Danfoss AR05, Operation Instructions Manual

21

Reglermanual - VMGFF102

5.9 - Capacity control

This function adjusts the number of active compressors to 
keep the heat exchanger water temperature at its setpoint. 
The precision with which this is achieved depends on the 
capacity of the water loop, the fl ow rate, the load, and the 
number of stages available on the unit. The control system 
continuously takes account of the temperature error with 
respect to the setpoint, as well as the rate of change in this 
error and the diff erence between entering and leaving water 
temperatures, in order to determine the optimum moment at 
which to add or withdraw a capacity stage.

If the same compressor undergoes too many starts (per hour) 
or runs below one minute each time it is started this 
automatically brings about reduction of compressor starts, 
which makes leaving water temperature control less precise.

The high pressure, low pressure or defrost unloading functions 
can also aff ect temperature control accuracy. Compres-sors are 
started and stopped in a sequence designed to equalise the 
number of start-ups (value weighted by their operating time).

5.10 - Head pressure control

The head pressure is independently controlled for each 
circuit, based on the saturated condensing temperature value.

5.11 - Defrost function

This function only applies to heat pumps. Defrost is activated, 
when the unit is in heating mode to reduce frost build-up on 
the air heat exchanger. The defrost cycle can only be applied to 
one circuit at a time. During the defrost cycle the fans of that 
circuit are stopped, and the four-way refrigerant valve is 
reversed, forcing the circuit to cooling mode. The fan can 
temporarily be restarted during the defrost cycle. The defrost 
cycle is fully automatic and does not require any setting.

5.12 - Desuperheater option

In units with desuperheater it is possible to reclaim the hot 
water. To optimise this option, the condensing setpoint 
should be increased (chapter 4.9.4 setpoint menu, sct_min) 
while the desuperheater heat exchanger is used. Optimis-
ing desuperheater condensation is activated via dry contact 
DSHTR_SW (see chapter 3.6.1).

5.13 - Additional electric heater stage control

The heat pump units can control up to four additional 
electric heating stages (accessory).

The electric heating stages are activated to complement the 
heating capacity when the following conditions are satisfi ed:
• 

The unit uses 100% of the available heating capacity, or 
the unit is limited in its operation by a protection mode 
(low suction temperature, hot gas or defrost sequence in 
progress protection), and in all cases cannot satisfy the 
heating load.

• 

The outdoor temperature is below a confi gured 
threshold (see HCCONFIG confi guration).

• 

The unit demand limit is not active.

The user may confi gure the last available electric heating 
stages as a safety stage. In this case, the safety stage is only 
activated in addition to the other stages if there is a machine 
fault, preventing the use of the heating capacity. The other 
electric heating stages will continue to operate as described 
above.

5.14 - Control of a boiler

NOTE: The control of the electric heating stages or of a 
boiler is not authorised for slave units.

The unit can control the start-up of a boiler, if it is in heating 
mode. When the boiler is operating, the unit water pump is 
stopped.

A heat pump unit and a boiler cannot operate together. In 
this case the boiler output is activated in the following 
conditions:
• 

The unit is in heating mode, but a fault prevents the use 
of the heat pump capacity.

• 

The unit is in heating mode, but works at a very low 
outdoor temperature, making the heat pump capacity 
insuffi

  cient. The outdoor air temperature threshold for 

use of the boiler is fi xed at -10°C, but this value can be 
adjusted in the HCCONFIG menu.

5.15 - Master/slave assembly

Two control units can be linked to produce a master/slave 
assembly. The two machines are interconnected over the CCN 
bus. All parameters required for the master/slave function 
must be confi gured through the Service confi gura-tion 
menu.

Master/slave operation requires the connection of a tempe-
rature probe at the common manifold on each machine, if the 
heat exchanger leaving water temperature is controlled. It is 
not required, if the entering water temperature is controlled.

The master/slave assembly can operate with constant or 
variable fl ow. In the case of variable fl ow each machine must 
control its own water pump and automatically shut down the 
pump, if the cooling capacity is zero.

For constant fl ow operation the pumps for each unit are 
continuously operating, if the system is operating. The master 
unit can control a common pump that will be activated, 
when the system is started. In this case the slave unit pump is 
not used.

All control commands to the master/slave assembly (start/
stop, setpoint, heating/cooling operation, load shedding, etc.) 
are handled by the unit which is confi gured as the master, 
and must therefore only be applied to the master unit. They 
will be transmitted automatically to the slave unit.

The master unit can be controlled locally, remotely or by CCN 
commands. Therefore to start up the assembly, simply 
validate the Master operating type (Master) on the master 
unit. If the Master has been confi gured for remote control 
then use the remote volt-free contacts for unit start/stop.