Making hot water green
Environmental performance, efficiency, installation quality and running costs are key factors in selection of the most efficient and effective water-heating solution for multi-dwelling residential buildings. Consumption and wastage of water should also be part of the equation for determining the most suitable option.
In Australia, centralised hot water services for apartment buildings tend to be gas-fired, while stand-alone instantaneous units that heat water are either gas-fired or electrical. In terms of heating efficiency, electric instantaneous hot water systems are a better option, because they have a heating efficiency of 99.5 percent when compared with gas-fired hot water systems, which have burner-heating efficiencies of 80 to 85 percent.
Electrical systems have the advantage in terms of heating efficiency, but there are other factors that contribute to the overall system efficiency and environmental performance: system architecture, installation quality and the ‘cleanness’ of the energy source. Operating costs must also be considered in system selection. While most environmental impact studies focus solely on energy usage, water wastage becomes an important factor to be considered, given the fact that most parts of Australia regularly face water restrictions.
According to Darren Fletcher, sales director at Stiebel Eltron, there are several aspects to the cost of producing hot water. “While fossil fuels are consumed to generate electricity or burned directly to heat water for domestic use, the greater cost is the use and wastage of water in the systems.”
There is mounting pressure to make buildings as green as practical and to meet the challenge of evaluating which hot water system is the most suitable. The alternatives are centralised boilers – servicing the whole building – or individual instantaneous water heaters installed in each apartment.
Multi-dwelling residential developments provide an ideal setting in which to demonstrate the water-saving and other benefits of instantaneous hot water systems. An instantaneous electric hot water solution can be positioned exactly where it is needed without the restrictions associated with gas feeds, flues or venting.
“Apartment buildings generally allow a choice of instantaneous electric, instantaneous gas, or centralised gas water heating systems,” Fletcher notes. “The instantaneous electric option has the least infrastructure requirements and, therefore, represents the most cost-efficient choice for installation.”
WHY CENTRALISED HOT WATER SUPPLY IS INEFFICIENT
Centralised gas-fired hot water heating systems are generally assumed to have the lowest carbon footprint of the three systems available, but by having to operate continuously, regardless of demand, centralised gas-fired systems can be inefficient in their design and costly to operate. Typically, a centralised hot water plant comprises a ring main that extends throughout the building, with individual feeds to each apartment. With this type of system, heat is lost from the storage tank and from the long pipe runs.
Water in each of the apartment feeds is often significantly colder than the water circulating in the ring main. This necessitates draining the cooler water from the feed – often referred to as ‘dead legs’ – every time hot water is wanted, resulting in significant water wastage.
Such systems also often present a more expensive initial cost for a property developer, given the requirements for a dedicated plant room, large commercial gas boilers, hot water storage tanks, circulating pumps and the reticulation pipework. Moreover, these systems are often associated with higher maintenance costs.
THE ADVANTAGES OF INSTANTANEOUS WATER HEATERS
Many advantages are available through choosing stand-alone instantaneous systems. As water is only heated at the time of use, energy is not wasted in maintaining a stored volume of hot water and operating costs are reduced. With the heating units located closer to the point of use, pipe runs are shorter and less water is wasted by the user while waiting for sufficiently hot water to arrive at the outlet.
Studies have shown that a typical apartment fed from a centralised gas-fired system wastes 7000 litres of water over the course of a year simply through waiting for hot water to arrive at the tap. An instantaneous electric water heater system can reduce this down to less than 1600 litres.
The cost-effectiveness and the environmental performance of the various water heater technologies needs to be weighed up to determine the most appropriate hot water service option for multi-dwelling residential blocks. The opportunity to minimise equipment and installation costs, while delivering efficient, reliable, water savings and environmentally responsible hot water for building residents is there.