Viessmann VITOTRONIC300 Installation And Service Instructions Manual page 80

Function Description
Boiler Water Temperature Control
Return temperature raising in conjunction with heating
systems that are not connected with the Vitotronic via
LON
Plug sÖA1 of the Vitotronic is used as switching contact.
Required code: "4C:2" in the "General" group.
Legend
sÖA1 Closing the mixing valves
A Contactor relay
Downstream heating circuit controller, switching
B
contact closed:
Signal for mixing valve "Close"
80
Vitotronic 300, GW2B Installation and Service
(continued)
Return temperature control via shunt pump and 3-way
mixing valve
For heating systems where downstream heating circuits
cannot be controlled, e.g. older heating systems
or commercial nurseries.
Function
Temperature sensors T1 and T2 capture the return
temperature at different test points.
The shunt pump is started via temperature sensor T2
if the factory-selected minimum return temperature
(defaulted by the boiler coding card) is not achieved.
The flow rate will be reduced via the 3- way mixing valve
if, in spite of the return temperature raising facility,
the minimum return temperature captured by temperature
sensor T1 is not achieved.