Unvented System Regulation

Approved Document G Part L Part P
  • Building Regulations Document G
    • Section 2 - Water Efficiency
      • MAX daily internal usage - 125L
      • 5L - external usage
      • Water Efficiency Calculator
    • Section 3 - Hot Water Supply and Systems
      • Should incorporate MIN 2 independent Safety Devices
      • MAX delivered temperature is 48°C
      • Safety discharge
        • Calculate the diameter of the discharge pipe
  • Approved document L - Conservation of fuel and power
    • Recovery time of HWSC [BS 7206]
  • Water Supply (Water Fittings) Regulations 1999
  • 1986 Water Byelaws - accommodating of the expansions
  • Manufactures Recommendations
  • Code of Practice Benchmark 30 - commissioning of hot water system

  • British Standards:
    • BS6700:2006 - Design, installation, testing and maintenance of services supplying water for domestic use within buildings and their cartilages
    • British Board of Agrement BBA - approved testing body
    • BS 6283 - Safety and control devices for use in hot water systems
      • Part 2 - Specifications for temperature relief valves (pressures from 1 bar to 10 bar)
      • Part 4 - Specification for drop-tight pressure reducing valves (of nominal size up to and including 12 bar)
    • BS 6282: Part 1 - Devices with moving parts for the prevention of contamination of water by backflow
    • BS 6144 - Specification for expansion vessels using an internal diaphragm, for unvented hot water supply systems
    • BS 3955: Part 3 - Specification for electrical controls for household and similar general purposes
    • BS 7206 - Specification for Unvented Hot-water Storage Units and Packages (or be approved by the British Board of Agrement)
    • BS 7291 - Thermoplastics pipe and fitting systems for hot and cold water for domestic purposes and heating installations in buildings

Unvented Systems

  • The system is operating at a pressure of usually between 2 bar and 3.5 bar

    Unvented Hot Water Storage System
    Use of expansion vessel
  • Storage Heaters (over 15L)
    • Directly Heated
      • MIN 2 temperature activated safety devices (operating in sequence)
        • Non-self resetting thermal cut-out (BS 3955 or BS 4201)
        • Temperature relief valve (BS 6283: Part 2 or 3)
      • Overheat Thermostat alongside the normal control thermostat
    • Indirectly Heated - through a coil inserted inside the cylinder
      • all the controls listed for a direct system
      • plus Control Thermostat with ECO at 65°C

  • Any <500L and <45kW should be in the form of Unit or Package

Instantaneous Heaters (under 15L)

  • Multipoint Water Heaters Plumbing pictures
    • Outlet-controlled Plumbing pictures
    • 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

  • 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

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

  • Water-jacketed Tube Heaters
    • indirectly heated via an internal coil
    • thermal storage and combination boilers

  • Cistern-fed Storage Heater
    • wall mounted Plumbing pictures
    • floor mounted Plumbing pictures

Installation of Instantaneous Water Heaters


10% on market
  • 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


90% on market
  • 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

Unvented Hot Water Storage Cylinders

  • BS EN 12897:2006 - reqirements for UHWSS
  • Approved Document L1 in terms of thermal insulation and surface area of heating coil

    • Copper
      • grade 2 cylinders (1.6mm thick)
      • operating at 2 bar (less than following types)
      • have a sacrificial anode (~£50) installed
    • Mild steel lined
      • 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
    • Stainless steel
      • majority of new unvented cylinders
      • may or may not require a sacrificial anode
      • operates between 3 and 3.5 bar

  • Water has to be stored above 60°C
  • Circulated to outlets at >50°C
  • Typical operating pressure is 1.5-2.5bar

  • if located in a cellar/basement:
    • a self-priming pump (~£150) 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

  • Cylinder's Lable:
    • 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

Size of Unvented Cylinder Calculation

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

Unvented System Components

  • Safety devices are included to protect the user and the property
    Unvented System Design
    Unvented System Design
  • Functional devices are used to protect the supply of water
  • Isolating/stop valve
  • In-line strainer – preventing foreign bodies from passing into the system
  • Pressure Control Valve (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
    • Pressure Limiting Valve
      Modern Unvented System Control Layout
      • Spring-loaded device
      • Old installations (2 bar)
      • Could provide a higher then required pressure!

