Why Is Water Coming Out of My Tundish? Causes, Fixes & When to Take Action
- Introduction
- What Is a Tundish?
- Should Water Ever Come Out of a Tundish?
- The Most Common Reasons a Tundish Is Dripping
- How Can You Tell Which Component Is Causing the Problem?
- Can You Ignore a Dripping Tundish?
- Common Replacement Parts for Tundish Discharge Problems
- Final Thoughts
- Frequently Asked Questions
Water Coming Out of Your Tundish? Here's What It Could Mean
If you've noticed water dripping or flowing from a tundish connected to your hot water cylinder, it's natural to be concerned. While an occasional discharge may not always indicate a serious fault, continuous dripping or regular water flow is usually a sign that something within the system requires attention.
A tundish is an important safety component found on unvented hot water cylinders such as Megaflo systems. Its purpose is to provide a visible indication that a pressure relief or temperature relief device has operated. In other words, it's designed to show you when your system is discharging water for safety reasons.
In this guide, we'll explain what a tundish does, why water may be coming out of it, the most common causes of discharge, and the components that may need inspection or replacement.
What Is a Tundish?
A tundish is a small fitting installed within the discharge pipework of an unvented hot water system.
Its transparent section allows water discharge to be seen easily if a safety valve activates. Building Regulations require tundishes on unvented cylinders because they provide a visible warning that the system is relieving excess pressure or temperature.
Typical locations include:
- Beside an unvented hot water cylinder
- Near a Megaflo cylinder
- Adjacent to expansion relief valves
- Connected to discharge pipework from safety devices
A properly functioning tundish should normally remain dry.
Should Water Ever Come Out of a Tundish?
Occasional discharge can occur during certain operating conditions, particularly if the water within the cylinder expands during heating.
However, a tundish that drips regularly or discharges continuously should not be ignored.
Common signs of a problem include:
- Constant dripping
- Water running after heating cycles
- Water discharging every day
- Visible staining beneath the tundish
- Increased water consumption
- Pressure fluctuations within the system
If any of these symptoms are present, further investigation is recommended.
The Most Common Reasons a Tundish Is Dripping
Several different faults can cause water to discharge through a tundish. Identifying the underlying cause is important because replacing the wrong component can waste both time and money.
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A Failed Expansion Vessel
One of the most common causes of tundish discharge is a faulty expansion vessel.
As water heats, it expands. The expansion vessel absorbs this increased volume and prevents excessive pressure from building within the system.
When an expansion vessel loses its air charge or develops an internal diaphragm failure, pressure can rise rapidly during heating. This often causes the expansion relief valve to open and discharge water through the tundish.
Symptoms of a failed expansion vessel:
- Water only discharges when heating is active
- Relief valve opens frequently
- Pressure fluctuates significantly
- Expansion vessel feels unusually heavy
Expansion vessels can sometimes be recharged, but older or damaged vessels often require replacement.
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A Faulty Expansion Relief Valve
The expansion relief valve protects the system by opening if pressure exceeds its design limit.
Over time, debris, limescale or general wear can prevent the valve from sealing correctly.
Once this happens, the valve may continue allowing water to pass even after pressure has returned to normal.
Signs of a faulty relief valve:
- Constant dripping from the tundish
- Water discharge even when heating is off
- No obvious pressure problems elsewhere
- Evidence of scale deposits around the valve
In many cases, replacing the relief valve resolves the issue.
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Problems with the Inlet Control Valve
The inlet control valve regulates incoming mains water pressure to the cylinder.
If the pressure reducing section fails or becomes contaminated with debris, excessive pressure may enter the system.
This can trigger safety devices and cause regular discharge through the tundish.
Possible symptoms include:
- Frequent relief valve activation
- Increased system pressure
- Intermittent discharge patterns
- No visible faults with the expansion vessel
Many manufacturers supply replacement cartridges or complete inlet control valve assemblies depending on the model.
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Excessively High Mains Water Pressure
In some properties, mains water pressure can exceed the operating limits of the cylinder's control equipment.
This is particularly common in areas where network upgrades have increased supply pressure over time.
If incoming pressure becomes too high, relief devices may activate more frequently.
Signs include:
- Strong flow at taps
- Repeated discharge from safety valves
- Pressure-related issues elsewhere in the property
A qualified installer can test incoming mains pressure and determine whether corrective action is required.
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A Temperature and Pressure Relief Valve Operating
Every unvented cylinder includes a temperature and pressure relief valve (T&P valve).
This safety device is designed to operate only under abnormal conditions.
If the valve activates regularly, it may indicate:
- A thermostat fault
- An immersion heater problem
- Excessive cylinder temperature
- A faulty T&P valve
Because this valve forms part of the cylinder's critical safety system, repeated discharge should always be investigated promptly.
How Can You Tell Which Component Is Causing the Problem?
Diagnosing tundish discharge usually involves checking:
- Expansion vessel pressure
- Relief valve condition
- Inlet control valve performance
- System operating pressure
- Cylinder thermostat operation
The discharge pattern often provides useful clues.
A qualified engineer can carry out pressure testing and confirm the exact cause.
Can You Ignore a Dripping Tundish?
No.
While a small drip may not seem serious, ongoing discharge usually indicates an underlying fault.
Ignoring the problem can result in:
- Increased water bills
- Premature wear on safety components
- Damage to surrounding areas
- Reduced system efficiency
- Potential failure of other components
Early diagnosis is normally the most cost-effective solution.
Common Replacement Parts for Tundish Discharge Problems
Depending on the fault identified, replacement parts may include:
- Expansion vessels
- Expansion relief valves
- Pressure reducing valve cartridges
- Inlet control valve kits
- Temperature and pressure relief valves
- Tundish assemblies
Always ensure replacement parts are compatible with your specific cylinder model.
Final Thoughts
A tundish is designed to provide an early warning that something within your unvented hot water system requires attention.
While occasional discharge may occur under certain conditions, regular dripping or continuous water flow is usually a sign of a pressure, temperature or control valve issue.
The most common causes include failed expansion vessels, faulty relief valves and inlet control valve problems. Identifying the root cause early can help prevent unnecessary water waste and avoid more costly repairs later.
If you're replacing components on a Megaflo or other unvented hot water cylinder, using genuine replacement parts helps ensure compatibility and maintains system safety.
FAQ Section
Why is my tundish dripping constantly?
A faulty expansion relief valve commonly causes a constantly dripping tundish, failed expansion vessel or excessive system pressure.
Is it normal for water to come out of a tundish?
Occasional discharge during heating cycles is normal, but regular or continuous discharge usually indicates a fault requiring investigation.
Can a failed expansion vessel cause a tundish to leak?
Yes. A failed expansion vessel is one of the most common reasons for water discharging through a tundish on an unvented cylinder.
Does a dripping tundish mean my cylinder is faulty?
Not necessarily. In many cases, the issue lies with a valve, expansion vessel or pressure control component rather than the cylinder itself.
Can I stop a tundish leaking by replacing the tundish?
Usually not. The tundish is normally only indicating a problem elsewhere in the system. The underlying cause must be identified first.
When should I call an engineer?
If the tundish is discharging regularly, continuously, or releasing large amounts of hot water, professional investigation is recommended.