Technical data

126
Function description – Vitocontrol-S, VD2/CT3/CM2
Cascade control (continued)
Sequential type of control sequence
(coding 3b: 4)
With the sequential type of control, the
cascade control unit regulates to the
required set supply temperature by
pre-setting the output for each
individual boiler. The lead boiler can be
operated in modulating mode. All other
boilers are operated in two stages via a
defaulted output (incl. modulating
burners).
The control sequence is subject to the start-up and shutdown criteria set by the control strategy (coding address 3C):
Gross calorific strateg.
Net calorific strategy 1.
Net calorific strategy 2.
With supply temperature sensor:
The control deviation is calculated from
the set supply temperature and the
actual supply temperature to determine
the start-up and shutdown criteria.
Without supply temperature sensor:
The control deviation is calculated from
the set boiler water temperature and
the estimated average actual boiler
water temperature of the active boiler
to determine the start-up and
shutdown criteria. Only those boilers
are taken into consideration whose
isolation valve is not closed.
Condensing strategy
It is the aim of the gross calorific
strategy to operate as many boilers as
possible at the lowest output level.
Advantages:
Optimum utilization of the
condensing effect (as many
boilers active as possible).
Long burner operating times.
Start-up criteria
The boiler is started via an output
statement (coding 3d: 1).
An additional boiler will be started,
when the currently required output can
also be achieved by the currently active
boiler plus the next boiler in the boiler
sequence (see page 129).
Shutdown criteria
The boiler is shut down via a shutdown
integral. The shutdown criteria is met
when the shutdown integral exceeds a
limit set via coding address 46, and the
boiler started last will be shut down.
5351 340 v1.4