If the pressure or temperature inside the hot water cylinder gets too high and the relief valve opens, water or steam is forced out. The tundish allows you to visually monitor the system so you can easily spot discharge and take the appropriate corrective action. In addition to this, a tundish provides a physical gap (“air gap”) between the drainage pipe and the heating system thus preventing waste water or odours contaminating the clean (potable) water supply,

Seeing water continuously dripping or flowing through the tundish means the relief valve is working, but it signals an underlying fault (like a failing expansion vessel) that requires a professional engineer. Visibility of discharge is also a requirement of Part G3 of the Building Regulations in England and Wales.

Traditionally, tundishes have been passive components. However, modern innovations such as hotun and hotun detect are transforming the role of the tundish from a simple compliance component into an active fault detection solution.

As plumbing and heating systems become increasingly concealed within buildings, making discharge events visible is becoming more important than ever.

In most circumstances, a tundish should remain dry. If water is flowing through it, the system is alerting you to a problem in the system. This is typically a failing expansion vessel.

A tundish is a safety device used in unvented hot water and central heating systems. It acts as a visible visual warning and an air break between pipes. If a fault causes pressure or temperature to rise, the pressure relief valve opens, allowing the tundish to catch and safely redirect escaping water

Expansion vessel problems

One of the most common causes of tundish discharge is a failed or incorrectly charged expansion vessel. Without adequate expansion capacity, system pressure rises as water heats up, causing the relief valve to operate.

Faulty pressure reducing valves

If incoming mains pressure exceeds design limits, excessive system pressure can result.

Debris in relief valves

Limescale or debris may prevent a relief valve from fully reseating after operation, leading to continuous dripping.

Thermostat or control issues

Temperature control faults can cause overheating, activating temperature relief protection.

Even a small but continuous discharge can waste significant amounts of water over time.

Because that water has already been heated, energy is also wasted.

In some installations, discharge can continue unnoticed for weeks or months before a fault is discovered.

This is one reason why discharge monitoring solutions such as hotun detect and hotun detect remote are becoming increasingly popular.

A faulty expansion vessel is one of the most common causes of discharge through a tundish.

Unfortunately, it is also one of the easiest faults to overlook.

Understanding the Problem

As water heats up, it expands. An expansion vessel absorbs this increase in volume and helps maintain safe system pressure.

When the vessel loses charge or fails completely, pressure can rise significantly during heating cycles. The system then protects itself by discharging water through the pressure relief valve.

The hidden cost

A small drip may not seem like a problem. However, a continuous drip over days, weeks or months can result in substantial water loss. More importantly, every litre discharged has already been heated. That means energy has been consumed before the water is wasted.

The real impact

The consequences can include:

• Increased water bills
• Increased energy bills
• Reduced system efficiency
• Additional wear on components
• Customer dissatisfaction
• Unnecessary service visits

Early detection

The sooner discharge is identified, the sooner the underlying fault can be corrected.

Historically, many discharge events have remained unnoticed because tundishes are often installed in lofts, cupboards or concealed locations.

Innovations such as hotun detect remote help address this challenge by providing an audible and visual alert when discharge occurs.

Making faults visible is one of the most effective ways to reduce unnecessary water and energy waste.

G3 refers to Part G3 of the Building Regulations in England and Wales, which covers hot water supply systems and, in particular, unvented hot water storage systems.

Because unvented cylinders operate under mains pressure, they must incorporate specific safety devices to protect occupants and property.

These include:

• Temperature and pressure relief valves
• Expansion control devices
• Correct discharge pipework
• A visible means of observing discharge

The purpose of these requirements is to ensure any excess pressure or temperature can be relieved safely.

Why is compliance important

A properly installed G3-compliant system protects against potentially dangerous operating conditions.

However, compliance should not be viewed as a box-ticking exercise.

A compliant installation that develops a fault may still discharge water unnoticed for extended periods, leading to water waste, energy loss and avoidable maintenance issues.

Beyond compliance

At RA Tech, we believe compliance should be the starting point, not the finish line.

Innovations such as hotun detect and hotun detect remote help installers and building owners identify discharge events sooner, supporting better maintenance and improved system performance.

Unvented hot water cylinders are popular in homes, apartments and commercial buildings across the UK.

They offer excellent hot water performance, but they also require specific safety measures and professional installation.

Unlike traditional vented systems, an unvented cylinder is supplied directly from the mains water supply.

This allows it to deliver hot water at mains pressure, often providing:

• Stronger showers
• Faster bath filling
• Improved flow rates
• Better overall user experience

Because the system is sealed and operates under pressure, safety devices are required to control pressure and temperature.

Safety features

Typical safety features include:

• Pressure reducing valves
• Expansion vessels
• Temperature and pressure relief valves
• Discharge pipework
• Tundishes

These components work together to protect the system.

Monitoring system health

One of the challenges with modern installations is that safety components are often hidden from view.

This means faults can remain unnoticed until they become more significant.

Technologies such as hotun detect remote help bridge this gap by providing audible and visual notification when discharge occurs.

Early warning helps prevent wasted water, wasted energy and unnecessary disruption.

The plumbing and heating industry has embraced smarter controls, connected devices and energy-efficient technologies.

Water safety monitoring is beginning to follow the same path.

A changing industry

Historically, many safety components have been passive.

They operate only when needed and rely on someone being present to notice.

As buildings become more complex and more energy conscious, that approach is becoming less effective.

The rise of early fault detection

The next generation of products focuses on:

• Earlier warning
• Improved visibility
• Reduced waste
• Better maintenance planning

Rather than simply reacting to faults, systems can help identify them sooner.

Benefits of smarter monitoring

Early detection can lead to:

• Reduced water waste
• Lower energy consumption
• Improved system reliability
• Reduced maintenance costs
• Better customer outcomes

Looking ahead

The future of water safety is about visibility, awareness and proactive action.

As the industry evolves, solutions that make hidden faults visible will become increasingly important.