Can variable speed pumps be used on compensated heating circuits, in place of three port and pressure relief valves. Are they cost effective, and how is the pump controlled?
Variable speed driven pumps can be used in compensated circuits, but not as a substitute for the three port mixing valve.

The variable speed pump would normally be installed in the usual position (after the three port valve0, and then vary the flowrate as the TRV or zone valves open and close.

Questions have been asked, if this circuit now classes as variable volume, and could the three port valve go out off authority. By installing the variable speed drive in this position and controlling it on constant pressure, the system could be classed as variable volume, but it was before! The VS drive will allo you to maximise power savings by slowing down the pump as TRV/Zone valve position changes, whereas a fixed speed pump would move up and down its fixed curve.

Exact calculation for payback and energy savings can be made by Grundfos on your behalf and pumps would be controlled by pressure sensors built into the pump body.

Source

Building Sustainable Design