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If you have set the system flow rate with one or more fixed flow pumps, you will need to leave one of the system loops uncontrolled (i.e. set the loop FCV to the fully open position).  The loop that you need to leave uncontrolled is the most hydraulically remote loop (the one where the control valve has the least amount of differential pressure available for control).

You can quickly determine which loop is the most hydraulically remote by running one lineup. In this lineup, take a guess at which loop is the most remote (a good guess is usually the loop furthest away from the pumps, or a loop that has a high required flow rate).  Run the lineup with the FCV in this loop set to fully open.  If the results do not show any FCVs with invalid results (in red text), you've found the most remote loop.  If only one FCV has invalid results, that one is the most remote loop.  If several FCVs have invalid results (i.e., they are acting as pumps), the easiest way to tell which one is in the most remote loop is to look at the FCV results.  The FCV with the largest negative (-) head gain identifies the loop that is the most remote.

NOTE:
If there are a lot of FCVs in your system with invalid results, a quick way to pick the one with the highest negative (-) pressure gain is to display the List Window for the controls and then sort the list by the control valve dP. The control valve with the highest negative dP is the most hydraulically remote.