Used to store the hot water you use to bathe, shower, do laundry and wash dishes, domestic hot water cylinders play a critical role in your day-to-day life. There are two different types of hot water cylinder - vented and unvented - both suited to certain types of properties and central heating systems. However, these days your cylinder is more likely to be unvented.

With a selection of design differences and installation factors to think about, from the size of your house and the type of heating system you have, to the water pressure in your home and your budget, in order to make the correct decision when it comes to your new hot water cylinder, you need to be as informed as possible. Here at Viessmann, we’ve put together this handy guide to help you understand the differences between vented and unvented cylinders, how each hot water cylinder works, and which would work best for you. 

Selection on unvented cylinders

Unvented cylinders

First we’ll look at unvented cylinders. Considered more modern than vented cylinders and only legalised in the UK in 1986, these water cylinders have become increasingly popular in recent years and are now fairly standard in new-build homes.

What is an unvented cylinder?

In basic terms, an unvented cylinder is a tank that stores hot water and provides it directly to taps, showers and other appliances on request. Unlike older vented cylinders, unvented solutions are fully sealed and take water directly from your main source of water. This means they can offer a higher and more consistent flow rate when it comes to water pressure, as well as not requiring an accompanying gravity-fed water tank to operate.

This absence of a loft tank and the fact the entire cylinder is sealed saves you valuable loft space, protects your household against nuisance frozen pipes in winter, and removes the threat of contaminated water. For this reason, it is considered a more hygienic system than its vented counterpart. It also means a quieter system, as there is no sound of the cold water tank refilling.

How does an unvented cylinder work?

Unvented hot water cylinders work by storing water directly from the mains water supply and heating it either through the home’s boiler or using a separate electrical heating element, such as an immersion heater. As the water is being taken directly from the mains, these cylinders deliver a consistent level of high pressure hot water to all of the feeding outlets attached to it, from kitchen and bathroom taps to showers and baths.

As mentioned above, unvented cylinders are completely sealed and do not need a cold storage tank located in the loft to work. This means less pipework is required and more space is saved. Additionally, as water is taken straight from the mains, these cylinders can be placed anywhere within a property. 

What size unvented hot water cylinder do I need?

When it comes to new cylinder installation and selecting the right size for your home, we would always recommend booking an inspection from a qualified heating engineer. They can assess the unique needs of your property and provide the best advice.

As a rule of thumb, however, the guidelines below provide you with a rough idea.

Size of property Number of bathrooms Cylinder size
1 bed 1 120 -150L
2 bed 1 150-180L
3 bed 1-2 180-200L
4 bed 2 200-300L
5 bed 2+ 300L+

Vented cylinders

If you don’t have an unvented cylinder in your home, the chances are you have a vented solution. Also known as gravity-fed hot water cylinders, vented hot water systems are considered the traditional option in the UK and were once the most common type of hot water cylinder found in domestic properties across the nation. 

What is a vented hot water cylinder?

A tank for storing hot water, vented cylinders are typically made from copper and take water from a big water tank located in the loft. Cheaper than unvented alternatives, these heating systems require gravity to work as no pressure is taken directly from the mains. This means water pressure in your home can be worse upstairs (in outlets located closer to the loft water tank) compared with those outlets located downstairs. However, electric pumps can be installed to help maintain good pressure. 

How does a vented cylinder work?

Vented, or open-vented, cylinders work by storing water that is supplied by a large tank of cold water typically located in the loft area of a property. These unsealed cylinders use gravity to pull the water down from the water tank, via a vent pipe, and then heat the water using either heat from the central boiler or a separate immersion heater within the cylinder.

As the water is heated it naturally expands. However, as this system is unsealed, unlike unvented cylinders, the vent pipe and cold tank provides an escape route for the excess heat, keeping the system safe at all times.

Can I convert a vented cylinder to an unvented cylinder?

Put simply, no, you cannot convert a vented cylinder into an unvented cylinder. While many parts are indeed compatible with both types of system, connecting to the pressurised mains water supply without professional help is not advised.

While converting a vented cylinder to an unvented cylinder is not really possible, paying to have your entire system upgraded from one that uses a vented cylinder and a loft tank to one which uses an unvented cylinder is possible, it might even be more cost effective as part of an overall heating system upgrade.  

Which cylinder should I choose?

Choosing the right hot water cylinder for your home comes down to a few key factors. Before deciding which is the best fit for you, it's important to go through each of these to weigh up the pros and cons. Deciding factors include:

  • Your current heating system

If you live in an older home, it's likely you currently have a vented cylinder. This means all the pipework around your property was installed with this low-pressure system in mind. If your home is over 30 years old, it is advised you seek professional advice as installing an unvented cylinder could create water pressure that is too powerful for your old network of pipes and radiators to handle.

  • Local mains pressure

If the water pressure of the mains supply to your home is typically low, an unvented cylinder is not a good option for your property. This is because an unvented cylinder is not aided by gravity and will therefore provide water at the same low pressure.

  • Property size

If your home is small or doesn't have an independent upstairs storage area such as a loft or attic, a vented cylinder is probably not for you. This is because, as discussed above, vented systems require a sizable cold water tank located in a high enough position to deliver the gravity the system needs to function at pressure. If you cannot facilitate this tank, a vented cylinder will not work in your home.

  • Installation and maintenance costs

Installation of an unvented cylinder is much less hassle in terms of home disruption when compared with that of a vented cylinder. This is because unvented cylinders can be placed anywhere in the home and there is less pipework, you also won’t be required to add a header tank in the loft. You may only need to use a vented cylinder in an area which suffers from low water pressure. 

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