Viessmann Vitocrossal 300 CA3 2.5 Installation And Service Instructions Manual page 124

Function Description
Cascade Control Unit
Boilers connected in parallel
H Without and with supply temperature sensor
(see the following table).
H The cascade control specifies the set boiler water
temperature for all currently active boilers. Every boiler
control unit regulates to the specified set value.
The following control strategies are available:
H Condensing strategy (see page 125)
H Conventional boiler strategy 1 (see pages 126)
H Conventional boiler strategy 2 (see pages 127)
With supply temperature sensor
Set coding "3b:1" or "3b:3" in the "Cascade" group.
The control deviation is calculated from the set supply
temperature and the actual supply temperature to
determine the start and shutdown criteria.
124
(continued)
Vitocrossal 300 CA3 Series 2.5 to 6.0 Installation/Service
Set supply temperature
The set supply temperature is determined from the
following coding:
H Set supply temperature of heating circuit without
mixing valve A1 (heating circuit 1) and heating circuits
with mixing valve M2 (heating circuit 2) and M3
(heating circuit 3).
H Set supply temperature of further consumers.
H DHW temperature target.
H External demands.
Upper control range limits:
Maximum system supply temperature limit
(coding "37" in the "Cascade" group).
Lower control range limits:
Minimum system supply temperature limit
(coding "36" in the "Cascade" group).
Standalone control
Note: Vitocrossal 300, type CA3, operate in parallel
and with condensing strategy
Boilers connected in series
H Without and with supply temperature sensor
(see the following table).
H The control strategy is set automatically to
conventional boiler strategy 2, coding "3c:2" in the
"Cascade" group (see page 157).
H The cascade control specifies the set boiler water
temperature for all currently active boilers. Every boiler
control unit regulates to the specified set value. If the
boiler section 1 (condensing boiler) does not achieve
the specified set value, it will only act to control
the return temperature of the conventional boiler
downstream. The conventional boiler is started late
and is also shut down late.
Without supply temperature sensor
Set coding "3b:0" or "3b:2" in the "Cascade" group.
To determine the start and stop criteria, the control
deviation is calculated from the set boiler water
temperature and the estimated average actual boiler
water temperature of the currently active boilers.
Only boilers with open motorized isolation valves are
taken into consideration.
Sequential control
H With supply temperature sensor.
H Set coding "3b:4" in the "Cascade" group.
H The control strategy is set automatically to
conventional boiler strategy 2, coding "3c:2"
in the "Cascade"group (see page 157).
H The cascade control unit regulates to the specified set
supply temperature by stipulating the output for each
individual boiler. The boiler section 1 can be operated in
modulating mode. All other boilers are operated in two
stages via a specified output (incl. modulating burners).