An unvented cylinder expansion vessel loses pressure for three reasons: the air inside it is gradually absorbed into the water, the rubber diaphragm perforates, or the Schrader valve leaks. Once that air cushion shrinks, the cylinder has nowhere to put the extra water it makes as it heats, so the expansion relief valve discharges and water shows at the tundish. Recharging or replacing the vessel sits inside the annual service and belongs to an installer qualified for unvented hot water systems. Your part is to recognise the symptom early and arrange an inspection.
Why does the expansion vessel on an unvented cylinder lose its charge?
The expansion vessel on an unvented cylinder loses its charge in one of three ways. Air slowly dissolves into the water it sits against, so the cushion shrinks. The rubber diaphragm that separates air from water thins and perforates, and water fills the air space. Or the Schrader valve leaks, the same type of valve fitted to a car tyre.
Water expands as it heats, and that is what the cushion is there for. A pressurised cylinder has no open vent pipe to the loft, so the extra volume has to go somewhere. Approved Document G defines an expansion vessel as a vessel that accommodates the increase in volume when water is heated from cold. Take the air away and the expansion has nowhere left to go.
Water at the tundish usually means the cylinder has lost its air cushion
Water dripping from the tundish is the clearest sign that an unvented cylinder expansion vessel has lost its charge. A tundish is the small open gap in the discharge pipework near the safety valves. It lets you see that water has left the system. It also keeps an air break, so waste water cannot be drawn back into the cylinder.
Heatrae Sadia's Megaflo manual links intermittent discharge from the expansion relief valve to a shrinking air volume. In plain terms, the vessel can no longer take the expansion. Note when the water appears and how often. Dripping while the cylinder reheats points at expansion rather than a stuck valve.
Leave the discharge pipe open to the atmosphere and do not cap or plug it. Baxi's guidance is clear that continued discharge after a service is a job for an installer qualified for unvented hot water systems.
External vessel or internal air bubble: which type does your cylinder use?
Unvented cylinders deal with expansion in one of two ways, and both can lose their charge. Some use an external vessel with a rubber diaphragm, mounted on the pipework near the cylinder. Others, including Megaflo models, hold an internal air bubble at the top of the cylinder under a floating baffle. Baxi describes that internal air gap becoming depleted over time as air is absorbed into the water.
Pre-charge figures belong to the installer rather than to you. A standard domestic Megaflo vessel is charged to 3 bar with the system depressurised, and PremierPlus cylinders leave the factory at 3.5 bar. Worcester Bosch gives the general rule: the pre-charge matches the setting of the pressure reducing valve. Checking either figure means depressurising the cylinder, which is service work.
What you can check yourself when the tundish drips, and what you cannot
Four checks are safe for a householder when hot water or heating goes wrong, and none of them involve the cylinder itself:
- Read the boiler pressure gauge and write down the figure
- Check the thermostat and programmer settings, including the hot water timer
- Press the boiler reset button once if the manual tells you to
- Bleed a radiator that is cold at the top
Recharging the air on an unvented cylinder, measuring pre-charge pressure, operating the temperature and pressure relief valve or opening a cylinder casing is not householder work. Manufacturers place recharging inside the annual service carried out by a trained, suitably qualified installer. Work on the gas appliance that heats the cylinder is separate again, and belongs to a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998 as amended in England.
Boiler losing pressure or cylinder vessel flat? How to tell them apart
A falling boiler pressure gauge and a dripping cylinder tundish are two different faults. The gauge on the boiler reads the sealed central heating circuit, which has its own expansion vessel inside or beside the boiler. An unvented cylinder runs at mains water pressure and does not show on that gauge. A flat cylinder vessel therefore appears as water at the tundish, not as a low reading on the boiler.
The failure modes are similar even though the components are separate. Vaillant lists a perforated diaphragm and a leaking Schrader valve among the causes of a boiler losing pressure. Ideal Heating treats a sudden drop with no visible leak as a reason to have the vessel examined. Our guide to low boiler pressure covers the gauge side of it.
Who is allowed to work on an unvented cylinder in England?
In England, unvented hot water storage falls under Part G3 of Schedule 1 to the Building Regulations 2010. G3 requires that a system with no vent pipe open to the atmosphere is installed by a person competent to do that work. It also requires precautions that stop stored water exceeding 100°C and that discharge hot water safely. Approved Document G adds that a cylinder over 15 litres needs a temperature relief valve or a combined temperature and pressure relief valve.
Approved Document G also requires the expansion valve to be fitted so that water is discharged properly if the expansion vessel or the system fails. That discharge route is the safety net behind a vessel that has lost its charge, which is why the tundish and its pipe must stay clear.
Notification depends on the installer's registration. Someone registered with a competent person scheme for unvented hot water systems self-certifies the work. You then receive a Building Regulations compliance certificate, usually from the scheme operator. If nobody holds that registration, the building control body has to be told before installation, and who can install an unvented hot water cylinder sets out the detail.
What to arrange when your cylinder's expansion vessel keeps losing charge
Arrange an inspection of the cylinder rather than waiting for the dripping to settle down. Before the visit, note when water appears at the tundish and how often. Record whether the cylinder had just reheated, and copy the model name from the label if you can read it. That saves guesswork on site.
Recharging the air is part of the annual service on an unvented cylinder, so the two jobs often happen in the same visit. Our guide on whether unvented cylinders need servicing explains what that service covers.
FM Power works across London and the surrounding counties, seven days a week, and puts a written fixed price in front of you before any work starts. If your tundish keeps running, take a look at how Megaflo and unvented cylinder work is handled and book a visit.
Sources
- Approved Document G: Sanitation, hot water safety and water efficiency (2024 amendments)
- The Building Regulations 2010, Schedule 3
- GOV.UK – Competent person scheme: current schemes and how schemes are authorised
- Baxi – How to recharge the air gap on a Megaflo unvented cylinder
- Vaillant – What is an expansion vessel?
- Heatrae Sadia – Megaflo Eco Plus 250/300 installation manual
- Heatrae Sadia – Megaflo Eco installation manual
- Heatrae Sadia – PremierPlus installation manual
- Worcester Bosch – Installation & Maintenance Instructions (1002647)
- Viessmann – What is a tundish for a hot water cylinder?
- Ideal Heating – Why is my boiler losing pressure?
- Heatrae Sadia – How does an unvented cylinder work?