Understanding Your Water Heater: How Do Unvented Cylinders Work?

Today, there are two types of water heaters used in residential applications. One type is older and has been around for a long time – vented cylinders. The other – an unvented cylinder – has been in use for several decades, but still represents newer technology and is harder for consumers to understand. At Easy Flow, our goal is to help our customers understand the systems and appliances installed within their home, including unvented cylinders.

Unvented cylinders are connected directly to your mains water supply. That ensures there is plenty of pressure when you open the tap, and it is heated using either a series of electric heating elements within the cylinder, or via a boiler. Because of the design, there is no need for a cold-water storage/feeder tank, and no need to install it high up within the home. That frees up plumbing space, plus offers additional room for other things you might want.

While an unvented cylinder does not need a cold-water storage/feeder tank, it does need what’s called an expansion vessel. Basically, water expands when it is heated. Because the cylinder is not vented, there must be somewhere for the excess water to go when it is heated. The expansion vessel is usually installed on top of the cylinder itself and serves this purpose. As the water expands, it moves into the expansion tank. When it cools, it flows back into the body of the cylinder itself. The tank should be inspected annually for safety.

Some unvented cylinders use a different expansion method. Rather than an external expansion tank, they use an internal air bubble. In this situation, air is trapped within the cylinder itself. As the water heats and expands, the bubble becomes compressed, providing additional space for heated water. Like the expansion tank, your internal air bubble should be maintained annually to ensure safe operation.

Vented cylinders allow all the pressure that builds up as a result of heating water to escape into the atmosphere. Unvented cylinders do not. Because of that, they have several safety systems installed. The first is a temperature relief valve that activates if the unit were to overheat and allows excess heat and steam to be vented safely. The second is a pressure relief valve that activates if the pressure reducing valve on the inflow side were to fail.

Unvented cylinders offer excellent performance and better pressure in showers and at the taps throughout your home. However, they should never be installed by amateurs. In fact, only plumbers licensed to install unvented cylinders should be allowed to work in your home. A qualified plumber understands how to install unvented cylinders safely and quickly, and can also provide annual inspections to ensure ongoing safe operation of your hot water system.

At EasyFlow, our engineers are fully trained and qualified to install unvented cylinders. Call us on 0161 941 5571.

EasyFlow can send an engineer to you if you’re located in Greater Manchester, Warrington, Liverpool and Chester.