47
WATER
The heating system must be filled and topped up with water in ac-
cordance with VDI guideline 2035 “Avoiding damage in hot water
heating systems”.
U
SING AN ACCUMULATOR When calculating the thermal requirements of buildings, e.g. accor-
ding to DIN EN 12831 “Method for calculating the normal heating
load”, the lowest outside temperature of the relevant climate zone
(e.g. -15 °C) is used. This condition only applies a few days per year,
thus, the thermal performance of the heating system is greater than
required on most days when heating is needed. For this reason, the
HDG Compact 25 - 80 is fitted as standard with power control and au-
tomatic ignition.
It is, however, highly recommended to use an accumulator even
with automatic boiler systems.
The size of the accumulator will depend on the nominal thermal
power of the boiler and on the thermal requirements of the building.
As a benchmark value, 20 litres per kilowatt boiler power can be
used. This results in a boiler burning duration of approximately one
hour at full-capacity operation, during which the accumulator is
completely filled. The emptying time of the accumulator at 25% no-
minal load amounts to 3.7 hours for this type of design, with an as-
sumed usable temperature difference of 40 Kelvin.
An advantage of using an accumulator lies in the lower operating
hours of the system and fewer start-up phases due to the extension
of the heating intervals. This leads to a reduction in the proportion of
external energy supplied and to lower wear of the mechanical com-
ponents.
One further advantage of the accumulator is in summer operation
when only hot water is required. When operating in this mode, the
accumulator helps avoid frequent ON/OFF switching.
For the above-mentioned reasons we recommend an accumu-
lator, even for automatic boiler systems.
S
AFETY DEVICES Safety devices are to be installed in accordance with DIN EN 12828:
2003 “Design of water-based heating systems in buildings”.
R
ETURN TEMPERATURE
CONTROL
Operating temperatures which are too low significantly shorten the
service life of the boiler. Water vapour contained in the flue gas could
be released in the form of condensation if the temperature drops
below the dew point (approx. 50 - 55 °C), especially in the area
around the water-cooled heat-exchanger surfaces. This condensa-
tion, in combination with combustion residue, can lead to corrosion.
For these reasons, a return temperature control must be installed for
the HDG Compact 25 - 80 heating system. The return temperature
control causes the water from the boiler return flow to be mixed with
the water from the boiler supply flow until the minimum return tem-
perature has been reached.
✎ The minimum return temperature is listed in chapter “3 Mode of
operation”, section “3.3 Technical data”.
Antifreeze agents may only be used after consultation with HDG.
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