Common Hot Water System Compliance Failures and How to Avoid Them
A hot water system can be working perfectly well and still have a compliance problem. With unvented cylinders, the issue is often not a failed component but something within the wider installation, such as missing safety controls, unsuitable discharge pipework, poor expansion provision, incomplete commissioning or an alteration made since the system was first fitted.
For installers, hot water system compliance means looking at the whole arrangement rather than checking individual components in isolation. Here, we look at some of the most common hot water system compliance failures, what the G3 regulations require and where problems are most likely to arise.
This guide is intended as an overview for trade professionals. Always refer to current manufacturer instructions, the applicable regulations and Building Control guidance where required.
What Does Hot Water System Compliance Mean?
In England, Part G of the Building Regulations covers sanitation, hot water safety and water efficiency. Approved Document G gives practical guidance on meeting these requirements, with Requirement G3 dealing specifically with hot water supply and systems.
In simple terms, the G3 requirements are there to make sure a hot water system can cope with the temperatures and pressures it may reasonably experience. Stored hot water must also be protected from exceeding 100°C, while water released by safety devices must be discharged somewhere visible without putting people or the building at risk.
Other requirements sit alongside Part G. The Water Supply (Water Fittings) Regulations 1999, for example, cover areas including expansion, temperature control, pressure and relief arrangements.
Manufacturer instructions are just as important when dealing with a particular cylinder or packaged system. Regulations set the overall requirements, but installers still need to follow the approved configuration for the equipment they’re working on.
Approved Document G applies in England, so work elsewhere in the UK should be checked against the relevant national building standards and regulations.
Why Do Hot Water Systems Fail Compliance Checks?
Problems can be introduced at almost any point in a system’s life.
Some are there from the initial design or installation. Others only appear later, perhaps after a component has been replaced, a control altered or a different heat source connected.
Typical hot water system compliance failures include:
- Insufficient or bypassed safety controls
- Discharge arrangements that don’t meet the relevant requirements
- Problems with expansion or pressure control
- Incomplete commissioning or poor records
- Replacement parts or alterations that change the original safety arrangement
- Heat pump systems being treated differently simply because the heat source has changed
It’s worth drawing a distinction between a compliance problem and a conventional hot water system failure. A customer may still have plenty of hot water even though part of the installation no longer meets the required safety standard.
Common Hot Water System Compliance Failures
Incorrect, Missing or Bypassed Safety Controls
Safety devices on an unvented cylinder aren’t there simply as a back-up to normal operation. They’re part of the system’s protection against excessive temperature and pressure.
Approved Document G states that an unvented storage vessel should have at least two independent safety devices, in addition to the thermostat, to prevent stored water exceeding 100°C. It also gives guidance on temperature relief and combined temperature and pressure relief valves.
Problems can arise when a control is removed, bypassed, moved or replaced without checking whether the new arrangement is supported by the manufacturer.
A cylinder continuing to produce hot water isn’t proof that the system remains compliant. If you come across a non-compliant unvented cylinder, the full safety arrangement needs to be considered.
Approved Document G also advises that factory-fitted temperature relief valves on hot water storage units or packages shouldn’t be disconnected, except for replacement, or relocated elsewhere in the installation.
When replacing unvented cylinder safety devices, check the current cylinder instructions rather than assuming a component used on another system will be suitable.
Unsafe Discharge Pipework and Tundish Arrangements
If a safety valve operates, the discharge arrangement has to take potentially very hot water somewhere safe. It also needs to make the discharge noticeable so a fault isn’t allowed to continue unnoticed.
That’s why unvented cylinder discharge pipe regulations cover more than whether a pipe is present.
Approved Document G provides guidance on tundishes, D1 and D2 pipework, suitable materials, routing and termination. Among other requirements, discharge should be visible at the tundish and the pipework should terminate where it won’t create a danger.
Issues can develop where unsuitable materials have been used, the termination isn’t safe, the discharge can’t readily be seen or later alterations no longer follow the cylinder manufacturer’s instructions.
Rather than relying on a remembered version of the tundish regulations, check the current guidance for the system you’re working on. Differences between installations matter.
