Basic Documentation

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Siemens Modular Heating Controller RMH760B CE1P3133en
Building Technologies 10 DHW heating 2017-09-29
10.6.1 General
Legionella viruses develop significant growth in the temperature range of 35…45 °C. At
temperatures above 50 °C, they stop growing.
Legionella viruses are killed at temperatures above about 55 °C; the higher the
temperature, the shorter the time required to kill them.
There are different opinions regarding the effectiveness of thermal disinfection.
Control measures, such as the legionella function, are only effective in connection with
other measures (primarily building construction measures, but also chemical
disinfection and UV radiation).
The legionella function ensures thermal disinfection of the storage tank. It is important
here that the entire DHW storage tank will be brought to the required temperature. This
poses problems in connection with certain types of storage tanks (with electric
immersion heater or coiled heat exchanger) where cold water accumulates beneath the
heat exchanger. These problems can only be solved by taking adequate measures.
In addition to the legionella function, it should be made certain that the DHW setpoint
and the switching differential are adjusted such that the switch-on point will not be too
low (e.g. 55 °C).
It is also important to thermally disinfect not only the storage tank but also the entire
piping network. It must be made certain that there are no dead pipes or piping that has
not been used for longer periods of time.
If possible, the circulating pump should run during the legionella program.
Ideally, during the legionella program, the taps are in use.
The legionella protection function contradicts with requirements in terms of energy
savings, the formation of scale (the higher the storage tank temperature, the more
scale) and protection against scalding (above 60 °C).
Attention must be drawn to the risk of scalding when opening taps on completion of the
legionella function.
10.6.2 Sequence of legionella function
Using the legionella program, the DHW storage tank and, optionally, the circulation piping
(with the help of the circulating pump) can be maintained at the legionella protection
setpoint for the required period of time.
Legionella protection is also available with direct DHW heating, but a holiday time (period
of time legionella protection is provided) is possible only when the circulating pump runs.
The legionella program can be enabled either daily or weekly at a selectable point in
time.
As with forced charging, storage tank charging is started as soon as the storage tank
temperature (or one of the 2) lies 1 K below the legionella protection setpoint.
The legionella program will not be performed in the following cases:
• When the DHW operation selector is set to Protection mode
• During holiday mode when the selected DHW holiday mode is Protection
• When the DHW operating mode contact forces DHW heating to Protection mode
• When the plant operation selector is set to "Off"
• When storage tank charging is effected with an electric immersion heater, but without
storage tank sensor
Thermal disinfection
Piping network
Circulating pump
Practical problems in
connectio
n with legionella
protection
Starting the
legionella
program