Hot Water Systems & Safety (Unvented - G3)

The legal and technical framework that governs hot water work in the UK. This module gives you the knowledge to stay compliant and to justify your decisions on site to Building Control, customers and insurers.

• G3 of the Building Regulations and Approved Document G in detail
• Water Supply (Water Fittings) Regulations 1999
• BS EN 12897 (unvented hot water storage) and BS 6700 / BS 8558 (system design)
• Manufacturer instructions and how they sit alongside the Regulations
• Notification routes: Building Control vs Competent Person Scheme self-certification
• Where the rules differ in Scotland, Wales and Northern Ireland

Water expands by roughly 4% when heated from 10°C to 60°C. In a sealed (unvented) system that expansion has nowhere to go unless you design for it and getting this wrong is what causes cylinders to fail dangerously. This module covers the physics and the practical implications.

• How and why water expands when heated
• Calculating expansion volume for different cylinder sizes
• Why unvented systems need either an expansion vessel or an expansion pipe
• Charge pressure, system pressure and how they interact
• Common faults caused by undersized or failed expansion vessels (drips, weeping PRVs, blown discharge)

The differences between vented and unvented storage, how each system works, and when to specify one over the other. This module lets you advise customers correctly and choose the right system for the building.

• Vented (open) systems: cold-water storage cistern, gravity-fed, low pressure
• Unvented (sealed) systems: mains-pressure, balanced hot and cold flow
• Direct vs indirect cylinders and where each is appropriate
• Thermal store systems and how they differ from conventional cylinders
• Stratification, heat loss and cylinder insulation standards (ErP labelling)
• Choosing capacity based on occupancy, bathroom count and demand patterns

How to design a hot water system that performs well, meets the Regulations and integrates cleanly with the rest of the building services. Good design upfront avoids callbacks and warranty disputes later.

• Sizing the cylinder, expansion vessel and pipework for the property
• Cold water supply requirements: mains pressure, flow rate, balanced cold feeds
• Locating the discharge pipe (D1 and D2) so it terminates safely and visibly
• Combining with secondary circulation for larger properties
• Designing for renewables integration: heat pumps, solar thermal, twin-coil cylinders
• Designing for legionella control (storage temperature, dead legs, distribution temperature)

Every safety device and control on an unvented cylinder has a specific job. You need to know what each one does, how to test it, and how to spot when it has failed. This module walks through each component in turn.

• Expansion vessel: function, charge pressure, sizing and replacement
• Pressure-reducing valve (PRV) and inlet control group
• Expansion relief valve (typically 6 bar)
• Temperature & pressure relief valve (T&P, typically 90°C / 7 bar)
• Cylinder thermostat and high-limit (energy cut-out)
• Immersion heater, motorised valves, system controls and the tundish
• Discharge pipework (D1 from the safety devices, D2 from the tundish to outside)

The three layers of protection on an unvented system, how the discharge pipework must be installed, and how everything ties together when you add renewables or secondary heat sources. This is the highest-stakes module. A mistake here is what G3 was written to prevent.

• The three-tier safety strategy: thermostat, energy cut-out, T&P relief valve
• Why no single device is ever relied on alone
• D1 discharge pipework: minimum size, materials, fall and termination
• D2 discharge pipework: routing, length, equivalent length calculations, visibility requirements
• Tundish positioning and the 300mm rule
• Integration with solar thermal (twin-coil cylinders, drain-back protection)
• Integration with heat pumps (high-recovery coils, larger cylinder sizing)
• Wiring of safety controls and safe electrical isolation

Unvented cylinders should be serviced annually. This module covers what to check, what to test, and what to record to keep keep your customer’s system safe and to maintain the manufacturer’s warranty.

• Annual service checklist for unvented cylinders
• Testing the expansion vessel charge pressure
• Testing both relief valves (PRV and T&P) and verifying they reseat
• Checking the discharge pipework is clear and correctly terminated
• Identifying and replacing failed components
• Descaling and dealing with hard-water areas
• Completing the Benchmark service record and handover documentation
• Common fault diagnosis: weeping valves, no hot water, lukewarm water, noisy operation

The HWSS course takes one full day in-centre, including theory training, practical installation assessment and an open-book multiple-choice exam. Re-assessment candidates complete the same day with a slightly shorter practical element.

The HWSS course costs £230 + VAT at Logic4training. The fee includes a full day of training, the LCL Awards certification fee, all tools and materials, a course manual to keep.

Yes, the HWSS course replaced the standalone Unvented Hot Water course. HWSS covers both vented and unvented systems in one qualification, in line with G3 of the Building Regulations. If your old certificate was titled ‘G3 Unvented’ or ‘Unvented Hot Water’, the renewal route is now the HWSS re-assessment.

The HWSS certificate is valid for five years from the date of issue. To remain qualified, you must re-assess within five years.

If your certificate expired within the last 12 months, you can take the re-assessment route. If it expired more than 12 months ago, you must complete the full initial assessment again.

Yes. Under G3 of the Building Regulations (England and Wales), only competent persons may install, commission or service unvented hot water systems with a capacity greater than 15 litres. The HWSS certificate is the recognised proof of competence and is required to join a Competent Person Scheme and self-certify your work.

Yes, in most cases. The Microgeneration Certification Scheme (MCS) requires installers working on systems that include an unvented hot water cylinder to hold an HWSS certificate. If you want to progress to MCS-registered air source heat pump or solar thermal work, HWSS is a prerequisite.

G3 is the section of the Building Regulations that governs hot water safety. HWSS is the training qualification that proves you are competent to work to G3. You don’t pass ‘G3’ as a qualification – you pass the HWSS course, which is assessed against the G3 standard.

Logic4training runs the HWSS course at all four of its centres: Northolt (West London), Luton (Bedfordshire), Basildon (Essex) and Sittingbourne (Kent). Dates run weekly across the centres.

Yes, provided it expired within the last 12 months. The re-assessment route takes one day and costs the same as the initial assessment (£230 + VAT). If your certificate expired more than 12 months ago, you must take the full initial assessment instead.

On successful completion, you receive the LCL Awards Hot Water Systems & Safety Certificate of Competence. LCL Awards is one of the UK’s leading awarding bodies for building services engineering qualifications, and the certificate is recognised by every Competent Person Scheme approved to offer self-certification under Approved Document G.

Yes, once you have joined a Competent Person Scheme (CPS) using your HWSS certificate. Membership of a CPS lets you notify your own installations directly to the local authority instead of going through Building Control.

The course is open to any trade, but candidates must hold one DCLG-recognised competency in a conventional fuel. Electricians without a plumbing or gas qualification will not be eligible. Electricians working on immersion heaters or controls who do hold a plumbing or gas qualification can attend.