Daikin McQuay AGZ-D, Operating And Maintenance Manual, Page: 34 / 82

34 AGZ 025D through 190D OMM 1087
EXV Control
Auto Control
Any time the circuit is not in the run state, the
EXV position should be 0. The EXV control state
should display that the EXV state is closed when
this is the case.
While a circuit is in the run state, the EXV should
go into superheat control. While in superheat
control, the EXV controls suction superheat. The
suction superheat target is set by a set point. A
PID function will be used to control the superheat
to the target value.
The EXV should also prevent the evaporator
pressure from exceeding the Maximum Evaporator
Pressure set point. This is done by using another
PID function to control evaporator pressure to the
maximum evaporator pressure.
The EXV position should be lesser position output
from the two PID functions.
EXV Position Range
The following table shows the EXV range based
on the number of compressors running and the
total number of fans on the unit.
Compressors Running
1 2 3
EXV Min 8% 8% -
Num Fans = 4
EXV Max 40% 60% -
EXV Min 8% 8% -
Num Fans = 6
EXV Max 60% 100% -
EXV Min 8% 8% 8%
Num Fans = 8
EXV Max 40% 55% 70%
EXV Min 8% 8% 8%
Num Fans ≥ 10
EXV Max 30% 40% 50%
On units equipped with 10 or more condenser fans
(shell and tube type evaporator):
When staging down a compressor the
maximum position is reduced by 10% for
one minute to prevent liquid from getting
to compressors. After this initial one
minute delay, the valve’s maximum is
allowed to return to its normal value at a
rate of 0.1% every six seconds. This
offset to the maximum position should not
occur if the stage down is due to a low
pressure unload.
In addition, the expansion valve maximum
position may be increased if after two
minutes both the suction superheat is
greater than 7.2°C (13°F) and the
expansion valve has been within 5% of its
current maximum position. The maximum
should increase at a rate of 0.1% every six
seconds up to a total of an additional 10%.
This offset to the maximum position
should be reset when the EXV is no longer
in the Superheat Control state or a
compressor on the circuit stages.
Manual Control
The EXV position can be set manually. Manual
control can only be selected when the circuit is in
the run state. At any other time, the EXV control
set point is forced to auto.
When EXV control is set to manual, the EXV
position is equal to the manual EXV position
setting. If set to manual when the circuit state
transitions from run to another state, the control
setting is automatically set back to auto. When in
manual control, the EXV control state displayed
should reflect that it is manual control.