Both as a tool for starting an offline chiller and as a reliable indicator for stopping an online chiller, it makes sense to use chilled water (CHW) flow reading through a decoupled bypass leg.
A decoupled CHW plant is one where flow and head produced by pumps in the primary loop are unaffected by flow and head produced by pumps in the secondary loop. The section of CHW piping, common to both the primary and the secondary loops, is called the bypass leg. If the bypass leg sees negligible pressure drop, it is considered a decoupled bypass leg.