Types of Water Cylinders: Unvented and Vented

Many homes come equipped with a hot water cylinder, but not all cylinders are built the same and are divided into two camps: unvented and vented. 

Many homes come equipped with a hot water cylinder, but not all cylinders are built the same and are divided into two camps: unvented and vented. 

It’s worth mentioning that, depending on your heating system, you may or may not need a hot water cylinder at all. For example, smaller homes or flats can opt for a combi boiler that doesn’t require a cylinder. 

However, if you need a cylinder, you might be confronted with a choice between unvented or vented cylinders. This article explores the differences, strengths and limitations of both. 

The three types of boilers

To explain this, it’s essential to understand that there are three main types of gas boilers: combi boilers, system boilers and conventional boilers. 

With a combi boiler, one unit heats water and delivers it to taps and appliances without needing an extra water tank. On the other hand, system and conventional boilers both use separate water tanks to meet higher hot water demands in larger properties. 

System boilers don’t require a gravity-fed cold water feed tank (typically mounted in the loft). In essence, conventional heating systems involve three core parts: the boiler, hot water tank and cold water feed, whereas modern system boilers don’t require the gravity feed tank, so they have just two main components. 

In either case, it’s very common for hot water tanks to be installed in larger homes with three bedrooms or more.

Unvented cylinders

Unvented cylinders form part of pressurised heating systems that provide hot water at mains pressure. By ‘unvented’, this pretty much means these cylinders form part of closed, pressurised systems. 

They’re typically made of durable stainless steel and can deliver high-pressure hot water to larger households. They’re typically used in conjunction with system boilers. 

Unvented boilers often come equipped with their own immersion heater to provide hot water even when the boiler is not used. This also reduces the likelihood of hot water running out when many people are using it, as may be the case with conventional systems.


  1. Increased water pressure: Unvented cylinders provide hot water at mains pressure, which means you get a strong and consistent water flow throughout your property. This is ideal for 3 or 4-bed houses and larger when multiple showers/baths/taps are in use at any one time. In such situations, a combi boiler is unlikely to deliver sufficient pressure, and a conventional boiler is unlikely to deliver enough hot water. 
  2. More efficient: Since unvented cylinders are pressurised, they do not require a pump to provide hot water (but this can sometimes be added if the main’s water pressure is low). 
  3. Space-saving: Unvented cylinders are typically smaller than vented cylinders and do not require a cold-water tank in the loft, which frees up space. This is essential for larger properties that don’t have a usable loft (e.g. the loft has been converted or forms part of another property). 


  1. Installation complexity: Unvented cylinders require professional installation and may require additional plumbing work to connect them to the mains water supply. The installation engineer must possess the G3 Unvented Hot Water qualification in addition to being Gas Safe registered.
  2. Higher cost: Unvented cylinders are generally more expensive than vented cylinders.
  3. Risk of leak: Unvented cylinders are pressurised, which means there is a risk of a leak if the pressure relief valve fails. Any leak will likely be more damaging due to the higher pressures involved. 

Vented cylinders

Vented cylinders are gravity-fed systems that require a cold water tank in the loft to provide hot water. They are typically made of copper and are connected to the mains water supply via a vent pipe. 

You have a vented cylinder if you have a conventional heating system with a tank in the loft. They’re much simpler than system boilers as they aren’t under pressure, though some are equipped with immersion heaters to boost the hot water supply. Water pressure is assisted by gravity.


  1. Lower cost: Vented cylinders are generally less expensive than unvented cylinders. They’re simpler as they aren’t pressurised. 
  2. Simple installation: Vented cylinders are relatively easy to install and do not require any additional plumbing work if one is being used to replace another. Replacing a vented cylinder with an unvented cylinder involves significant upgrades and possibly a new boiler.
  3. No risk of leakage: Vented cylinders are not pressurised, which means that there is a very small likelihood of leaks. 


  1. Lower water pressure: As they’re assisted by gravity and not the mains pressure, vented cylinders provide hot water at relatively low pressure, which means that the flow rate may be slower, especially if you use more than one outlet at the same time.
  2. Space-consuming: Vented cylinders require a cold-water tank in the loft, which takes up space. If you want to use or convert your loft, you’ll probably have to consider upgrading the heating system to an unvented system. 
  3. Inefficiency: Vented cylinders are generally less efficient than unvented cylinders because they require a pump to provide hot water.

Choosing the right water cylinder

When choosing a water cylinder, consider the following:

Age of the home and heating system 

The age of the home and heating system is a major consideration. 

For example, if you have a loft and your heating system is more than about 20 years old, you likely have a conventional heating system with a tank in the loft and vented cylinder (typically in an airing cupboard).

In this situation, there are a few reasons why you might want or need to change this setup:

  • Your existing heating system fails or becomes too costly to maintain 
  • You want to convert the loft space
  • You simply want a modern, efficient heating system 

When upgrading from a traditional heating system to a pressurised system, It's essential to be aware that the new pressure could be higher than your old network of pipes and radiators can handle. In this case, you’ll need significant upgrades to replace your conventional heating system with a modern system boiler and unvented tank. 

Mains pressure

Another crucial factor to consider is the mains water pressure. If your mains pressure is low, you need to consider whether introducing an unvented cylinder is the best option. 

The pressure will reflect your mains pressure. This is because it will provide water directly from the mains. 

Some people with vented cylinders who have problems with low mains pressure will install an additional pump to boost the flow. However, installing a pump is a straightforward solution in most cases. 

Size of home and space

The size of your house is another important consideration. If you have limited space, especially in an upstairs area like a loft, a vented cylinder may not be the best option for you. 

This is typical in homes with converted lofts or no usable loft space. Unvented cylinders save space while delivering high performance. This is popular for homes that want to convert the loft, thus removing the gravity feed tank. 

Summary: Types of water cylinders: unvented and vented

Ultimately, vented and unvented cylinders are different and are appropriate for different heating systems. 

Conventional heating systems aren’t pressurised and use vented cylinders connected to feed tanks (typically installed in the loft). Modern pressurised heating systems with system boilers use unvented cylinders and don’t require that extra component. 

So, for saving loft space, unvented cylinders are inarguably the way to go. But otherwise, upgrading from a vented to unvented system isn’t a no-brainer. 

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