A Critical Warning message in AFT Arrow

Full message is:

FCV/PRV/PSV could not control pressure/flow at Junction X. Pressure was added to achieve control flow. See Valve Summary for more information.

A flow or pressure control valve could not control pressure or flow and the setting is to Always Control (Never Fail). In order to achieve the control flow or pressure, the valve must add pressure and act like a compressor/fan.

The Valve Summary in the Output window gives more information.

This warning is useful for resolving operational problems in your system.