The unvented cylinder industry has enjoyed unprecedented sales growth is recent years, Stuart Elsy examines the reasons for this success.

Mains pressure unvented hot water cylinders are the fastest growing sector of the water heating industry. A technology widely used across the developed world for generations, but unavailable in the UK until the late 1980's, the system has enjoyed unprecedented growth in the past decade. The industry trade association MODUS (Manufacturers Of Domestic Unvented Systems) has submitted sales records since 2000 and results show that business has more than doubled in that time, with the current situation being that over a third of all water storage heaters sold are now unvented and this figure predicted to be over a half by 2008/9.

In the new build sector unvented systems are especially dominant with virtually every major housebuilder now specifying the products. On the buoyant replacement market unvented cylinders are also now becoming increasingly popular as the public recognises the performance advantages and the nation's installers become more skilled at fitting the cylinders.

System Advantages

The advantages of unvented cylinders are clear to any installation engineer who chooses to examine the options available. The traditional UK hot water system was a roof tank supplying a low pressure vented copper cylinder. This cylinder is heated by a boiler and water distributed at the pressure created by the head of water - typically not more than half a bar. The resulting poor shower pressure and low hot water flow rate at all outlets was a source of constant frustration to UK citizens who experienced high pressure invigorating showers when travelling abroad.

The unvented concept is simplicity itself, with the water main piped directly into the hot water cylinder. In most domestic situations the cylinder is heated either by electric immersion heaters or indirectly by a central heating boiler - exactly the same as traditional system. The water is then distributed under pressure and at high flow rates to all outlets. No booster pumps are used and the result is a silent, high performance and energy efficient system that removes roof tanks and the risk of freezing pipes. Typical shower pressures vary from 2 to 3.5 bar - many times that of the traditional system. It is little wonder that the sales of unvented cylinders are growing at the rate of 20% per year, while sales of vented systems are falling.

It is also important to clarify the differences between unvented units and that other great water heating phenomenon of the past 20 years - the combination boiler. Combi boilers undoubtedly have their place and have advantages in certain situations, however they are not the solution to all problems that they have on occasions been billed. Combis are great where space is at a premium and hot water demand is modest. However many houses, both new and old, now boast ensuite showering facilities as well as a family bathroom. It is not unreasonable to expect that 2 showers or a bath and a shower to be used simultaneously. Combi boilers that can produce the required 35 litres per minute of hot water that this situation demands are few and far between, hence the advent of the unvented cylinder to fill this gap in the market. Unvented cylinders will, with sufficient incoming mains flow rate, supply at least 50 litres of hot water per minute.

Cylinder features and installation

All unvented cylinders are supplied with factory fitted insulation to conform with Part L of the building regulations, the units are invariably cased as well. A full fittings pack is provided, including pressure control valves, safety valves and an expansion system. The cylinder is fitted with a metal discharge pipe to an outside point, but other than that is very similar to a traditional cylinder.

Since the sales of unvented cylinder really started to pick up about a decade ago, one trend in the design of cylinders has been obvious even to the most casual observer. Stainless steel has become the material of choice for installers and specifiers. This search for quality is a credit to the industry in the UK and not only gives the lie to the oft quoted stereotype of the cowboy plumber, but also stands in contrast to the industries in other European countries where cheaper mild steel cylinders are still dominant.
Hot water cylinders were invariably copper in this country but the number of stainless steel units available in the UK has grown from 2 or 3 a decade ago to up to 20 now, with new products constantly being launched.
The advent of stainless steel units seen lengthy guarantees offered by many manufacturers and an overall drive for reliability and ease of use that has benefited installers and householders alike.

Renewable energy

One future trend that is already making a strong impact is the growth of renewable energy with hot water storage. Many renewable technologies, such as wind, ground source and air source heat pumps are compatible with unvented water heating, however it is undoubtedly solar technology that offers the strongest benefits. A well designed solar water heating system can provide over 50% of all hot water needs over a year in the UK. During the summer, the overwhelming majority if hot water needs can be supplied from solar gain, whilst during the winter a solar preheat can significantly reduce fossil fuel usage.

The most common form of solar water heating system is via a twin coil cylinder. The lower coil will typically be connected to solar collectors and will be able to heat the entire cylinder, provided sufficient solar energy is available. The upper coil will then be connected to a boiler. If required the boiler will be used to raise the temperature of the top portion of the cylinder if solar gain is insufficient.

There is considerable political, media and public interest in solar and other renewable energy heating, and manufacturers of twin coil and other renewable compatible products report rapidly increasing levels of business in recent times. With the government increasingly using building regulations to promote heating with zero or low carbon impact, and increasing fuel costs affecting us all, the long term future for renewable energy installations is assured.

Training & regulations

Unvented cylinders should always be fitted by a competent installer as defined by Part G3 of the building regulations, effectively meaning an engineer who has completed a course run by a recognised training provider.
With the introduction of the house sellers' pack in 2007, it will become more important than ever before that installers have achieved competence. Self certification schemes operated by CORGI and others are being launched this year to allow approved installers to self-certify their work and many more installers are completing courses.
Unvented courses typically last for one or two days and some manufacturers now offer assistance to installers to encourage more engineers to become approved operatives.

Most manufacturers offer a range from around 100 to 300 litres in direct and indirect heating formats. The indirect system connected to a central heating boiler is the most popular type, especially in the replacement market. However the increase in popularity of new build urban apartments, often on brownfield land has opened a large new market for electric unvented systems. Stand-alone direct unvented cylinders lend themselves perfectly to this situation, where developers are anxious to avoid the huge logistical problems of bringing in a gas main, and householders are looking for the showering performance simply not available from either electric showers or a vented system with severely restricted head.

Unvented cylinders are most certainly here to stay. Because of the higher price of the cylinders, it is already the most important water storage sector in the UK economy, however with two thirds of the market still there to win, the opportunities for growth, both for the manufacturers and the nation's installers, remains extremely high.