Viessmann Vitocal 350-A Installation And Service Instructions Manual page 33

Function description for the system examples
If enabled via parameter "Max. e heat-
ing stage", the heat pump control unit
starts stages 1, 2 or 3 of the instantane-
ous heating water heater, subject to heat
demand. As soon as the maximum flow
temperature in the secondary circuit
"Max. flow temp" is reached, the heat
pump control unit stops the instantane-
ous heating water heater.
Parameter "Stage at power-OFF"
restricts the output stage of the instanta-
neous heating water heater for the dura-
tion of the power-OFF period.
External heat source
The heat pump control unit enables the
heat pump to operate in dual mode with
an external heat source, e.g. oil boiler
("Ext. heat source").
The external heat source is hydraulically
connected to let the heat pump also be
used as a return temperature raising
facility for the boiler. System separation
is provided either with a low loss header
or heating water buffer cylinder.
For optimum heat pump operation, the
external heat source must be integrated
via a mixer into the heating water flow. A
quick reaction is achieved by directly
controlling this mixer via the heat pump
control unit.
If the outside temperature (long-term
average) is below the "Dual-mode
temp.", the heat pump control unit starts
the external heat source. In case of
direct heat demand from the consumers
(e.g. for frost protection or if the heat
pump is faulty), the external heat source
is also started above the dual mode tem-
perature.
Preparing for installation
(cont.)
To limit the total power consumption, the
heat pump control unit stops the instan-
taneous heating water heater for a few
seconds directly before the compressor
starts. Each stage is subsequently star-
ted individually one after the other in
intervals of 10 s.
If the instantaneous heating water heater
is on and the differential between flow
and return temperatures in the secon-
dary circuit does not rise to at least 1 K
within 24 h, the heat pump control unit
displays a fault message.
In addition, the external heat source can
be enabled for DHW heating ("Ext. heat
source for DHW").
Note
The heat pump control unit does not
contain any safety function for the exter-
nal heat source. To prevent excessive
temperatures in the heat pump flow and
return in case of a fault, high limit safety
cut-outs must be provided to stop the
external heat source (switching thresh-
old 70 °C).
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