  • Single Check Valve (BS 6282: Part 1; 22mm~£6)
    • Located on the cold water inlet upstream to the Expansion Vessel
    • Stops stored hot water entering the cold water supply pipe

  • Composite Valves ( Valve ~£120)
    • 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
      • Pressure reducing valve at 3.5 bar
      • Line Strainer
      • Expansion valve at 6 bar
      • Balanced cold water take-off
      • Check valve

  • Expansion Vessel (BS 6144; 18L~£30)
    • 'Dead leg' overcoming:
      • use a through flow expansion vessel
      • provide an "Anti-legionella Valve"
        • uses venturi effect
        • incorporated isolating and drain-down valves
    • External, internal or air gap (bubble top)
      • 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
    • Volume - MIN 4% of the system
    • No valve MUST be fitted between this device and storage vessel

  • Expansion Valve (BS 6283: Part 2; 1/2"~£12)
    • Lever-top and Twist-top patterns
    • To relief over pressurization of a system (malfunction of PRedV, 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)
    • Fitted if capacity of the system > 15L
    • No valve must be fitted between this device and storage vessel
    • Adjusted by manufacture

  • Three-tier Level of Safety Protection:
    1. Control Thermostat
    2. High Limit Thermostat (Non-resetting Energy Cut Out Device (ECO); BS 3955: Part 3; ~£55)
    3. Temperature Relief Valve

  • Tundish (~£5)
    • 600mm MAX from the TRV
    • AUK3 type air gap (twice the diameter of the inlet)
    • 300mm min straight drop below the tundish
    • 1:200 – continuous fall
    • 300mm – between pipe clips for HEPvO method

  • Vacuum Relief Valve (Anti-vacuum Valve; 1/2"~£50)
    • Incorporated into some early temperature-relief valves
    • Unnecessary at present
    • For light gauge copper cylinders (old installations)

Safety Devices for Unvented System

There are 3 levels of safety protection (three-tier level of safety protection) to guard against overheating

  1. Control Thermostat (operating cylinder thermostat;~£25, Plumbing pictures)
    • to switch off at 60-65°C
  2. High-limit Thermostat (non-resetting energy cut-out device)
    • to shut down at 80-85°C
    • wired to a primary heater motorized valve
    • Operates at 80-85°C
    • 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

  3. Temperature Relief Valve (BS 6283-2:1991; combined temperature/pressure relief valves; ~£50; Plumbing pictures)
    • 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 [BS 6283]
    • Pressure Relief Valve operates at a higher pressure (9 bar) than that on the Expansion Valve
    • 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
  • Preventing risk of scalding :

Sizing of Pipes and Components

  • Supply Pipe from the Pressure Reducing Valve (both hot and cold) - 22mm
  • Cylinder feeding pipe - 22mm
  • Hot Water draw off to the point at which it branches - 22mm
  • Cold Water (balanced) draw off to the point at which it branches - 22mm
  • Supplies to individual appliances - 15mm or less

Unvented System Discharge Pipework

  • 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


Insulation of Unvented System Pipework

  • Regulations
    • TIMSA "HVAC guidance for achieving compliance with part L of the Building Regs"

  • Standing heat loss in 24 hours < 1.6 (1.28 for high performance)x(0.2+0.051V)kWh
    • Cylinder: 120L; 15L/min - 20min to reheat
  • MAX heat loss – 1W/L
  • Vessel Insulation
    • 35mm thick of PU-foam
    • MIN density – 30kg/m2
  • 15mm thickness for Thermal Conductivity (K) – 0.045 W/mK
  • MAX uninsulated HW pipes
    • 12mm – 20m
    • 12-22inc – 12
    • 22-28inc – 8
    • Over 28 – 3
  • Two piece spacing clip (10Pk~£5)– for insulating pipes

