Are Unvented Cylinders Safe?

Why does an unvented cylinder need safety devices at all?

A vented cylinder is open to the air through a vent pipe, so the water can never be at more than a small pressure above the weight of the loft tank. An unvented cylinder is sealed and holds hot water at mains pressure. That is why the taps and showers are stronger. It is also why a pressurised vessel full of hot water is treated more seriously than an open one.

The way this is managed is by layers. No single device carries the safety. If the first one fails, the next one takes over, and the last in the chain, the relief valve fitted on the cylinder discharging through a tundish, is built to vent safely. The design principle is that a fault should become visible long before it becomes dangerous.

Unvented hot water storage is covered by Building Regulations Part G in England, which sets out what must be fitted and tells owners to choose installers with the necessary skills. The cylinder manual adds the manufacturer settings. The combination is what makes the arrangement safe, so a cylinder that has been altered or had a device removed is no longer the system that was assessed.

What are the protections, in order?

Think of five layers, from the everyday control to the last resort.

• The control thermostat holds the water at its set temperature, which the manual and your own needs decide, and switches the heat source on and off.
• The high-limit thermal cut-out opens if the water reaches a limit temperature well above the thermostat setting, and has to be reset by hand. It is not self-resetting by design.
• The pressure reducing valve and the expansion relief valve in the cold inlet group keep the supply at the cylinder pressure and release excess pressure.
• The expansion vessel, or the internal air gap in cylinders without one, takes up the water that expands on heating, so the relief valves are not asked to do it.
• The temperature and pressure relief valve is the last layer. It opens on high temperature or pressure, and its discharge travels down a pipe to a tundish and out to a safe place.

What does each failure look like?

The reassuring thing about the system is that the usual failures announce themselves.

A failed expansion vessel shows up as the tundish dripping on every heat-up, usually from the expansion relief valve. That is the system working, and the fault is described in waterlogged expansion vessel. A temperature and pressure relief valve that drips hot water, or constantly through a scarred seat, is covered in TPRV dripping on an unvented cylinder. A failed thermostat shows as water that is too hot and a thermal cut-out that trips, covered in unvented cylinder tripped cut-out. A drifted pressure reducing valve shows as a noisy supply, pressure that swings, or a discharge at night.

The cases that deserve concern are the ones where nobody sees the warning. A tundish hidden behind a boxed-in panel, a discharge pipe that has been capped or restricted by someone tidying up, or a thermal cut-out wired out of the circuit remove the visibility the system depends on. These are the faults that turn a nuisance into a danger.

A less dramatic failure is corrosion. A cylinder shell that leaks at a weld, or heavy limescale on the element and the valve seats, shows the cylinder has reached the end of its life. That is a repair-or-replace question, not an acute safety one.

What is the scalding or steam safety limit?

If steam or very hot water comes from the tundish, the discharge pipe or a relief valve, the cylinder is overheating and a safety device is doing its job. At that point your job is to stop heating it and stay away.

Turn off the heat source at the isolator. That means the immersion heater switch, the fused spur and the boiler or heating call where the cylinder has a coil. Keep clear of the cylinder and the discharge pipe, keep children and pets away from the area, and call an engineer. Do not open taps to relieve it, do not touch the valves and never try to stop the flow by capping, plugging or tightening anything.

The mains water can usually be isolated at the stopcock, but with a hot cylinder discharging, the priority is to cut the heat first and stand back. Isolating the water on an overheated cylinder does not make it safe if the heat is still going in.

Can an unvented cylinder explode?

A properly installed and maintained unvented cylinder does not explode in normal use, because it is built so that excess heat or pressure has a way out. Approved Document G, the guidance to Building Regulations Part G, expects the cylinder to carry a thermostat, a non-resetting thermal cut-out and relief valves, with a discharge pipe and a visible tundish that carries any release to a safe place. Each layer is there so that a fault ends as water at the tundish, not as pressure building in a sealed vessel.

The risk case is a cylinder where those layers have been removed or defeated: a relief valve blocked or replaced with a plug, a discharge pipe capped, a cut-out wired out of the circuit or a thermostat bypassed. Stored hot water at mains pressure with no way out is the hazard the regulations exist to prevent, which is why work on these devices is for an engineer with the skills for unvented cylinders and why nothing on the discharge route is ever blocked.

For an owner the practical point is to watch the tundish. A dry tundish in normal use is a good sign. Water running into it means a valve is opening and the cause needs finding, but that is the system doing what it was designed to do. Source: Approved Document G (sanitation, hot water safety and water efficiency), 2015 edition with 2016 and 2024 amendments, read October 2026.

What are the real disadvantages of an unvented cylinder?

The honest list is not about danger. It is about cost and dependency.

An unvented cylinder needs an installer with the skills for it, because Part G treats installation as notifiable and expects the safety devices to be fitted properly, so a do-it-yourself installation is not an option. It also needs a decent mains supply, with enough pressure and flow, or the extra pressure never arrives. Where the mains is weak a vented cylinder may serve the house better, and our comparison of vented and unvented cylinders sets out when.

It has more parts to wear. The inlet group, the vessel, the relief valves and the tundish all age, and that is why manufacturers set a service interval in their instructions, and why the unvented cylinder service check looks at each of them in order. Skipping that check is the main way an otherwise safe unvented system drifts towards trouble.

And it is less forgiving when the heating controls fail, because a failed thermostat has a pressurised vessel to push against. That is the reason the layers exist.

None of this makes a well-installed, serviced cylinder unsafe. It means the safety depends on the devices being fitted, working and left alone.