Cylinder types explained – vented, unvented, direct and indirect

You might be used to having a combination boiler – better known as a combi boiler – but if you’re looking at changing to a heat only boiler or updating to an air source heat pump system, then you will need to have a hot water cylinder as well.  

 
When it comes to selecting the perfect hot water cylinder for your home, understanding the difference between the various types is crucial. Vented cylinders rely on gravity and an open vent, while unvented cylinders work under mains water pressure, providing a consistent supply of hot water. 

You will also need to know some key differences between indirect and direct hot water cylinders.

What are vented hot water cylinders?  

A vented cylinder draws cold water stored in a tank, usually located in the loft, and feeds it into the cylinder through gravity. This straightforward connection ensures a steady and reliable supply of hot water for daily water use throughout the home.

Often made of copper or stainless steel, vented cylinders are known for their durability and heat retention. They can be a practical choice if you’re seeking a simple and cost-effective hot water solution.

What are unvented hot water cylinders? 

Unvented cylinders are a modern alternative to vented cylinders that directly connect to the mains water supply for an all-in-one solution. These systems maintain hot water at consistent pressure, eliminating the need for a separate storage tank.

Equipped with a thermostat and immersion heater, unvented hot water cylinders are key for having an efficient system which removes the pressure fluctuations that occur when using a vented hot water cylinder. Because the system doesn’t need a separate tank, unvented cylinders might be the right solution for properties where there is little or no loft space.
 

Pros and cons of unvented cylinders

Unvented cylinders are favoured for their ability to deliver hot water at a constantly high pressure. This makes them a good choice for larger homes with multiple bathrooms on different floors or ones that have en-suites. They are also suitable for both new build properties and ones where the cylinder is being fitted into an existing system, which makes them a flexible option. 

However, systems that use unvented cylinders require additional specialist safety mechanisms like pressure relief valves, which is why they should always be installed and maintained by a qualified professional.

The differences between direct and indirect hot water cylinders

Whether a hot water cylinder is “direct” or “indirect” depends on where it draws its heat energy from. A direct cylinder has its own immersion heater installed and can be programmed to operate during off-peak hours, leading to a reduction in energy consumption and costs, which can be especially beneficial for larger households or properties with high hot water demands. An indirect cylinder uses an external source, such as a boiler. It is also worth noting that they can also be vented or unvented, so remember to keep in mind which one is the most suitable for your property. 

Here at Baxi, we have both direct and indirect unvented cylinders in our range. Let’s take a look at them in more detail.