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to the building by a physical barrier, such as a filtration system. A minimum water temperature of 50˚C is therefore considered safe. The water leaving the water heater has a temperature of 55˚C. In most systems, a gas or diesel fired storage water heater is applied. We based our calculations on a gas fired water heater with a thermal efficiency of 80%. The insulated circulating system is designed to ensure a minimum temperature of 50°C in all return legs to the water heater. Temperatures between 25°C and 50°C occur only temporarily, in the non-insulated spurs to the user outlets, which have a maximum length of 5 m. Local legislation may require a different system design or different temperatures. The centralised system has several

disadvantages. It requires extensive control measures to prevent proliferation of Legionella, including continuous monitoring of the temperatures in the system, yearly inspections and maintenance on the various components. Also, the high temperatures lead to a risk of burn injuries. Temperature fluctuations, which result from pressure variations in the system and subsequent displacement of hot or cold water in mixer taps, can lead to uncomfortable or even dangerous situations. Concerning energy consumption, the

centralised system is highly inefficient. Long pipe runs, high temperatures and continuous circulation result in substantial energy losses. The concept does not conform to modern heating systems, which use relatively low temperatures and large temperature differences, often combined with the use of renewable energy resources. Optimisation of the conventional system is possible, but leads to increasing complexity and more extensive control measures.

Decentralised system An alternative to the centralised hot water supply system is the DSS hot water concept, which largely eliminates the problems associated with the former system. The concept is based on individual water heating near each user outlet, using electric instantaneous water heaters. The efficiency of these water heaters is up to 99%. The amount of water to be heated, as well as the temperature, is exactly adjusted to the actual demand. Due to the low water content of the (non-insulated) unit, any remaining water will quickly cool and Legionella bacteria have no chance to incubate. Hence, there are no restrictions for the temperature settings. For most applications, a temperature of about 40°C is sufficient. This eliminates the risk of burn injuries. Also, a hot water distribution

Figure 2: Estimation of the primary energy consumption of the two hot water supply systems, accounting for the various losses in the system.

system is no longer required. Feed water to the water heaters is supplied via the cold water distribution system. The water heaters are connected to the user outlets by pipes with a length of 2 m or 3 m. Overall, this setup leads to a system which is safe, simple and – provided the feed water is softened – nearly maintenance free. An electric instantaneous water heater is

characterised by a large momentary power demand. It is necessary to adjust the electric infrastructure of the building accordingly. Electrical outlets, including supply cables, need to be provided and distribution boards need to be expanded. Furthermore, the transformer capacity may need to be expanded. Whether expansion of transformer capacity is necessary depends on the simultaneous power demand of all water heaters, as well as the timing of their operation in relation to the power demand of other electrical equipment. The required transformer capacity is determined based on an estimation of the peak demand. In a hospital equipped with electric chillers, this peak demand appears to occur when the cooling load reaches its maximum, typically in the middle of a warm summer day. At any other moment, the power demand is (much) smaller and excess transformer capacity is available. In contrast, the simultaneous use of hot

water exhibits a peak in the morning and a smaller peak in the evening, mainly caused by showering. Although the peak load resulting from the simultaneous use of electric water heaters may be substantial, this peak load does not coincide with the peak load resulting from the cooling system, as illustrated in Figure 1. This means that the required expansion of transformer capacity is

‘An alternative to the centralised hot water supply system is the DSS hot water concept, which largely eliminates the problems associated with the former system.’


minor. If the power supply and demand were to be controlled interactively by application of a so-called smart grid, further optimisation is possible. When applying a decentralised hot water

system, attention is required to control Legionella in the cold water supply system. In most conventional systems, the piping network is periodically flushed with hot water in order to kill any Legionella bacteria that may be present. Without the availability of a central water heater, however, this is not an option. Therefore, an alternative method for preventing Legionella colonisation needs to be applied. A suitable method, for example, is incorporating a physical barrier at the point of entry of the building, which prevents bacteria from entering the system. If, despite the physical barrier, a Legionella outbreak were to occur, the pipes should be disinfected using, for example, a mobile water heater or, less preferably, chemical disinfection.

Energy consumption The energy performance of the DSS concept, applied in a fictional building, was modeled and compared to the performance of a conventional centralised system. The building under study is part of a hospital complex. It has a rectangular shape and encloses a patio. The two bottom floors are occupied by polyclinics – the three floors above accommodate wards. The building has a total floor area of 13,065 m2

and accommodates

156 beds. The hot water consumption was estimated to be about 9,300 l/day, or about 60 l/day per bed. For both systems a theoretical model was developed in order to calculate the energy consumption. The calculations show that major energy losses occur in the distribution system and, to a lesser extent, the gas-fired water heater of the centralised system. The smaller the hot water consumption, the larger the relative energy losses, due to continuous circulation. In the system based on the DSS concept,

energy losses in the building are reduced to a minimum. However, depending on the energy source and conversion method, the


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