Incorrect Expansion Provision or Pressure Control
Heating water increases its volume. In an unvented system, there has to be suitable provision for that expansion while keeping the system pressure under control.
The Water Supply (Water Fittings) Regulations include requirements for expansion arrangements and relief valves, including provision for safe discharge if the normal expansion arrangement fails.
Problems can come from incorrect component selection or set-up, checks being missed during commissioning or servicing, or changes elsewhere in the installation affecting how expansion is accommodated.
There isn’t one pressure setting, vessel size or component arrangement that applies to every job. These details should always be taken from the manufacturer’s instructions and the requirements applying to that installation.
Poor Commissioning or Incomplete Documentation
An unvented cylinder isn’t finished just because it’s commissioned.
Proper unvented cylinder commissioning confirms that the completed system and its safety controls operate as intended. Approved Document G covers commissioning of fixed building services, including testing, adjustment and the recording of relevant results.
Paperwork matters here too. Missing commissioning records, incomplete handover information or a notification that hasn’t been completed can all create avoidable problems later.
The installation of an unvented system is also notifiable building work. Where an installer is registered with an appropriate Competent Person Scheme, work covered by that registration can usually be self-certified rather than notified to the Building Control Body in advance.
Non-Compliant Alterations During Servicing or Replacement Work
A system may have started out compliant and then changed over several years of servicing and repair. A control may have been disconnected, a replacement component fitted or another part of the heating system altered without checking how the change affects the original design.
This is where treating components individually can cause problems.
Before replacing or changing anything that forms part of the safety arrangement, check its role within the system as a whole. Two components may appear to do the same job during normal operation while behaving very differently under fault conditions.
Approved Document G makes the wider principle clear: safe operation depends on the system, its safety devices and their correct installation working together.
Applying Boiler-Era Assumptions to Heat Pump Hot Water Systems
This has become a bigger talking point as more heating engineers move into heat pumps.
In 2026, the Hot Water Association and HPA UK published guidance to clarify G3 compliance for unvented hot water systems. One of its key points is that G3 doesn’t stop applying because the heat source has changed.
An unvented cylinder connected to a heat pump, boiler, solar thermal system or another heat source still needs to meet the relevant safety requirements.
Where things can differ is the control strategy. Some heat pump and cylinder packages use arrangements that have been assessed and approved as a complete system. In those cases, installers should work to the manufacturer’s approved design.
A different heat source isn’t, by itself, a reason to remove or bypass a safety provision. Where an alternative arrangement is proposed, it needs suitable manufacturer support and, where applicable, agreement from the relevant Building Control Body.
For engineers working across boilers, heat pumps and hybrid systems, that makes current G3 compliance knowledge particularly important.
What Are the Risks of a Non-Compliant Hot Water System?
The first concern is safety. The temperature, pressure, expansion and discharge provisions on an unvented system exist for a reason.
There are also practical consequences for the installer and customer. Compliance issues identified during Building Control checks may need remedial work, while missing commissioning or notification records can make sign-off more difficult.
There’s also the inconvenience of returning to completed jobs, tracing changes made by other engineers or trying to establish how an installation was originally intended to work when paperwork is missing.
Good records don’t prevent every problem, but they make the history and configuration of a system far easier to understand.
How Can Installers Avoid Hot Water System Compliance Failures?
Follow Approved Document G and Manufacturer Instructions
Avoid installing from habit when the current documentation is available.
Check the relevant hot water safety regulations, Approved Document G and the instructions for the actual cylinder or package being installed. A layout you’ve used dozens of times on one product shouldn’t automatically be copied to another.
This is particularly important where manufacturers use alternative control strategies that have been assessed as part of a complete package.
Check the Complete Safety and Discharge Arrangement
When checking compliance, look beyond the individual valve or control you’ve been called to inspect.
Consider temperature protection, pressure and expansion provision, safety devices and the full discharge route together. If something has been replaced or altered, confirm that the system still matches the intended arrangement.
The same applies during servicing. Normal operation at the time of the visit doesn’t rule out a missing or unsuitable safety provision.
Commission, Document and Hand Over Correctly
Commissioning and paperwork should be part of the job, not an afterthought once the tools are back in the van.
