Viessmann VITOCAL 250-A - Flow Rate Control with Hydro Autocontrol

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To ensure fault-free operation of an air source heat
pump, a minimum flow rate is required.
If the flow rate in the secondary circuit is too low, not
enough heat is transferred and the condensing tem-
perature rises. Depending on the operating situation,
the increase in condensing temperature can have the
following effects:
■
High pressure fault: The refrigerant circuit switches
off.
■
The instantaneous heating water heater switches to
a lower output stage, switches off or does not start
up.
■
The defrost process is cancelled.
■
The refrigerant circuit unit is locked out.
The flow rate depends on the current hydraulic pres-
sure drop in the secondary circuit. If, for example, a
thermostatic valve is closed in the directly connected
heating/cooling circuit 1, the hydraulic pressure drop in
the secondary circuit increases and the flow rate
through the condenser decreases. If the minimum flow
rate is undershot here, heat generation in the refriger-
ant circuit switches off even though there is a heat
demand.
To ensure the minimum flow rate, heat pumps with
Viessmann One Base are equipped with Hydro Auto-
Control. Hydro AutoControl regulates the flow rate
through the condenser depending on the hydraulic
pressure drop of the system via the position of the 4/3-
way valve.
If the hydraulic pressure drop increases and the flow
rate in the secondary circuit decreases as a result,
some of the heating water is channelled via the buffer
cylinder integrated into the indoor unit. Due to the low
pressure drop in the integral buffer cylinder, the mini-
mum flow rate can be maintained for operation of the
heat pump. The refrigerant circuit only switches off if
heat transfer from the secondary circuit is too low.
Flow diagram: Example with heating mode
D
E
G
F
H
A
B
C
K
Fig. 13
Air source heat pump
Heating/cooling circuit 1
Indoor unit with 2 integral heating/cooling cir-
cuits:
Heating/cooling circuit 2
Instantaneous heating water heater
(not available with all heat pump types)
DHW cylinder
(integrated in floorstanding indoor units)
A
Return temperature sensor, secondary circuit
B
Condenser
Function description
Flow rate control with Hydro AutoControl
6218234
Functions