mechwarrior-

You have two control systems fighting against each other. The CPC drives the pump speed, therefore the gpm and system pressure drop. The control valves react to the pressure drop and modulate accordingly to control for setpoint and their actuation changes the system pressure drop, etc.(assuming 2-way valves). As one system attempts to control to its setpoint, it affects the second system which, again, affects the other.

Solution:

The CPC should ramp the pump speed to control only the system pressure with a differential pressure transmitter at least 2/3 of the total pipe distance from the pump. (I prefer the sensors to be at the last valve.)

You should have one valve at the end of the loop to bypass a minimum of 20% of the total flow of water from the supply side to the return side. The valve should be actuated by a "high limit" when the pump is at its minimum frequency (or on bypass) and still cannot reduce the pressure to setpoint. This valve is 2-position actuated by the high limit.

The control valves should only control from a temperature sensor with proportional control, only. Leave integral and derivative at "0".

Send more info concerning the system for a detailed solution.

1. Number of valves, Cv of each valve, designed system pressure drop.

2. Pump specifications, GPM, etc.

3. Pipe size of main system.

4. Original design of the piping system & local valves.