A vented hot water cylinder is fed by a cold water tank in the loft and works at low pressure. An unvented cylinder is fed straight from the mains and stores hot water under pressure. That one difference affects water pressure, safety parts and who can install the system.
How a Vented Cylinder Works
In a vented system, a cold water storage cistern sits in the loft, often higher than the cylinder. Gravity pushes water down from the cistern into the cylinder, where it is heated. The pressure at your taps depends on how high the cistern is above them.
A vented cylinder also has an open vent pipe. This runs up and over the cistern, so the system is open to the air. If the water gets too hot and expands, it has somewhere safe to go.
Vented systems are common in older UK homes. They are fairly simple, but upstairs showers can feel weak because the cistern is not far above them.
How an Unvented Cylinder Works
An unvented cylinder connects straight to the cold mains supply, so there is no loft cistern. The hot water is stored at close to mains pressure. This often means stronger showers and the ability to run more than one tap at a time.
Because the water is stored under pressure, an unvented cylinder needs safety devices. These include a temperature and pressure relief valve, an expansion vessel or space for expansion, and a tundish with a discharge pipe. These parts are there to protect the cylinder and the people in the home, and they must never be blocked, capped or altered.
An unvented cylinder also needs good mains pressure and flow. If the supply to the home is weak, the cylinder may not perform as expected, so this is usually checked before one is chosen.
Installation Rules and Competence
The rules are much stricter for unvented cylinders. They are covered by Building Regulations requirement G3, set out in Approved Document G. Installation must be carried out by an appropriately competent installer, and the work needs to be certified through a competent person scheme or building control.
Unvented cylinders are not a DIY job. Their safety parts are there for a reason, and mistakes can be dangerous. Vented systems are less complex, but changes to them should still be done by someone who understands the whole system.
Which Type Suits Your Home?
The right choice depends on your mains pressure, the space you have, how much hot water you use and how your water is heated. Some homes suit a vented system well, while others benefit from switching to unvented.
If you are thinking about a change, it helps to understand what the work involves first. Our page on replacing a hot water cylinder covers this in more detail.
If you are in Swindon and are unsure which type of cylinder you have, Emergency Plumber Swindon can talk it through with you and look at the plumbing side. Call 01793 569521.