18
Economizer Control
If you have configured the GPCPlus as a very simple Air Handling Unit, it has the ability
to control the outside air dampers in a true first stage economizer cooling mode. This
mode requires a Minimum Ventilation position that it maintains whenever the
economizer is not enabled for cooling. It also needs to know which relay has been
configured as the first compressor stage. Use the Outdoor Air Enable limits to set when
the free cooling mode can be used in conjunction with scheduling and temperature
demands.
Alternate Override
There are situations where we want normal control to be suspended temporarily
whenever an unusual situation occurs. For example: we are using the economizer control
method and we have installed a CO² sensor on this controller to use for Indoor Air
Quality. If the CO² reading exceeds a specified level, this Alternate Override can take
charge and move the output signal to a pre-designated level to bring the IAQ back under
control.
Proportional Reset Signal
As the Reset Source goes from its Maximum Reset Source to its Minimum Reset
Source the Controlling Setpoint goes from its Minimum Reset Setpoint limit to its
Maximum Reset Setpoint limit.
Since the reset limits can be set to any desired value, the user can initiate a reverse acting
proportional reset or a direct acting proportional reset of the setpoint simply by crossing
the min and max values if direct acting is required.
Direct Acting = As Temperature Rises the Setpoint Drops
Reverse Acting = As Temperature Rises the Setpoint Rises
An example of using Proportional Reset would be for Boiler Control. As the outside air
temperature rises, we would like the boiler output temperature to drop. We would use the
outside air temperature as the control source and set the Max Setpoint and Min Setpoint
to the range we want to vary the voltage from 0 to 10.0 vdc. If we set the Max Setpoint to
50° and the Min Setpoint to 60° then as the outside air rose from 50° to 60°, the output
signal would drop from 10.0 vdc @ 50° to 0.0 vdc @ 60°. This is because we made the
output Reverse Acting by setting the Max Setpoint lower than the Min Setpoint.