Working With Unvented Cylinders

Date - January 2010

Unvented cylinders are generally recognised as the preferred water heating solution for new build and refurbishment projects. Here Jon Cockburn, head of marketing for Santon, looks at the benefits of these systems, how they work and what to look for when choosing products.

Unvented water heating cylinders deliver a powerful supply of hot water, without significant loss of performance if more than one tap or shower is used simultaneously, making them ideal for modern dwellings which include several bathrooms. 

In addition, more homeowners are having luxury bathroom products installed, such as multi-jet showers and jetted air baths, which require large quantities of hot water and fast flow rates to operate effectively. 

As well as meeting the needs of the domestic consumer, there is also a demand for unvented hot water storage in the commercial market.  A large unvented cylinder - or several installed in sequence - provide an ideal solution where there's high demand at peak times in applications such as hotels, leisure centres, offices, schools and hospitals. 

The success of unvented cylinders has lead to a wide range of products being available on the market, but it is important to remember that they aren't all the same.  As unvented cylinders store water under pressure, they need to be strong and robust and choosing a reputable brand ensures quality and reliability.  Ultimately an installer's reputation depends not only on the quality of their work, but on the products they recommend.  When selecting equipment, the key points to look out for are; cylinder material; the hot water performance; standing heat losses/insulation; flow rates; the length, terms and conditions of the guarantee and the quality of the support package offered by the manufacturer. 

It is advisable to look at the type of expansion vessel provided as these also differ between brands.  All unvented water heating systems require an expansion vessel to accommodate the water which expands when it is heated. In a traditional vented system, the expansion occurs in the cold feed pipe, resulting in only a small water level rise in the cold feed cistern. In an unvented water heating system, the cold water supply is taken from the mains, so a different solution is needed.

Water is non-compressible - therefore water heating systems need to allow space for expansion, otherwise the pressure will rapidly rise.  In some countries this phenomenon is used to control expansion in water heating systems and is known as 'expansion to waste'.  Under UK Water Regulations, this is not allowed and with unvented cylinders, it is necessary to allow for this expansion within the water heating system.

In the majority of unvented systems the expansion is accommodated by providing an 'expansion volume' and again this differs between brands, as it is either provided internally within the water heating cylinder (commonly termed an 'air gap' or 'bubble top' system) or through an external expansion vessel.
 
With many domestic customers demanding 'greener' solutions, unvented cylinders also work with solar water heating systems.  Solar panels collect heat from the sun, which then heats a liquid contained in the system's pipework.  This is then circulated through a coil located in the base of a specially designed cylinder, which then transfers heat to the stored water - producing domestic hot water.