Unit installation

IOMM ACZ/AGZ-3 ACZ / AGZ-BM 69
Stage Up Error Step = Saturated Condenser Refrigerant temperature – (Target + Stage Up
deadband).
The Stage Up Error Step is added to Stage Up Accumulator once every Stage Up Error Delay
seconds. When Stage Up Error Accumulator is greater than the Stage Up Error Setpoint another
stage is started.
When a stage up occurs or the saturated condenser temperature falls back within the Stage Up
deadband the Stage Up Accumulator is reset to zero.
Normal Operation - Staging Down
There are four Stage Down deadbands, one for each stage.
When the saturated condenser refrigerant temperature is below the Target – the active deadband, a
Stage Down error is accumulated.
Stage Down Error Step = (Target − Stage Down deadband) − Saturated Condenser Refrigerant
temperature.
The Stage Down Error Step is added to Stage Down Accumulator once every Stage Down Error
Delay Second. When the Stage Down Error Accumulator is greater than the Stage Down Error
Setpoint, another stage of condenser fans turns off. The last stage on will not shut off until the
circuit is in an off state.
When a stage down occurs, or the saturated temperature rises back within the Stage Down deadband,
the Stage Down Error Accumulator is reset to zero.
Forced Fan Stage At Start
Fans may be started simultaneously with the compressor based on outdoor ambient temperature.
When the compressor starts, a Fantrol stage is forced based on the following table.
Table 46, Forced Fan Staging
Fantrol Stage At Start
> 75
o
F Forced Fan 1 SP
> 90
o
F Forced Fan 2 SP
> 105
o
F Forced Fan 3 SP
Up to four fans may be forced on when the compressor starts. If the unit has the Optional Low
Ambient VFD option, then only three fans can start with the compressor, and the VFD will start
normally when the saturated condenser temperature is higher than the target.
After forcing fans on, the saturated condenser temperature may temporarily stay below the target by
some amount. In order to keep these fans from staging off, no stage down error can be accumulated
until either the OAT drops below 75
o
F, or the saturated condenser temperature goes above the target.
Optional VFD Low Ambient Fan Control
Low ambient air temperature fan control is accomplished by using the optional Low Ambient VFD
to control the speed of the first fan on each circuit. This VFD control uses a proportional integral
function to drive the saturated condenser temperature to a target value by changing the fan speed.
The target value is normally the same as the saturated condenser temperature target setpoint.
The fan VFD always starts when the saturated condenser temperature rises higher than the target.