If your hot water cylinder is leaking into the tundish or you see puddles of water below the cylinder, it is because the external expansion vessel has failed, causing the PRV (pressure relief valve) or TPRV (temperature and pressure relief valve) to open. If these valves open repeatedly, they can become damaged and start leaking constantly.
The tundish is a key safety component, which is a ‘viewing window’ to alert you that there is a problem with your cylinder safety components. The tundish may overflow onto the ground if the D2 pipe from the tundish cannot dispel water quickly enough if the leak is consistent or bad. Do not ignore leaks into your tundish or from your tundish overspilling onto your floor.
If the tundish is dripping, it means the system is not controlling pressure correctly for these reasons:
Expansion vessel failure. An expansion vessel is designed to absorb excess heat Expansion when a cylinder is heating. This is the first safety device
PRV (Pressure relief valve). The PRV is a safety device that is designed to open if the Expansion vessel cannot do its job
TPRV (temperature and pressure relief valve). The PRV is a safety device that is designed to open if the Expansion vessel cannot do its job
This discharge must be diagnosed properly. It is not normal for an unvented system to discharge regularly. The Expansion vessel, PRV and TPRV are three key safety devices on Unvented Cylinders which are there specifically to prevent the cylinder for over-pressuring and overheating, which can result in disastrous results such as split cylinder or in extreme cases – cylinder explosion.
This is the correct chain of events in most unvented cylinder leaks:
The external expansion vessel fails
Pressure rises as the water heats up
The PRV or TPRV opens to relieve excess pressure
The valve opens repeatedly and becomes damaged
The PRV or TPRV begins leaking constantly
This is why the correct starting point is always the external expansion vessel.
When cold water heats up, it expands.
In an unvented cylinder system, this heat expansion must be absorbed by an expansion vessel. Expansion vessels must be completely full of air and not water. If yous expansion vessel feels heavy or is appears to be falling off the wall, your expansion vessel is full of water and need replacing.
If the external expansion vessel fails, there is no longer enough space for expansion.
This causes pressure to rise every time the cylinder heats.
Common external expansion vessel faults include:
Loss of air charge
Ruptured diaphragm and expansion vessel fills with water instead of air.
Incorrect pre-charge pressure
Incorrect sizing
Some cylinders, most notably Megaflo, use an internal expansion method (often referred to as an internal air gap or air bubble) rather than an external expansion vessel. The airgap will first be recharged. However, if recharging the air gap fails, there is NO way of replacing the diaphragm as it is internal ie. inside the cylinder itself.
There are two potential options if recharging the air gap does not work: Replace the cylinder or install an external expansion vessel.
The principle is the same:
If expansion is not controlled correctly, pressure rises during heat-up and the safety valves discharge.
So even on Megaflo cylinders, a dripping tundish still indicates an expansion-related problem, even though there is no external vessel to replace.
Yes, in some cases, an expansion vessel can be recharged (re-pressurised) by pumping it back up to the correct air pressure.
However, it is important to understand that this is not a guaranteed solution.
The reason is simple: the expansion vessel contains an internal rubber diaphragm (similar to a balloon). If that diaphragm has developed a pinhole rupture, split, or tear, the vessel may still accept air when pumped up — but it will slowly deflate again over time.
A good way to think of it is this:
If a balloon has a tiny hole in it, you can refill it with air, but it will still deflate again afterwards.
This means the system may return to the same symptoms and require a repeat call-out.
For this reason, it is usually best practice to replace the expansion vessel for certainty, and to prevent the PRV and TPRV being forced to open repeatedly again.
Boilergurus can recharge an expansion vessel where appropriate, but it is important to understand that this comes with a 60-70% risk that the vessel will deflate again and cycle of leaks into tundish will return as the PRV & TPRV activate again to protect the cylinder from over pressurising and excess heat.
Once the external expansion vessel has failed (or the cylinder’s internal expansion method is not working properly), system pressure rises.
To protect the cylinder from over pressuring due to a failed expansion vessel, there are two safety devices that activate next:
PRV (Pressure Relief Valve)
The PRV opens due to pressure only.
This is the most common valve to discharge when the external expansion vessel fails.
TPRV (Temperature & Pressure Relief Valve)
The TPRV can open due to:
Excess pressure
Excess temperature
Or both
A PRV or TPRV is designed to open and then reseal.
But once a valve has opened repeatedly, it may begin leaking constantly due to:
The washer hardening and losing elasticity
Scale or debris trapped in the valve seat
Wear from repeated activation
This is why many homeowners notice:
A drip that starts only during heating
Then becomes more frequent
Then becomes constant
This is the key rule that prevents repeat callouts:
If the PRV or TPRV is leaking, the external expansion vessel is faulty.
Because the safety valves should not be opening repeatedly in normal operation.
If a PRV or TPRV is opening, it is because the expansion vessel has failed and is not doing its job, the expansion vessel is the FIRST safety device in an unvented cylinder.
Repair in the correct sequence
Correct repair order is:
Recharge or Replace the external expansion vessel
Then test the PRV and TPRV
Replace the PRV, TPRV or both depending on test results
Recharging the expansion vessel is not a guaranteed repair and has a 30-40% chance of success.
Unvented cylinders are regulated under G3 Building Regulations.
The discharge system is a safety requirement.
If a tundish is dripping, the system is telling you something is wrong — and the root cause must be corrected.
Because the external expansion vessel has failed, causing the PRV or TPRV to discharge water.
Yes. If a PRV is leaking, the external expansion vessel should be usually replaced.
Yes. Megaflo cylinders do not have an external expansion vessel, but they can still discharge from the PRV or TPRV if expansion is not being absorbed correctly.
Yes to a degree. Expansion vessels must be completely full of air and not water. If your expansion vessel feels heavy or is appears to be falling off the wall, your expansion vessel is definitely full of water. However this is not always so obvious. Just because it is not falling off the wall does not mean it is not faulty. Boilergurus have specialist equipment to test the air charge within the expansion vessel.
PRV has been damaged and cannot reseal, usually after repeated discharge caused by a failed external expansion vessel.
TPRV (temperature and pressure relief valve) has been damaged and cannot reseal, usually after repeated discharge but the root cause is a usually a failed external expansion vessel.
Yes. This requires urgent diagnosis. It can indicate severe overpressure or overheating. However often the Unvented cylinder D2 pipe is terminated within the property – discharging into an internal bathroom or kitchen waste pipe or soil stack and therefore this constant discharge of water will not be visible. This is why the tundish is a key safety component, which is a ‘viewing window’ to alert you that there is a problem with your cylinder safety components. Do not ignore leaks into your tundish or onto your floor.
If your unvented cylinder is leaking from the tundish, Boilergurus can diagnose and repair it properly.
We carry out unvented cylinder work using our trio of skills: plumbing, heating, and electrics.
We cover Surrey and South-West London including:
Wimbledon, Raynes Park, Kingston, New Malden, Esher, Cobham, Weybridge, Woking, Guildford, Leatherhead and surrounding areas.
20th August, 2026
18th March, 2026
