Meeting the challenges of design specifications

Andrews Water Heaters pioneered the break with traditional systems when it entered the market with direct gas-fired water heaters 30 years ago. Gavin Watson, Business Development Manager, explains the benefits this brought to system designers and building operators alike. He also examines the continuing development of modern water heating technology and discusses the large variety of options now available to meet today’s commercial building requirements.
Traditional versus modern

Although the proportion of traditional systems being installed in the UK is diminishing we have all seen examples of the combined hot water and space heating plant approach to providing these services in commercial and industrial buildings. We know that whereas heating requirement is seasonal, hot water is required year round with peaks and troughs throughout the time the building is in use.

It is now generally accepted wisdom that the incompatibility of the demand for these services, if supplied by the same boiler, will result in inefficiencies over the year. Given two such specific and totally different requirements, it therefore makes fundamental economic sense wherever possible to separate the hot water and space heating loads and to install plant dedicated to their individual needs. In separating the systems, dedicated equipment can be sized to match each load resulting in greater operating efficiency for both.

Direct-fired storage water heaters

The equipment that first made this an economically viable option was the independent gas or oil-fired storage water heater. This has a highly insulated cylinder heated by an externally fitted burner that fires only when the water temperature in the tank falls below a certain level following hot water draw off. Available in various sizes, these days numbering at least eleven, gas-fired appliances typically have outputs ranging from 178 litres/hr to 1998 litres/hr and very fast recovery rates. It is therefore possible to keep storage capacity and therefore standby heat loss to a minimum, while immediately satisfying any peak load as needed.

Larger units can be accurately sized to provide centralised hot water for the whole building while additional energy savings can be made by installing smaller units close to the point of use, thereby virtually eliminating standing losses from long pipe runs. Typical efficiencies achieved are between 73% to 80%, depending on siting and usage. As technology has advanced the traditional storage open flue water heaters have become available as balanced flue appliances with fan assisted flue option, providing flexibility in design.

Direct-fired non-storage water heaters

Large gas-fired instantaneous water heating appliances have been available now for many years, originally introduced as a very efficient means of generating high volume hot water. Typically, outputs range from 75kW to 1000kW with recovery rates from 1117 litres/hr to 20,000 litres/hr (?T44ºC). Full gas and air electronic and Hi-Lo-Off modulation achieves typical efficiencies of between 89% and 93% with full load efficiency maintained, and more often exceeded, under part load conditions. Temperature accuracy is generally up to +1ºC and some units of this type can provide hot water temperatures as high as 90ºC, necessary for some industrial and hygiene applications.

This type of non-storage water heater is of lightweight construction, and characteristics such as low water content and finned copper tube heat exchangers with stainless steel headers mean scale build-up is reduced. Designed for long service life and to allow replacement of all components, even the heat exchanger, these units are an appropriate choice where whole life cost is important.

Continuing with the instantaneous non-storage theme but on a smaller scale, the market place has recently seen the introduction of wall hung continuous flow water heaters for large residential and light commercial applications. Whilst extremely compact these heaters can deliver surprisingly large amounts of hot water and provide a cost effective alternative to point of use electric water heating. As well as the internal version allowing installation in the smallest of spaces, there is also the option of external weather-proof versions. From 42 to 56kW of heat output, a single unit typically can deliver between 13 and 32 litres/min ?25ºC and when up to six heaters are installed in line they can provide a maximum flow rate of circa 192 litres/min. It attains its typically high efficiency of 89-90% by sensing the inlet and outlet temperatures and then modulating either the burner or water flow to deliver hot water at the pre-set temperature.

High efficiency condensing water heaters

Returning to the larger instantaneous type water heaters, the fact that their mode of operation achieves higher efficiencies at the lower boiler output position means that they could easily adapt to the application of condensing technology. There are now many installations of this type and Figure 1 shows an instantaneous condensing water heater with a series of heat exchangers that can be either finned or plain stainless steel tubes, and a downward firing pre-mix burner. This results in a high turn down ratio, very high efficiencies of up to 107% and low Nox, with the added benefit of single wall flueing.

A more recent refinement of this technology is the continuous flow condensing water heater. These units have an extremely small space-saving footprint and some models can be wall mounted. A water-cooled burner control achieves ultra low Nox and CO2 emissions applicable to the Energy Technology Listing and whole life costing.

Similar outstanding contributions to efficiency and lifecycle can now be matched and even exceeded by a revolutionary design of condensing storage water heaters. This is due to the application of advanced technology such as the water storage tank being of stainless steel, as are the externally mounted finned tube heat exchangers, which can number up to four, also modulating burners, advanced integrated diagnostics and built in anti-legionella function.

Ensuring the maximum amount of condense time is utilised during heat up produces a net efficiency of 109% and in addition, the burners remain on high fire longer, thus giving faster recovery. Therefore, provided the system flow is equal to, or less than, the total burner capacity, the unit will operate almost as an instantaneous water heater and still condense.

The units have a small footprint base supporting one of three sizes of stainless steel storage tank and have up to four externally mounted 30kW modular burners within a protective rectangular casing. Utilising two or more modules, for up to 120kW output from one stand-alone unit, provides in-built stand-by facilities saving the cost and space of additional individual units.

Environmental factors have brought about no less than four pieces of legislation relevant to hot water installations; the Climate Change Levy, revisions to Building Regulations (April 2006), the Energy Performance of Buildings Directive and EU Emissions Trading Scheme. High efficiency condensing water heaters meet the current requirements but there are signs that the Government will need to tighten controls further in order to meet their emissions target. In anticipation, some of the industry’s leading manufacturers are researching renewable and sustainable technologies and developing products that will achieve even greater efficiency and lower emissions than the current high efficiency appliances. As the penalties for inefficient use of energy rise, so the payback for better performing services plant becomes less and water heating will play an increasingly significant role in the mix.