Complete the required checks, record the relevant information and leave the customer with the appropriate handover documentation. Where Building Regulations notification is required, make sure it goes through the correct route for the work and your registration.
Those records are useful for the next engineer too, particularly if the system needs attention several years later.
Keep Hot Water Safety Knowledge and Competence Current
Product designs change. Heating technologies change. Guidance changes too.
The questions raised by heat pumps and unvented cylinders are a good example of why installers need to keep their knowledge current rather than relying solely on experience with older systems.
Approved Document G highlights the importance of installers having the necessary skills for unvented hot water work, while current industry guidance also recommends recognised training alongside relevant manufacturer training.
If you want to update or refresh that knowledge, the Intergas HWSS Hot Water Systems Safety / Unvented G3 course covers areas including installation, commissioning, servicing, maintenance, safety controls and the regulations surrounding unvented hot water systems.
What Should You Do if You Find a Non-Compliant Unvented Cylinder?
If something within the safety arrangement doesn’t look right, don’t simply reset the system and move on.
First establish how the cylinder is supposed to be configured. Check the current manufacturer’s instructions and any relevant product documentation against what’s actually installed, then consider the applicable Building Regulations and Water Supply requirements.
Where components or controls have been changed, look at the effect on the entire safety arrangement rather than the most obvious fault alone.
Any remedial work should follow the manufacturer’s approved configuration. If an alternative approach is being considered or there’s uncertainty about how the regulations apply, get appropriate manufacturer, technical or Building Control advice before proceeding.
Build Compliant Hot Water Systems with the Right Knowledge
Good unvented hot water work is about understanding how the complete system protects against excessive temperature and pressure, not simply knowing where each component fits.
That becomes even more relevant when engineers are moving between boilers, heat pumps and hybrid systems, where the control strategy can look quite different from one installation to the next.
For installers looking to strengthen or refresh their understanding of unvented hot water safety and G3 compliance, the Intergas HWSS Hot Water Systems Safety / Unvented G3 course provides practical training focused on the work engineers encounter on site.
[VIEW THE INTERGAS HWSS COURSE]
Frequently Asked Questions
What Are the Most Common Hot Water System Compliance Failures?
Common issues include missing or bypassed safety controls, unsuitable discharge arrangements, incorrect pressure or expansion provision, incomplete commissioning and alterations that no longer match the manufacturer’s approved system.
Importantly, a compliance problem doesn’t always stop the system from working. An installation can continue providing hot water while still falling short of the relevant safety requirements.
What Makes an Unvented Cylinder Non-Compliant?
There’s no single fault that makes every unvented cylinder non-compliant. Problems may relate to the safety devices, control strategy, pressure and expansion arrangement, discharge pipework, commissioning or Building Regulations notification.
The installation needs to be assessed as a complete system against the manufacturer’s instructions and the applicable requirements.
What Are the Main G3 Requirements for Unvented Hot Water Systems?
At a high level, G3 requires hot water systems to withstand the temperatures and pressures they can reasonably be expected to experience. Stored hot water must be protected from exceeding 100°C and discharge from safety devices must be taken somewhere visible without creating danger.
Approved Document G provides more detailed guidance on meeting those requirements.
Do G3 Requirements Apply to Hot Water Cylinders Connected to Heat Pumps?
Yes. Current HWA/HPA guidance confirms that the relevant G3 requirements continue to apply when an unvented cylinder is connected to a heat pump.
Some approved heat pump and cylinder packages use different control strategies. Where that’s the case, the manufacturer’s approved system design and instructions should be followed.
What Are Common Tundish and Discharge Pipe Compliance Problems?
Problems can include unsuitable materials, an unsafe termination point, poor visibility of discharge, incorrect routing or arrangements that don’t follow Approved Document G and the cylinder manufacturer’s instructions.
The discharge arrangement needs to carry water from the safety devices somewhere safe while making it clear that a discharge has occurred.
Who Is Responsible for Ensuring an Unvented Hot Water System is Compliant?
People carrying out building work, including designers, builders and installers, have responsibilities for ensuring their work complies with the relevant Building Regulations. Building owners may also have responsibilities.
For installers, that means considering compliance throughout design, installation, commissioning, documentation and any later alterations to the system.
