Viessmann VITODENS 300 WB3A, Service Instructions Manual

Page: 112 / 148

Heating mode
The control unit determines a set boi-
ler water temperature subject to out-
side temperature or room temperature
(if a room temperature-dependent
remote control is connected) and the
slope/level of the heating curve. The
determined set boiler water tempera-
ture is then transferred to the burner
control unit. From the set and actual
boiler water temperatures, the burner
control unit calculates the modulation
level and controls the burner accord-
ingly. The burner control unit limits the
boiler water temperature: through the
control thermostat to 84 °C and
through the electronic temperature
limiter to 82 °C. The thermocouple in
the safety chain locks out the burner
control unit at a boiler water tempera-
ture of 100 °C.
DHW heating
The DHW heating will be activated if
the cylinder temperature falls 2.5 K
below the set cylinder temperature.
The burner and the cylinder loading
pump are switched ON. In the deliv-
ered condition, the set boiler water
temperature lies 20 K above the set
cylinder temperature. If the actual
cylinder temperature rises 2.5 K
above the set cylinder temperature,
the burner will be switched OFF and
the cylinder loading pump run-on time
will be activated.
Supplementary DHW heating
The backup heating function is acti-
vated, if a switching period is selected
for the fourth time phase. This period
must lie outside the switching times
for standard DHW heating to enable
the control unit to recognise the sig-
nal. The set temperature value for
backup heating is adjustable in coding
address 58.
Weather-compensated control units
96
Function description
56
92
550
GB