Improving hot water generation
Hot water is one of those things that we all take for granted and is just as important in the workplace (hotels, restaurants, sports and leisure facilities, schools, hospitals, offices and factories) as it is within the home.
The term Domestic Hot Water (DHW) means any hot water used for normal hygiene uses and is sometimes referred to as Sanitary Hot Water (SHW). Nonetheless, it’s a big mistake to think that DHW is simply a matter of using existing technology and practices just because it’s a given in daily life. In fact the design of both system and equipment is subject to continuous improvement and massive investment by the industry.
Traditional direct fired storage water heaters have used atmospheric combustion technology with simple on/off control, but like modern boilers, water heater technology has moved on and modulating burners with condensing heat exchangers will be the future.
Building Regulations are driving designers towards tighter buildings with better insulation and modern construction methods, which have made a big difference in recent years, reducing energy demands for space heating. In some applications it is likely that the energy required to generate the sanitary hot water will now exceed the space heating load.
The objectives in designing a modern hot water system should focus on reducing the cost of generating hot water, ensuring safe and reliable operation. Improving the efficiency of hot water production will then automatically reduce the carbon emissions and associated environmental impact.
While replacing atmospheric water heaters like for like is common industry practice, it may minimise the replacement cost, but the efficiency of such units is far less than their modern condensing equivalents. Over the lifetime of the unit, the increased energy costs are likely to be far greater than the savings made at installation. Of course this raises the common debate of replacement coming from capital or maintenance budgets and energy running costs from an operating budget, something that historically often over-rides good business decision making.
The fact is that in future, with demands to reduce emissions and the likely high increases in energy costs, these factors will be difficult to ignore.
A traditional atmospheric water heater will be operating at nominally 77% gross combustion efficiency, compared to a modern condensing water heater with modulating burner control and operating in excess of 96% gross efficiency. Upgrade and annual fuel savings of 30% or more would not be an unreasonable target, and at that rate the additional cost of installing a condensing storage water heater will soon be recovered. From thereafter, real savings will be realised over the life of the product. Using less fuel naturally reduces emissions and supports carbon reduction to minimise impact on the environment.
Fortunately manufacturers like Hamworthy have risen to the challenge. New products such as our Dorchester DR-FC Evo Condensing Water Heater have been introduced not only to comply with all the latest regulations, but also to provide customers with products that will deliver what they need. This includes reduced energy bills, comprehensive built-in controls, low NOx emissions, good safety features and whisper quiet operation at less than 45dBA, plus the option of an open flue or room sealed flue system, all of which are easy to install.
Alongside carbon footprint reduction, the protection of human health is of paramount importance. Ensuring adequacy of supply and reliability of operation are vital when it comes to commercial considerations; and there are health, cost and commercial dangers inherent in both oversizing and undersizing plant.
Risk of Legionella infection must be minimised and water temperature is a significant factor in controlling risk, with optimum conditions for bacterial growth occurring between 20°C and 45°C. Legionella bacteria are eliminated progressively above 46°C with survival time reduced to just minutes at 60°C. One recommended method for reducing risks associated with Legionella is to ensure delivery temperatures are maintained at 60°C or above with adequate temperature protection for prevention of scalding employed at all relevant outlets.
The control system for the Hamworthy Dorchester DR-FC Evo water heater incorporates an anti-Legionella function which can be programmed to run regular purge cycles which operate the heater at a higher temperature, above 60°C for one hour. In many applications this is best done at night when there is least likelihood of users drawing off excessively hot water, allowing the whole hot water system to be sufficiently heated.
Where hot water temperature is the primary anti-Legionella control measure, re-circulating circuits should be designed to provide a return temperature to the cylinder of 50°C or above, in normal operation.
During operation of a condensing storage water heater, it may be desirable to allow stratification across the water heater, delivering set-point temperature from the top of the cylinder, while the cold water supply is introduced at the bottom. During the anti-Legionella purge cycle, it is important to thoroughly mix the contents of the cylinder to ensure 60°C is achieved throughout. For this purpose, a top to bottom recirculation kit should be fitted to the water heater and operated during the purge cycle. The Health and Safety Commission publish the Approved Code of Practice & Guidance Document L8, for the control of Legionella bacteria in water systems.
Wastage of both energy and water can be reduced by using re-circulating systems as far as possible and avoiding long dead legs. Mandatory regulations insist on insulated pipework where runs exceed certain specified lengths, which in turn are defined according to the diameter of the pipes.
It is important to minimise draw off times so that as little water as possible cools within the spur pipes that connect individual sinks, basins or showers to the hot water ring main. The distribution pipe work design should enable the hot water to reach all outlets at 50°C within one minute of turning on the tap. Thermostatic mixing valves should be used to comply with regulations and manage the risk of scalding.
Installing the water heater close to the point of use will help to minimise system losses and wasting energy. The whisper quiet operation for the Dorchester DR-FC Evo means the unit can be installed in a wide variety of locations to suit most buildings, especially as the flue system can be room sealed with flue runs up to 40m in concentric and 100m in twin duct and open flue arrangement.
Hamworthy Heating’s contribution to improving the sustainability and economy of DHW systems is centred on such products as the Dorchester DR-FC Evo condensing gas fired storage water heaters and its high performance Powerstock storage calorifiers and storage tanks, for including solar thermal energy in the hot water equation. – all of which meet the many demands of the modern building.