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Vented vs Unvented Hot Water Cylinders Advantages and Disadvantages


Domestic hot water cylinders are used to store hot water for various uses in a home set-up. So, if you are planning to install a new hot water system or upgrade your existing one, you will need to purchase a new hot water cylinder. However, like many homeowners, you might find it challenging to decide which water cylinder type best suits your home’s heating needs.

Hot water cylinders come in two types; vented and unvented ones. In this guide, we will look at how these cylinders types work and explore the advantages and disadvantages of each. We will also cover some of the factors you need to consider before settling on a cylinder type.

Vented hot water cylinders

Vented hot cylinders, also known as gravity-fed hot water cylinders, is the traditional hot water cylinder option in the UK. It is the most common hot water cylinder type found in most older properties in the country.

Vented hot water cylinders are usually made from copper or stainless steel and draw water from an open cold water storage tank located in the loft area of the property.  The water cylinder uses gravity to draw water from the loft water tank via a vent pipe. Once the water gets to the hot water cylinder, it is heated directly by an immersion heater or indirectly using a heat exchanger connected to a central boiler.

When the cold water is heated, it naturally expands. However, the excess heat in the hot water cylinder usually escapes through the vent pipe and into the cold water tank. This ensures that the hot water cylinder is safe and at no risk of an explosion at all times. The vent pipe also allows air in the hot water cylinder to escape through the open cold water tank.

Once a hot water tap is opened, the cold water flowing into the water cylinder displaces the hot water, which then flows towards the open hot water outlet.

Benefits of a vented hot water cylinder

  • Vented hot water cylinders are usually cheaper to install and require low maintenance.
  • It is not affected by a low mains pressure, provided the cold water tank is at a sufficient height.
  • They are compatible with all types of water outlets, including modern power showers and mixers.
  • Provides hot water even when the mains supply is cut off, thanks to the water stored in the loft storage tank.
  • Replacement is easy as a “like for like” swap is possible.
  • Can supply hot water to several outlets at once without affecting pressure head

Disadvantages of a vented hot water cylinder

  • The cold water storage tank is open at the top, meaning the stored water can easily be contaminated.
  • Since it relies on gravity, the height of the cold water tank will determine the pressure of water coming out at the outlets.
  • The cold water storage tank is usually large and takes up a lot of loft space.
  • Hot water pressure is usually lower at upstairs outlets and may require an additional pump to boost water flow.

Unvented hot water cylinders

Unvented hot water cylinders are a modern water cylinder solution that was legalized in the UK in 1986. Since then, they have gained popularity countrywide and are currently the standard hot water cylinder in all new homes.

An unvented hot water cylinder stores water from the cold mains, heats it and supplies it to hot taps and other hot water outlets on request. As such, an unvented hot water cylinder can provide hot water at consistent high pressures to all hot water outlets in your home.

Since the unvented hot water cylinder is a sealed system, it has no vent pipe to provide an escape for the excess volume of water produced by the expansion of cold water when heated. Instead, they have an expansion vessel that stores the excess water during the heating process.

Benefits of unvented hot water cylinders

  • They don’t require cold water tanks; hence they are a great fit where space is limited.
  • They can be placed anywhere on the property as they draw water from the mains and don’t rely on gravity.
  • The system is completely sealed, meaning there is no risk of frozen tanks in winter or contamination.
  • Unvented hot water cylinders provide good water flow rates as they operate at the mains pressure.
  • The system provides drinking water at all water outlets.

Disadvantages of unvented hot water cylinders

  • Unvented hot water cylinders are more complex and expensive to install as they require a G3 specialist.
  • They require regular and expensive maintenance to ensure they meet safety regulations.
  • They aren’t compatible with modern power showers and mixers.
  • Since they draw water from the mains, there will be no hot water when the mains water is turned off.

Factors to consider while choosing a type of hot water cylinder

Choosing the right water cylinder for your home is a complex process that precedes the mere advantages and disadvantages of each cylinder option. You need to keep in mind some underlying critical factors if you want to make the best possible choice.  Here are some of these crucial factors you need to take into consideration.

The existing heating system you have

One key point to consider when shopping around for a replacement hot water cylinder is the current hot water cylinder you have. It is usually cheaper and safer to install a swap for a swap hot water cylinder, especially if you currently have a vented one.