Pipework Heat Loss*

Pipe outside diameter mm 8 10 12 15 22 28 35 42 54
Maximum heat loss W/m 7.06 7.23 1.35 7.89 9.12 10.07 11.08 12.19 14.12
*Based on: horizontal pipe at 60°C in still air 15°C

Installation Requirements for Unvented System

  • The floor where the cylinder is to stand:
    • a support for a typical 100L cylinder should be capable of withstanding 150kg
    • chipboard flooring material could withstand water penetration

  • 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)
  • !!!Should never be supplied from uncontrollable heat sources:
    • NO! solid-fuel boilers
    • NO! solar water-heating circuits

  • 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

Termination of Discharge Pipes

  • 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 hot 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]

Secondary Circulation Systems

  • MAX length of un-insulated hot water pipes [Water Regulations]
    Secondary Circulation
    Secondary return layout
    • 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)


  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


  1. all components are approved for the vessel and are functional
  2. all valves are in the correct operation position and all thermostats are properly wired
  3. evidence of recent discharge and ask the owner if they have noticed any
  4. Temperature Relief Valve is re-seals (can be not resettable and a replacement might be required)
  5. manually check that Expansion Relief Valve re-seals
  6. check for obstruction of the discharge pipe and termination point
  7. check D1 and D2 comply with the Building Regulations
  8. cylinder's Operating Thermostat setting is 60-65°C
  9. isolate gas, water and electrical supplies open at least one hot tap to release the pressure
    1. check, clean or replace the Line Strainer Filter
    2. check the Expansion Vessel pressure
    3. check the Expansion Vessel diaphragm
    4. reinstate the air gap in line with a manufacture instructions
  10. reinstate water, electricity and gas supply and run the system
    1. check that the relief valves are not discharging
    2. check incoming mains water pressure and flow rate upstream of the Pressure Reducing Valve
    3. check the pressure, flow and temperature (min 55°C) at the tap (furthest away) and compare against mains
  11. complete any maintenance records and Benchmark forms
Vented System Maintenance Requirements
Component Interval Procedure
Vented Hot Water System 12 months General inspection
Control valves 12 months close tightly; operate smoothly; clearly labelled; customer is aware of isolation valves
Storage cisterns 6 months clean out; lid is squarely fixed; signs of stagnation; cistern base is sound; blockage and correct fall of the overflow pipe; insulation is intact
Float operated valve 6 months operation and shut off; correct water level; re-washer or replace
Terminal fittings 6 to 12 months rewasher, reset or renew; tighten packing glands; spindles for wear; clean spray heads on taps
Pipework 12 months signs of leakage or corrosion; tighten loose fittings; replace vulnerable sections
Thermostat 6 months boiler/cylinder/immersion heater thermostat/cut outs are working to specification; cylinder and terminal temperature conforms to regulations
Earthing and bonding At time of other inspection is intact; get electrician to check if in doubt

Water Discharge

Expansion Valve Discharge
  1. Manually reset/seat expansion valve
  2. Check pressure downstream of pressure control valve
  3. Correctly size expansion vessel and relief valve
  4. Are boiler and cylinder thermostat cutting off at recommended temperature?
  5. Check charge of pressure in expansion vessel. Does pressure hold?
  6. Exchange expansion vessel
  7. Exchange expansion valve
TRV Discharge
  1. Manually reset/seat TRV
  2. Check its temperature is set between 90°C and 95°C
  3. Check energy cut-out device on cylinder
  4. Is motorised valve feeding cylonder operating correctly?
  5. Check operating cylinder thermostat setting/operation is correct
  6. Exchange temperature relief valve
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Deteriorating Flow from Taps
  1. Check incoming mains pressure
  2. Check line strainer
  3. Check pressure control valve
  4. Check system for blockage

Advantages & Disadvantages of Unvented Systems

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