Approved Document G Sanitation: G3 Unvented Hot Water, Wholesome Water and Hot Water Supply Temperature

Summary

Approved Document G covers a broad range of water-related building regulations requirements: sanitation facilities (toilets, washbasins), hot and cold water supply quality, hot water temperature, and the safety of hot water storage systems. Section G3 — covering unvented hot water storage — is the most technically complex and has the most significant qualification requirements of any domestic plumbing regulation.

The reason G3 is heavily regulated is simple: an unvented hot water cylinder stores large volumes of water under pressure, typically at 60–65°C. If the safety devices fail — and no primary safety device is present — the cylinder can undergo a Boiling Liquid Expanding Vapour Explosion (BLEVE). This is a catastrophic failure: cylinders can be propelled through buildings and have caused fatalities. The safety device chain in a G3 system is designed to ensure multiple independent failures would be required for such an event.

Beyond G3, Part G establishes requirements for wholesome water supply (ensuring cold water to drinking outlets is direct from the mains, not re-circulated or contaminated), water efficiency requirements for new dwellings, and legionella control through hot water temperature.

Key Facts

• Approved Document G — Water efficiency, sanitation and hot water safety; current edition 2010 with amendments
• G1 — Cold water supply — Cold water for drinking must be wholesome; mains-fed where possible
• G2 — Water efficiency — New dwellings: maximum 125 litres/person/day (or 110 l/p/d in water-stressed areas)
• G3 — Hot water storage — Covers unvented cylinders and storage heaters; qualification and safety device requirements
• G4 — Sanitation — Minimum sanitary provision (WC and handwashing basin in each dwelling)
• G3 qualification — Must hold: either an Unvented Hot Water Systems qualification (City & Guilds 6189, EAL or equivalent) or be supervised by someone who does
• Building Control notification — Every G3 installation must be notified to Building Control; self-certification available via WaterSafe or relevant competent person scheme
• Mandatory G3 safety devices — Temperature relief valve (tpr), expansion relief valve, non-return valve, pressure reducing valve (PRV), expansion vessel
• PRV set pressure — Typically 3.5 bar; reduces mains pressure to safe working level
• TPR valve — Thermostatically-controlled; opens at 90–95°C to discharge scalding water before pressure builds
• Discharge pipework (D2) — From TPR and expansion relief valve; must terminate safely outside or to a drain; minimum 22mm for cylinder ≤15 litres/minute; see table in AD G
• Hot water temperature — Must reach ≥50°C at outlets within 1 minute for legionella control; maximum 48°C at baths to prevent scalding
• Thermostatic blending valve (TMV) — Required at baths to limit temperature to 48°C; TMV2 standard minimum
• WRAS approval — Fittings and materials in contact with water must be WRAS-approved (Water Regulations Advisory Scheme)

Quick Reference Table

Detailed Guidance

G3 Qualification Requirements

Anyone installing, commissioning or maintaining an unvented hot water storage system must be competent. AD G3 Regulation 2 defines competency as:

• Holding a recognised qualification in unvented hot water systems (City & Guilds 6189-11/21, EAL Award, or equivalent); OR
• Being supervised by someone who holds such a qualification; OR
• Working for a company with a relevant competent person scheme registration

What the G3 qualification covers:

• System design and sizing (cylinder volume, heat input)
• Safety device function, sizing and installation
• Discharge pipework sizing and routing
• Commissioning procedures
• Fault-finding

Building Control notification: Under Part G, work on unvented hot water systems must be either:

1. Notified to Building Control before starting (full plans or building notice)
2. Self-certified using a WaterSafe or equivalent competent person scheme (installer then notifies Building Control on completion)

Failure to notify is a Building Regulations offence.

G3 Safety Device Chain: Detailed Function

The safety devices in a G3 system work in series. Each device is the last line of defence in sequence:

1. Non-return valve / check valve (on cold feed): Prevents hot water flowing back into the cold mains. Without this, hot water could contaminate the cold supply (fluid category 3 risk under Water Regulations).

