Solar-supported heating networks in multi-storey residential buildings

7
System Hydraulics
The choice and design of system hydraulics has a major effect on ensuring satisfactory operation
while requiring as little supplementary heating energy and maintenance as possible. For this reason, it
is recommended that great care be taken during this planning phase and that the system hydraulics
selected is adapted to the specific requirements. A large number of different hydraulic concepts are
available, depending on the application (consumers, consumption profiles, type of conventional heat
generator, geometrical boundary conditions, etc.) and the system size. These concepts are described
in detail in the following sections, thus simplifying the selection of the most favourable system
hydraulics.
This section begins with a description of the requirements and conditions that apply equally to all
applications of solar-supported heat supply systems.
7.1
Basic information about solar-supported heat supply systems
7.1.1
Domestic water temperature and water hygiene – legionella bacteria
When defining the temperature of domestic water, the primary concern must be user comfort. As a
result, the minimum temperature at the tap should not drop below 45°C to ensure it is still usable. On
the other hand, domestic water temperature must not exceed 60°C in order to prevent scalding, lime
scale and corrosion, and to keep heat losses to a minimum. In addition to the aspects mentioned
above, hygienic requirements also have to be taken into account.
The most common bacteria in drinking water are the so-called legionella. These bacteria are difficult
to detect and they occur naturally in all types of fresh water. Healthy people are only rarely infected,
however, and this is generally only possible by inhaling small droplets of legionella-contaminated
water. In contrast, drinking this water is not harmful. It is very difficult to determine what kind of a
threat legionella pose, since it is not known whether there is a minimum infective dose and there are,
as a result, no clear threshold or guideline values.
Legionella growth is dependent on the water temperature, the surrounding materials (iron promotes
growth while copper hinders it), the bacterial culture medium and the water’s pH value. The optimum
growth temperature is between 30 and 45°C. Temperatures of at least 55°C cause the bacteria to die
off completely, provided they are subjected to this temperature for long enough. In addition to
thermal disinfection, chemical and UV disinfection can also be employed. Thermal disinfection is now
the most commonly used method for domestic water heating systems.
In order to minimise the risk of Legionnaires’ disease breaking out, the German Technical and
Scientific Association on Gas and Water (DVGW) has drawn up a corresponding set of regulations
(Worksheet W551 for new systems and Worksheet W552 for existing systems). The information is
summarised in Table 2. Austria has no comparable guidelines concerning legionella.
In general, it should be noted that legionella are not a problem affecting only solar-supported heat
supply systems for domestic water, but all conventional domestic water heating systems as well.
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