Apricus Installation Manual
64
Important Information
INTRODUCTION
The information contained herein is for guidance only with regard to the safe use and
operation of domestic hot water and heating systems. The installer / user should reference
the applicable National Standards in force that take precedence over any guidance
contained in this document.
BACTERIA IN WATER SYSTEMS
Legionella bacteria are always present in man-made water systems, however, it is only in
high numbers that the bacteria can cause illness. As the resulting Legionnaires’ Disease can
be fatal, there has been a considerable amount of research to define the characteristics of
the bacteria.
Whilst legionella is typically associated with larger heating systems, such as hotels, hospitals,
factories or air conditioning cooling towers etc. it can also be present in smaller domestic
and residential systems.
Research shows that the bacteria are encouraged to colonise and thrive in conditions where
the water temperature is between 20°C and 45°C, where the water is stagnant, an
accumulation of debris exists, or scale and corrosion are present.
Scientific research shows that the legionella bacteria in the cylinder is killed in a matter of
seconds at 70°C, and that 90% of the bacteria is killed after 2 minutes at 60°C, or after 2
hours at 50°C.
BACTERIA IN SOLAR SYSTEMS
Solar irradiation during the winter periods in the UK and Ireland may only contribute to
approximately 10-20% of the hot-water demand by pre-heating the incoming cold water
feed, by way of example, to temperatures of 20°C to 30°C. This lower energy contribution
from the solar collector can still reduce fossil fuel consumption during the winter periods.
However, the risk of lower temperature water, between 20°C and 45°C, being held in the
cylinder for a period of time is increased, as is the associated risk of legionella bacteria
growing.
Therefore, there is the requirement to incorporate a means to prevent bacterial growth
(legionella) at all foreseeable flow rates before DHW distribution. One way of achieving this
requirement (as detailed in MCS MIS3001 Standard 4.3.4) is through the use of a secondary
means of heating the water to at least 60°C.
The relatively low volume of cylinder storage in small systems combined with typically high
throughputs reduces the risk of bacterial proliferation. However, where a high risk of
bacterial proliferation exists or the end-users are in a high risk category or the installation is
a large system (e.g. hospital, nursing home, hotel, factory etc.) the solar store should be
designed to be regularly sterilised. This sterilisation should be accurately controlled by time
and temperature, and ideally occur at the end of the day to maximise solar contribution.
