Vented vs Unvented Cylinder: The Difference and Which to Choose

The difference comes down to where the cold water comes from. A vented cylinder is fed by gravity from a cold water storage tank in the loft, so the pressure at your taps depends on how high that tank sits above them. An unvented cylinder is sealed and fed straight from the mains, so it delivers hot water at close to mains pressure and needs no loft tank. Unvented gives stronger showers and more space, but it relies on a good mains supply and has stricter safety rules. Vented is simpler, cheaper to maintain, and still the better choice in some homes.

If you are not sure whether you need a cylinder at all, our combi vs system boiler guide covers when a combi with no cylinder is the better fit.

How each cylinder works

Vented (open vented) cylinder

Cold water from the mains fills a storage tank in the loft. That tank feeds the cylinder by gravity. The cylinder heats the water with a coil from the boiler, an immersion heater, or both. An open vent pipe runs from the top of the cylinder back up over the tank, so if the water ever overheats it has somewhere to go. The system is never under more pressure than the weight of water above it.

As a rough guide, every metre of height between the bottom of the loft tank and the outlet gives about 0.1 bar of pressure. A shower one floor below the loft may only get 0.2 to 0.3 bar, which is why vented systems often have a shower pump.

Unvented cylinder

The cylinder is connected directly to the incoming mains. A pressure reducing valve brings the mains down to a steady working pressure, usually around 3 bar depending on the model, and the cylinder stores and heats the water under that pressure. Because the system is sealed, it needs safety devices: a temperature and pressure relief valve, an expansion vessel or internal air gap to take up the expansion of heated water, and a discharge pipe with a tundish so any release can be seen and runs safely to a drain. Megaflo is a well-known brand; our article on Megaflo air gap problems covers one type of unvented design.

Vented vs unvented: side-by-side

When a vented cylinder makes sense

• Your mains pressure or flow is poor. Many older London streets and upper-floor flats have weak mains. An unvented cylinder cannot deliver more than the mains gives it, so a vented system with a suitable pump may perform better. Our guide to low water pressure at home shows how to test it.
• You are replacing like for like. If the existing vented system works and you are happy with it, a new vented cylinder is the simplest and usually the quickest swap.
• There is no safe route for a discharge pipe. Unvented cylinders need one. In some flats and awkward cupboards there is nowhere suitable to run it.
• You want low maintenance. Vented systems have fewer safety parts to check and fewer things to go wrong.

When an unvented cylinder makes sense

• You want better showers. With good mains, an unvented cylinder gives strong, balanced pressure at every tap and shower without a pump.
• You have more than one bathroom. Two showers running at once is where unvented systems shine, provided the mains flow can keep up.
• You want the loft back. Taking out the tank frees space for a loft conversion or storage and removes the risk of a loft tank leaking or freezing.
• You are renovating anyway. If walls and floors are open, it is the best time to run the discharge pipe and make any pipework changes.

Safety rules for unvented cylinders

Under Part G of the Building Regulations, an unvented hot water system with a capacity over 15 litres has to be installed by someone competent to do it, which in practice means an engineer holding a G3 unvented qualification, and the installation has to be notified to building control. That is because a sealed cylinder storing hot water under pressure is dangerous if its safety devices fail.

Once it is in, most manufacturers ask for an annual check. The engineer tests the relief valves, checks the expansion vessel charge or air gap, checks the pressure reducing valve and makes sure the discharge pipe and tundish are clear. Water running through the tundish is a warning sign, not a normal drip; see unvented cylinder tundish dripping for what it means.

Both types of cylinder should store water at 60°C or above to control Legionella, with thermostatic mixing valves where scalding is a risk.

Switching from vented to unvented

Most vented systems can be converted to unvented if the mains supply is good enough. The job involves fitting the new cylinder, adding the safety devices and discharge pipe, removing or decommissioning the loft tank, and checking that the existing pipework and taps can take mains pressure. Older joints and cheap taps that were fine at low pressure can leak when the pressure goes up, so a good installer checks them before the switch.

It is also the right moment to check the cylinder is the right size for your household. Our guide on what size hot water cylinder you need goes through the numbers.

Frequently Asked Questions