2. Pressure Reducing Valve (PRV): UK mains water pressure varies widely (1.5–10 bar). Most cylinders are designed for 3.5 bar maximum working pressure. The PRV reduces inlet pressure to the design working pressure. If the PRV fails open (common fault), cylinder pressure rises to mains pressure — potentially exceeding relief valve settings.

3. Expansion vessel: When cold water in the cylinder is heated from 10°C to 60°C, it expands by approximately 3%. In a sealed system, this expansion must go somewhere — the expansion vessel accommodates it. If the expansion vessel fails (waterlogged diaphragm), expansion pressure must be released through the expansion relief valve.

4. High-limit (overheat) thermostat: This is an energy cut-out device. If the primary thermostat fails and temperature rises above 80–90°C, the high-limit stat cuts power to the immersion heater or motorised valve to the boiler coil. This is the first safety device to activate on overheat.

5. Temperature and Pressure Relief Valve (TPR): Combined thermal/pressure relief valve. Opens at:

• Temperature: 90–95°C (to discharge hot water and prevent pressure build-up)
• Pressure: overpressure above working pressure (to prevent structural failure) Must be installed on the top third of the cylinder, never on the pipework.

6. Expansion Relief Valve (ERV): Installed on the cold feed; opens to release pressure if it rises above its set point. Provides additional protection alongside the TPR.

Discharge Pipework (D1 and D2)

The discharge pipe is safety-critical. It conveys scalding water (90°C+) when the TPR or ERV lifts. It must be routed to a safe discharge point.

D1 pipe (from safety devices to tundish):

• Minimum 22mm; must not be reduced
• Fall throughout towards tundish
• Air break at tundish (see below)

Tundish:

• Air break between the pressurised discharge and the downstream pipework
• Prevents siphoning and contamination
• Must be installed within 500mm of the safety device
• Must be visible (not concealed in cupboard) so discharge can be seen in operation

D2 pipe (from tundish to safe discharge):

• Must be copper (steel pipe must not be used — thermal movement)
• Minimum 22mm for most domestic cylinders; refer to AD G Table 3 for sizing by discharge rate
• Minimum fall of 1:200 throughout
• Must discharge to a safe visible location: over a tundish to a gully, over a drain, or outside 100mm above a drain
• Discharge pipe should terminate in a position where the householder can see if it is discharging (indicator that the system is faulty)

Common failures in D2 installation:

• Pipe terminates inside cupboard where discharge cannot be seen
• Discharge pipe diameter reduced below minimum
• Insufficient fall (D2 runs level)
• Discharge to over a domestic drain that could freeze in winter (Scotland/northern England consideration)

Hot Water Temperature: Legionella and Scalding Balance

Part G requires hot water to be:

Minimum temperature (legionella control):

• Stored at 60°C (kills legionella within 1 hour at this temperature)
• Distributed to outlets at ≥50°C (measured within 1 minute of opening the tap)
• This is the DHSC/HSE requirement per L8 (ACOP)

Maximum temperature (scalding prevention):

• Bath and shower outlets: maximum 48°C at point of use (AD G requirement; TMV2 or TMV3 valve required)
• Kitchen sink and washbasin: no upper limit mandated, but TMVs recommended for vulnerable users (elderly, children, disabled)

The conflict: To store at 60°C (legionella) but deliver at maximum 48°C (scalding), a thermostatic blending valve (TMV) must be fitted downstream of the cylinder. The TMV mixes hot and cold water to achieve the target temperature at the outlet.

