Download Viessmann VD2-860 Technical data

Function description – Vitocontrol-S, VD2/CT3/CM2
Cascade control (continued)
Net calorific strategy 1
Conventional boilers are preferably
operated at their upper output range, to
reliably prevent the formation of
condensate due to low return
temperatures.
With net calorific strategy 1, an
additional boiler will only be started if
the maximum output of all currently
active burners is insufficient to achieve
the set supply temperature.
A boiler will be shut down, when the
remaining boiler(s) can achieve the
required output on its/their own.
Benefit:
As few boilers as possible are active.
Start-up criteria
The boiler is started via a start-up
integral.
The start-up criteria is met when the
start-up integral exceeds a limit set via
coding address 45, and the next boiler
in the sequence will be started (see
page 129).
Shutdown criteria
The boiler is shut down via an output
statement (coding 3d: 1).
A boiler is shut down when the
currently required output can be
achieved without the boiler which was
started last.
5351 340 v1.4
Net calorific strategy 2
With the net calorific strategy 2, an
additional boiler will only be started if
the maximum output of all currently
active burners is insufficient to achieve
the set supply temperature.
A boiler will be shut down if the burner
was returned to its minimum output
because of a large negative control
deviation, and the output is then still
too high.
Benefit:
Long burner operating times
Start-up criteria
The boiler is started via a start-up
integral.
The start-up criteria is met when the
start-up integral exceeds a limit set via
coding address 45, and the next boiler
in the sequence will be started (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 3d: 1, and the
boiler started last will be shut down.
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