Boiler Water Temperature Control; Heating Circuit Control Unit - Viessmann Vitotronic 300 Installation And Service Instructions Manual

Vitotronic 300, GW5B Installation and Service

Boiler Water Temperature Control

Brief description
The boiler water temperature is regulated by
modulating the burner.
The set boiler water temperature is determined from
the following parameters:
– Set supply temperature of the heating circuits
connected to the control unit and the heating
circuits connected via the LON BUS
– External demand
– Set DHW temperature
When the DHW DHW tank is heated up, the default
set boiler water temperature is 20 K above the set
DHW temperature (adjustable via coding address "60"
in the "DHW" group).

Heating Circuit Control Unit

Brief description
The control unit has control circuits for one heating
circuit without mixing valve A1 (heating circuit 1)
and two heating circuits with mixing valve M2
(heating circuit 2)
and M3 (heating circuit 3).
The set supply temperature of every heating circuit
is determined from the following parameters:
– Outside temperature
– Set room temperature
– Operating mode
– Heating curve slope and shift
The boiler coding card specifies a minimum boiler
water temperature that must be maintained to protect
the boiler.
The supply temperature of the heating circuit without
mixing valve is equal to the boiler water temperature.
The supply temperature of the heating circuits with
mixing valve is regulated by the stepped opening
or closing of the mixing valves.
The mixing valve motor control changes the actuating
and pause times subject to the control differential
(control deviation).
Functions
The heating circuit without mixing valve is subject to the
boiler water temperature and its control range limits.
The heating circuit pump is the only actuator.
The supply temperature of the heating circuits with
mixing valve is captured by the supply temperature sensor
of the relevant heating circuit.
Emissions test function
The following functions are activated:
Burner start
Starting all pumps
Mixing valve remains set to control function
Function duration <30 min: The burner operates at 100
% output until the temperature limiter in the burner
control unit switches off. The burner (at 100% output)
then cycles by the value at the temperature limiter.
Function duration >30 min (uninterrupted): The boiler
water temperature is raised to the max. set value
[e.g. 194°F (90°C)]. The burner is operated in
modulating mode.
This prevents excessive cycling if emissions testing is
carried out over an extended period.
Time program
In accordance with the time program in the "Heating
and DHW" heating program, the control unit switches
between "Central heating with standard room
temperature" and "Central heating with reduced room
temperature".
Every operating mode has its own set level. Four time
phases per day can be selected.
Outside temperature
A heating curve must be set for matching the control
unit to the building and the heating system.
The heating curve characteristics determine the set
boiler water temperature subject to outside temperature.
The control unit uses an average outside temperature.
This comprises the actual and the adjusted outside
temperature.
Function Description
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