Generally speaking, if you live in a home older than 30 years, you probably have a vented hot water system and a regular heating system in place. If this is the case, then your home’s network of pipes and radiators are designed with the low water pressure of the vented hot water cylinder in mind. As such, replacing it with an unvented hot water cylinder might create a higher water pressure that might be too powerful for your old network of pipes and radiators.

The strength of your local mains pressure

If your home’s local mains pressure is typically low, you should consider installing a vented how water cylinder. This is because the vented hot water system will use gravity to boost the hot water pressure, thus ensuring the water coming out of your hot pipes is at a strong pressure. Additionally, you can use a pump to boost your hot water system’s water pressure, especially if there isn’t enough distance between the loft tank and your home’s hot water outlets.

On the other hand, if you install an unvented hot water system, you are bound to experience low hot water pressure issues. This is particularly the case as the water cylinder will be taking water directly from the mains and supplying it to your hot pipes at the same low pressure.

The size of your home

When it comes to the size of your property, there are two things to look out for; space restrictions and the number of bathrooms. If you live in a small property, chances are, there isn’t enough upstairs space such as an attic or loft for storing the large cold water tank required for a vented hot water system. As such, an unvented hot water system might be the only option you have.

If your home has two or more bathrooms, then a vented cylinder would be a better choice compared to an unvented one. The reason behind this is that the large cold water tank provides a steady supply of water without affecting the hot water pressure. An unvented hot water cylinder, on the other hand, relies on the mains pressure, which, when directed to many outlets at once, will split and significantly weaken.

The cost of installation and routine maintenance

Another aspect to consider when weighing whether to install a vented or an unvented hot water cylinder is the installation and maintenance costs. Installing a vented hot water system is generally cheaper and easier than installing an unvented one. Moreover, a vented water cylinder requires no maintenance once the installation is done.

If, however, you have a flexible budget, you should consider going for an unvented hot water cylinder. Although an unvented cylinder is more expensive to install and maintain, it generally involves less hassle as it doesn’t involve the installation of several pipes and a loft tank. Moreover, the cylinder can be placed anywhere in your home as it doesn’t rely on gravity.

What size water cylinder do I need?

The issue of the right cylinder size is an important one that you need to keep in mind. The ideal cylinder size is big enough to handle your home’s hot water demands. A small hot water cylinder will mean you will quickly run out of hot water, especially if several outlets are open at once. On the other hand, a large water cylinder will waste energy by heating and reheating more water than you need.

The simplest way to determine the size of the hot water cylinder for your home is to consider the number of bedrooms and showers or bathrooms you have. As a general rule, the more bedrooms and bathrooms you have, the higher your hot water needs, and the bigger the water cylinder you need.

Here is a rough idea of the cylinder size you need based on the above parameters.

Number of bedrooms Number of bathrooms/ showers Cylinder size
1 1 75-150L
2 1 150-180L
3 1-2 180-210L
4 2 210-300L
5 2+ 300L+

Best hot water cylinders

After seeing the difference between a vented and an unvented hot water cylinder and learning the pros and cons of each, you should already have decided which one suits you. After settling on the right type and size of hot water cylinder you need, it is about time you shop around for a reputable brand to purchase it from.

Here are some notable hot water cylinder models from leading brands in the UK.

  • Megaflo Eco by Heatrae Sadia – This cylinder model is available in sizes ranging from 75 litres to 300 litres and offers a lifetime warranty. The cylinder can also be heated directly using an immersion heater or indirectly using a boiler.
  • Greenstore by Worcester Bosch – This is an indirect only hot water cylinder with a capacity ranging from 90 litres to 300 litres. Greenstone has a B- energy efficiency rating and comes with a 25 years warranty.
  • Tempest by Telford – Tempest is a C-rated hot water cylinder available in capacities ranging from 120 litres to 300 litres. The cylinder can be heated directly or indirectly and comes with a 25-year warranty.
  • Vitocell 200-V by Viessmann – Vitocell-200 is usually heated indirectly using an external boiler but usually has a backup immersion heater. It usually comes with a 25-year warranty against corrosion and has a capacity ranging from 90-300 litres.
  • StainlessLite by Gledhill – StainlessLite is an excellent option for people looking for a large hot water cylinder as it can hold up to 400 litres of water.
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