G3 Building Regulation (G3)

G3 (Approved Document G) defines the safety requirements for hot water systems within UK Building Regulations, with a particular focus on unvented hot water systems.

The regulation ensures that systems are designed, installed and maintained to prevent safety risks, including excessive pressure, temperature and discharge hazards.

Clear definition of product requirements, certification scope and system design supports efficient progression and reliable compliance outcomes..

What is G3?

G3 forms part of Approved Document G of the UK Building Regulations and sets out the system safety & product compliance criteria for hot water systems, particularly unvented configurations.

It establishes:

  • Control of temperature and pressure within hot water systems
  • Safe operation of unvented hot water cylinders
  • Performance of safety devices and discharge arrangements
  • System design to prevent overheating, overpressure and scalding
  • Testing to relevant standards (e.g. BS EN 12897)
  • Assessment of system components and safety controls
  • Certification through accredited laboratories and certification bodies
  • Compliance with UK Building Regulations for product approval and use

G3 compliance requires coordination between product performance, system design and regulatory expectations.

Testing and compliance approaches may vary between laboratories and certification bodies, making early clarity on scope important to maintain progress.

Where G3 Applies

G3 applies to hot water systems installed within domestic and commercial buildings across the UK.

This commonly includes:

  • Unvented hot water cylinders
  • Indirect and direct storage systems
  • Packaged water heating units
  • Systems within residential, commercial and mixed-use developments

Early understanding of applicability is important, particularly where products are being specified for the UK market or incorporated into regulated building projects.

G3 Compliance Process

Early understanding of applicability is important, particularly where products are being specified for the UK market or incorporated into regulated building projects.

This typically includes:

  • Identifying applicable standards and regulatory expectations
  • Testing to relevant product standards (e.g. BS EN 12897)
  • Assessment of temperature and pressure control mechanisms
  • Verification of discharge arrangements and system performance
  • Compliance through accredited testing and certification bodies
  • Review of factory production controls and quality systems
  • Coordination with wider project requirements

Certification is delivered through certification body programmes, with testing conducted by accredited laboratories.

How AquaCompliance Supports

AquaCompliance provides structured advisory and project support across G3 compliance, helping manufacturers define product requirements and coordinate certification activities in line with UK Building Regulations.

  • Regulatory Pathway Definition — Identifying applicable standards, system requirements and approval routes for products entering the UK market
  • Documentation Review — Assessing product design, technical files and system data to identify gaps and readiness for evaluation
  • Laboratory Coordination — Working with laboratories and certification bodies to define testing scope, considering system performance and safety requirements
  • Certification Engagement — Supporting communication with certification bodies to address technical queries and maintain progress through approval
  • Commercial & Technical Alignment — Aligning product requirements with related UK frameworks (e.g. WRAS, BS 6920) to reduce rework and streamline parallel approval programmes
  • Application & Project Management — Coordinating activities across internal teams, laboratories and certification bodies to support delivery through to receipt of the accredited test report

Key Considerations

G3 compliance is influenced by both product design and system-level safety requirements, which should be clearly defined early:

  • Product design and system configuration
  • Temperature and pressure control mechanisms
  • Safety device performance and discharge arrangements
  • Integration of components within the overall system
  • Availability and completeness of supporting technical documentation
  • Laboratory capability and certification body expectations
In addition, several technical and commercial factors can influence scope, timelines and cost:
  • System complexity — Variations in configuration and component interaction may expand evaluation requirements
  • Performance verification — Safety devices and controls must operate reliably under defined conditions
  • Documentation quality — Gaps or inconsistencies within technical and supporting data can delay assessment and approval
  • Testing approach — Requirements may vary depending on the laboratory and certification body selected
  • Alignment of documentation across standards — Alignment with WRAS / BS 6920 can reduce testing, project cost and support on-time delivery

Clear definition of product scope, system requirements and certification pathway supports progression and improves confidence in delivery.

Related Requirements

G3 forms part of the UK regulatory framework and is often applied alongside additional requirements depending on product type and application.

This may include:

While G3 focuses on system safety and performance, these frameworks address product approval, material compliance and broader regulatory requirements.

A coordinated approach helps manage overlap and maintain consistency across standards.

If you are planning product approval or market entry for hot water systems in the UK, early clarity on G3 requirements is essential.

AquaCompliance supports manufacturers in defining certification pathways, coordinating testing and approval activities, and maintaining alignment with UK regulatory frameworks.

Explore our full range of water compliance services.