TMV standards:

• TMV2 — domestic use; suitable for baths, showers; manual thermal cutout
• TMV3 — healthcare use; more stringent; failsafe to cold on hot supply failure; required in hospitals, care homes, schools

Water Efficiency: G2 Requirements

For new dwellings, Part G2 sets maximum water consumption:

• Standard: 125 litres per person per day (l/p/d)
• Water-stressed areas (large parts of SE England): 110 l/p/d
• Must be demonstrated by WUIC (Water Use Impact Calculator) or Fittings Approach at Building Regulations stage

Fittings Approach limits (typical):

• WC: maximum 6 litres per flush (4.5 l dual flush)
• Washbasin tap: maximum 6 l/min at 3 bar
• Shower: maximum 8 l/min at 3 bar
• Bath: maximum 185 litres total fill

Compliance: Include schedule of fittings in building regs submission; no testing required but evidence of specified fittings required.

WRAS Approval for Fittings

All plumbing fittings and materials in contact with water supplied for domestic use (including hot water) must comply with the Water Supply (Water Fittings) Regulations 1999. WRAS (Water Regulations Advisory Scheme) maintains a list of approved products.

Key requirements for hot water systems:

• All fittings downstream of the meter must be WRAS-approved
• This includes the cylinder itself, TMVs, PRVs, TPR valves, expansion vessels
• Water company inspectors can require removal of non-approved fittings at the customer's cost
• Most major brands include WRAS approval in product data sheets; check before specifying

Frequently Asked Questions

Can a plumber without a G3 qualification work on an unvented cylinder?

No, unless supervised by a G3-qualified person. Any installation, commissioning, or replacement of components on an unvented hot water storage system requires G3 qualification. Replacing an immersion heater element in a vented cylinder does not require G3. Replacing an immersion heater in an unvented cylinder does require G3 competence (the system contains pressurised water; removing an immersion heater releases this pressure). If in doubt, the test is: is the cylinder sealed and pressurised? If yes, G3 competence is required.

Our unvented cylinder is dripping from the tundish. What does this mean?

A dripping tundish means one of the relief valves is weeping. Possible causes:

1. Expansion vessel failure (most common): waterlogged diaphragm means expansion has nowhere to go; ERV weeps on each heating cycle. Replace expansion vessel.
2. PRV set too high: mains pressure exceeds working pressure periodically. Replace or re-set PRV.
3. TPR valve faulty: seat damaged, weeping cold. Replace TPR valve.
4. System overpressure: supply pressure consistently too high. Add or replace PRV. A continuously weeping tundish that is producing hot water indicates the TPR is activating — this is an emergency; the system is overheating. Check the thermostat and overheat stat immediately.

Does a thermal store require G3 qualification?

A thermal store (where the DHW is produced by passing mains cold water through a heat exchanger coil in the store, not directly storing mains-pressure hot water) is typically not classified as an unvented hot water storage system under G3 if the DHW is heated and used immediately without accumulation. However, confirm this for specific products — some thermal stores do hold pressurised water. When in doubt, treat as G3.

Regulations & Standards

• Building Regulations Approved Document G (2010 + amendments) — Water efficiency, sanitation and hot water safety

• Water Supply (Water Fittings) Regulations 1999 — WRAS requirements for fittings in water supply systems

• BS EN 12897:2006 — Water supply: specification for indirectly heated unvented hot water storage systems

• HSE L8 ACOP — Legionella: The Control of Legionella Bacteria in Water Systems — legionella temperature requirements

• BS EN 1111 — Sanitary tapware: thermostatic mixing valves (PN10) — TMV general standard

• TMV2 Scheme — BEAMA industry standard for thermostatic mixing valves in domestic applications

• TMV3 Scheme — NHS Estates/BEAMA standard for healthcare applications

• GOV.UK Approved Document G — Full text of Part G

• WRAS Water Fittings and Materials Directory — Searchable database of approved products

• HSE Legionella L8 ACOP — Hot water temperature and legionella management

• unvented cylinders — Detailed G3 installation procedure and safety device specification

• heat pump cylinders — Heat pump cylinder legionella requirements

• septic tanks — Drainage requirements that interact with Part G sanitation provision