Combining gas with renewable energy

Condensing technology is used extensively in direct gas-fired water heaters. Shown here is a Lochinvar Eco Knight and two sizes of Turbo Charger.
David Pepper explains how direct gas-fired water heaters have an important role to play in reducing energy consumption and helping to exploit renewable energy.The use of renewable technologies to provide future energy requirements is a major challenge for property owners — and will be for the foreseeable future. Direct gas-fired water heaters may not command anything like the same profile or media coverage, but they still have an important part to play in the ongoing drive towards energy conservation and carbon reduction. This is particularly the case for commercial and industrial buildings, where water heating can often be a significant proportion of total energy use. DFWHs (direct gas-fired water heaters) have been providing hot water economically and efficiently for over 30 years and offer significant benefits when compared to a system comprising a boiler and indirect cylinder. The most recent revisions to Part L of the Building Regulations recognised this via the ‘Non-domestic heating ventilating and cooling compliance guide’, where minimum levels of efficiency for hot water generation are as follows. • Direct-fired water heaters, 73% (74% for LPG).
• Boiler with indirect cylinder, 80%. The trend towards condensing boilers has given further justification towards the separation of space heating and domestic hot water, and the use of DFWHs allows the condensing boiler to provide for the heating system only, enabling them to operate at best efficiency. The 7% difference in the minimum levels of efficiency takes into account the heat transfer loss from the boiler primary pipework to the cylinder, although these losses can be significantly higher in some situations. Accurate sizing of a water heater system is of paramount importance, not only in the drive towards conserving fuel resources, but also in providing a reliable supply of hot water when required. Traditional indirect systems were usually sized by providing sufficient storage capacity to meet peak demand. A drawback here is that large volumes of water are maintained at high temperatures during long periods of the day when there may be little or no demand. Instantaneous water heaters overcome this problem and have been extensively used in households for many years. The demands for hot water in most commercial and industrial buildings can be met by DFWHs which operate on the principles of low-storage but fast recovery of hot water. Typically, such products can have an output which allows for quick water heat-up, so the storage can be recovered in short periods of time (10 minutes to 1 h). This offers significant benefits to many establishments with a peak demand for hot water. Hotels, leisure centres, factories and commercial kitchens are amongst these properties where this occurs. Most DFWH manufacturers offer assistance on selecting water Heaters, either via sizing software or hard copy guides, but every application may have variations that should be considered, and the combination of storage and recovery is an important factor. The role of renewable technologies, particularly solar thermal is forefront in the mind of many building-services consultants when considering hot-water generation for a building. Combining DFWHs with solar installations is growing in popularity, and the evolving use of solar for hot-water generation in the UK lends itself ideally to a system based on using solar energy to pre-heat the feed water for a DFWH. A twin-coiled cylinder enables solar energy to be used with traditional boiler/indirect hot water generation, but this arrangement is generally regarded as being less efficient than the pre-heat alternative. Sizing of the pre-heat cylinder is extremely important, and a larger capacity will provide the opportunity to make the most of solar gain.

Solar thermal panels can work well with direct gas-fired water heaters by pre-heating the cold feed water.

Condensing technology is now used extensively in DFWHs, and a wide variety of condensing gas-fired water heaters is available, with hot-water recovery rates ranging from 600 to 10 000 l/h. These include a modification of the traditional vertical DFWH design where burner, storage tank and controls are all provided within the same casing. Also available are ‘generator-type’ models, where hot water is produced within a low-water-content heat exchanger, but stored separately in a direct cylinder. Such options offer the best efficiency, with some designs offering almost continuous operation in condensing mode. Installation flexibility is also provided by these models as various combinations of water heater and storage tank can be used. This allows for a design which will offer the best in terms of a reliable hot-water supply with efficient operation, but also provides alternative solutions — particularly where there are plant-room restrictions. Despite the high profile of renewable technologies and their ability to provide hot water, the direct gas-fired water heater has an important role to play in the future drive towards the better use of energy. Whether the DFWH is used as ‘stand alone’ or with a renewable package, equipment selection and design is crucial in order to maximise operating efficiency and reduce carbon emissions. David Pepper is managing director of Lochinvar.
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