • it is allowed to discharge onto a flat roof
  • Discharge at high level
    • gap between wall and pipe MUST be greater than ØD2

HEPvO water discharge

  • 32mm HEPvO MUST be vertical and adjacent to the water unit
  • water discharged to soil stack through boss adaptor
  • Pipe clips positioned MAX 300mm apart
  • 32mm Polypropylene Discharge pipe [BS 5254]
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SHOULD be installed by a person competent in the installation of UHWSS

  • current operative identity card
  • certificate for the installation of UHWSS issued by BPEC

Requires notice of Building Control Department BEFORE by means of a Building Control Notice

An installer who has joined Competent Person Scheme needs to give a Notice of Commissioning upon completion of the installation

< Go Back2014-04-16

Cold water feed pipe

OFWAT www.ofwat.gov.uk has declared standard of service indicator DG2 which requires to deliver a supply of water:

  • MIN external stop tap pressure and flow: 1bar and 9L/min
  • MIN for unvented system: 1.5bar at 20L/min (during peak usage time)
  • Flow Cup and  Weir Gauge are used to measure a flow rate
< Go Back2014-04-15

Heat Source

  • Indirect unvented cylinder uses a gas boiler as a heat source
  • Direct unvented cylinder uses an electric immersion heater 3KW as a heat source
  • Cylinder heat should never be supplied from uncontrollable heat sources:
    • NO! solid-fuel boilers
    • NO! solar water-heating circuits
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  • A floor where the cylinder is supposed to stand:
    • a support for a typical 100L cylinder should be capable of withstanding 150kg
    • The unit should be sited on a flat base capable of supporting the weight when full.
    • Where immersion  heaters (normally 375mm long) are fitted care should be taken to ensure that they can be withdrawn for servicing .
    • Care should be taken to ensure that the wiring centre and / or programmer are not sited such that they could be damaged by any escape of water.
    • Unvented systems operate at higher temperatures than conventional cylinders and so adequate ventilation is important in order to avoid excessive airing cupboard temperatures.
    • Directing the vents into the bathroom can provide useful background heating
    • Chipboard flooring material should withstand water penetration
    • If a cylinder installed in an airing cupboard then minimum cupboard size recommended is 650mm square
  • if a cylinder is located in a cellar/basement:
    • self-priming pump MUST be installed (to lift the discharged water to the atmosphere)
    • discharge pipe MUST be visible
    • discharge pipe MUST terminate as for low level discharge
< Go Back2014-04-12