An unvented hot water cylinder is a sealed storage tank fed directly by the cold mains, so it delivers hot water at mains pressure without needing a tank in the loft. That means strong, even flow at the taps and shower, fed straight from the same supply that feeds your kitchen cold tap.
How an unvented cylinder differs from the older approach
Traditional "vented" systems store cold water in a tank in the loft and let gravity push hot water down to the taps. The pressure depends on how high that tank sits above the outlet, which is why upstairs showers on older systems can feel weak.
An unvented cylinder skips the loft tank entirely. Cold mains water enters the bottom of the cylinder, is heated, and leaves the top at close to incoming mains pressure. Because both hot and cold are at mains pressure, you get balanced flow — useful for showers, and especially for more than one outlet running at once.
The trade-off is that storing heated water in a sealed, pressurised vessel introduces risks that a vented tank doesn't have. That is why the design includes several safety parts and why installation is tightly regulated.
The safety components and why they matter
An unvented hot water cylinder is a sealed storage tank fed directly by the cold mains, so it delivers hot water at mains pressure without needing a tank in the loft.
Heating water in a sealed container makes it expand and the pressure rise. If that pressure had nowhere to go, the cylinder could fail dangerously. A correctly built unvented system controls this through layered safety devices, each backing up the next.
- Expansion vessel — a separate chamber, usually with a rubber diaphragm and a cushion of air, that absorbs the increase in volume as water heats. This stops the pressure climbing too far during normal use.
- Pressure-reducing valve — limits the incoming mains pressure to a level the cylinder is designed for.
- Expansion relief valve — opens to release water if pressure rises beyond a set point, for example if the expansion vessel fails.
- Temperature and pressure relief valve — a final safeguard that discharges water if the temperature or pressure becomes unsafe.
When a relief valve opens, the released water has to go somewhere visible and safe. That is the job of the tundish — a small open funnel in the discharge pipe. It creates an air gap (so cylinder water can't be drawn back into the mains) and lets you see at a glance if a valve is leaking. From the tundish, a discharge pipe carries any hot water to a safe outside point where it can't scald anyone.
These parts together make up the G3 safety requirements, the section of the Building Regulations covering unvented hot water storage. By law, installing or making major changes to an unvented system must be done by someone holding the relevant G3 qualification, and the work is notifiable to building control. This is not optional, and it is the main reason these cylinders are not a DIY job.
When an unvented system suits a home
An unvented cylinder works well where the household wants strong, simultaneous hot water and the incoming mains can supply it. It is a common choice in homes with more than one bathroom, or where a powerful shower without a separate pump is wanted.
It also frees up the loft, since no cold storage tank or feed pipework is needed up there — helpful in a loft conversion or where roof space is limited.
It is less suitable where the mains supply itself is weak. If the incoming flow rate or pressure is low, an unvented cylinder cannot create pressure that isn't there, and the benefit largely disappears. A plumber will usually measure the mains flow rate and pressure before recommending one. For homes with poor mains, a vented system with a pump, or a different storage approach, may make more sense.
Matching storage size to how the household uses hot water
A stored-water cylinder holds a fixed volume of hot water, so the size needs to match peak demand. Run out, and you wait for the cylinder to reheat. Oversize it, and you pay to keep more water hot than you ever use.
The main factors are the number of bathrooms, how many people are likely to draw hot water at the same time, and habits such as baths versus showers. As a rough guide:
- A small household with one bathroom may be comfortable with a smaller cylinder.
- A family home with two bathrooms generally needs a larger capacity to cover back-to-back showers.
- Homes with several bathrooms, or a large bath, need careful sizing and may also need a check that the mains can refill the cylinder quickly enough.
The heat source matters too. A cylinder heated by a gas or oil boiler usually reheats faster than one relying only on an immersion heater. An immersion heater is an electric element inside the cylinder, much like a kettle element, that heats the water directly. Most unvented cylinders include at least one as a backup or as the main heat source in all-electric homes, though heating a full tank on electricity alone is slower and can cost more to run.
Servicing, lifespan and running costs to keep in mind
Unvented cylinders need an annual service to stay safe and efficient. The key task is checking the expansion vessel, because its air cushion gradually escapes over time. If it loses its charge, pressure has nowhere to go and the relief valves may start weeping water through the tundish — often the first visible sign that something needs attention.
A service also covers testing the relief valves and inspecting the discharge route. Many cylinders contain a sacrificial anode that protects the tank from corrosion; this is checked and replaced as needed depending on the model and water conditions.
A well-maintained cylinder can last many years, though hard water and neglected servicing shorten that. On running costs, the cylinder itself loses some heat through its insulated jacket even when no one is using hot water, so good insulation and a sensibly chosen capacity both help. Using a timed boiler or off-peak electricity, where available, can reduce the cost of keeping the stored water hot.
If you are weighing up an unvented cylinder, it is worth having the mains flow and pressure measured first, confirming the installer holds a current G3 qualification, and agreeing how the annual service will be arranged afterwards.