Viessmann VITOCAL 300-G Installation And Service Instructions Manual page 21

Function description for the system examples
The heat pump control unit regulates this
function ("Inst. heating water heater
7900"). The instantaneous heating
water heater can be enabled separately
for central heating ("Heating with elec-
tro 7902") and DHW heating ("DHW
with e heating 6015").
If enabled via parameter "Maximum
stage, electric heating 7907", the heat
pump control unit starts stages 1, 2 or 3
of the instantaneous heating water
heater, subject to heat demand. As soon
as the maximum flow temperature in the
secondary circuit "Maximum flow tem-
perature 200E" is reached, the heat
pump control unit switches the instanta-
neous heating water heater off.
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
("External heat source 7B00").
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.
Preparing for installation
(cont.)
Parameter "Stage at power-OFF
790A" restricts the output stage of the
instantaneous heating water heater for
the duration of the power-OFF period.
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 by at least 1 K
within 24 h, the heat pump control unit
displays a fault message.
If the outside temperature (long-term
average) is below the "Dual-mode tem-
perature 7B02", the heat pump control
unit starts the external heat source. In
case of direct heat demand from the con-
sumers (e.g. for frost protection or if the
heat pump is faulty), the external heat
source is also started above the dual
mode temperature.
In addition, the external heat source can
be enabled for DHW heating ("External
heat source for DHW 7B0D").
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 shut down
the external heat source and the secon-
dary pump(s) (switching threshold in
each case 70 °C).
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