Above the dual mode point (e. g. –11 °C), the heat pump covers the
entire heat load. Below the dual mode point, the heat pump increa-
ses the return temperature of the heating system and an instantane-
ous heating water heater in the heating flow provides additional
heating.
The system is sized in accordance with the performance diagrams.
55 °C
45 °C
35 °C
0
-20
-15
-10
-5
0
5
10
15
20
25
30
2
4
6
8
10
35 °C
45 °C
55 °C
Air intake temperature in °C
Output in kW
C
A
B
E
D
Curves subject to the flow temperature:
A
Heating output at flow temperatures of 35 °C, 45 °C, 55 °C
B
Power consumption for heating at flow temperatures 35 °C,
45 °C, 55 °C
C
Heat load
D
Dual mode point
E
Heating limit temperature
Example:
Heat load to EN 12831:
8 kW
Minimum outside temperature:
–20 °C
Heating limit temperature:
18 °C
Maximum flow temperature:
55 °C
Selected:
Air/water heat pump
Vitocal 200-A, type
AWO-M-E-AC 201.A08
As can be seen on the performance diagram, the dual mode point is
at –11 °C at an output of approx. 6.1 kW.
7.8 Hydraulic conditions for the secondary circuit
Minimum flow rate and minimum system volume
For trouble-free operation, heat pumps require a
minimum flow rate
in the secondary circuit.
In order to ensure the minimum runtimes for the heat pump, a
mini-
mum system volume
in the secondary circuit must also be consid-
ered. If the system volume is too small, the heat pump may switch
on and off too frequently if heat consumption in the building is low
(cycling).
There must be no means of shutting off the minimum system vol-
ume. The heating circuits that can be shut off via thermostatic valves
must therefore not be included in the calculation.
Values for minimum flow rate and minimum system volume
Values must be strictly observed: See tables on page 120.
In heat pumps with output control the heat transfer adjusts to the
building's heat load, which enables reduced cyclical operation in the
partial load range.
Even with these heat pumps, however, the minimum system volume
must be available when the heat demand in the building is very low,
e.g. towards the end of spring.
Providing the necessary defrost energy
Viessmann air/water heat pumps defrost efficiently by reversing the
refrigerant circuit. The defrost energy is taken from the secondary
circuit for a short period of time. To ensure the safe and long lasting
operation of the heat pump, a sufficiently high system volume must
be available to provide the defrost energy.
Systems with a heating water buffer cylinder connected in parallel
Heating water buffer cylinders connected in parallel to the heat pump
ensure a sufficient minimum system volume in the secondary circuit.
Hydraulic separation of the heating circuits also ensures the mini-
mum flow rate of the heat pump, regardless of the hydraulic condi-
tions in the heating circuits.
Design information
(cont.)
VITOCAL
VIESMANN
119
5831388
7
