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Specialists in Electric Hot Water Cylinders

How Does an Unvented Cylinder Work: Understanding Today’s Water-Heating Technology

15 June 2023

Once, vented cylinders were the only option. Manufacturing techniques and available materials simply offered no alternatives. Today, that’s not the case. The unvented cylinder has become the most popular way to heat water in traditional homes, flats, and even within businesses. Not sure how these cylinders work? We’ll give you a brief crash course.

What Is an Unvented Cylinder?

An unvented cylinder is exactly what it sounds like – a sealed system designed to heat water that’s then supplied to the taps within a home. Unlike a vented cylinder, these systems do not have a built-in vent that constantly allows pressure and heat to escape.

As you might imagine, that means they can build up very high amounts of pressure inside. They can also get quite hot. For those reasons, they’re equipped with several important safety devices, including thermostats and pressure relief valves.

How Does an Unvented Cylinder Work?

Unlike vented cylinders, unvented cylinders connect directly to the mains. Cold water flows in from the mains, where it is then heated. Unvented cylinders can connect to your boiler, but it’s increasingly common to use a direct heating method, such as immersion heaters.

These are special heating elements installed inside the cylinder that warm the water and maintain a consistent temperature. That said, indirect heating sources remain common, and most systems can be used with an external boiler, or even configured to use solar panels.

A thermostat ensures the unvented cylinder does not get too hot. When the internal temperature reaches the pre-set temperature, the heaters turn off. When the temperature drops, the heaters turn on once more.

Because the unvented cylinder maintains pressure, it does not need to use gravity to feed hot water into the home’s pipes. Instead, it provides mains-pressure water to every tap in the home. That helps avoid things like some taps having high pressure and others having little more than a trickle.

Unvented cylinders are very safe, despite the high pressures within the tank. Pressure is handled in several ways. One of those is a thermal expansion vessel. Since water cannot be compressed and expands when it’s heated, the expansion vessel provides space for this to occur. Another solution, depending on the manufacturer and cylinder, is the bubble top unit, which uses an internal air bubble.

The pressure relief valve also plays a vital role in safety. If the internal pressure becomes too high, the valve is designed to open and release pent-up pressure. That prevents the potential for an explosion.

However, because these cylinders have multiple safety systems and run at high pressures, they must be properly maintained. Most insurance schemes require an annual inspection, and only G3-certified heating engineers are allowed to service, maintain, repair, or install unvented cylinders.

EasyFlow are certified and authorised to handle all your unvented cylinder-related needs. Whether you need to arrange for annual maintenance, are experiencing a problem with your cylinder, or you want a new one installed, we can help.


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