Not sure I have a problem or not but wanted a confirmation on how interior loop plumbing should be set up: I have two Waterfurnace 7 Series Heat pumps (4 ton unit for downstairs zone and 3 ton unit for upstairs zone). Each HP has its own flow center; each flow center has a single variable speed circulator pump (Grundfos Magna GEO 32-140 in each). Both flow centers are plumbed together into a single interior HDPE header pipe that runs outside to the common closed ground loop. During initial installation, the installer had plumbed Johnson Controls modulating valves supplied by Waterfurnace on the output side of each heat pump and wired them into the HP control board on the same terminals as the signal for the flow center pumps. This didn't work - the modulating valves didn't modulate and the flow center pumps ran at maximum regardless of whether the HPs were calling for flow or not. I didn't understand why a modulating valve would be needed with variable speed pumping anyway and suggested just check valves in their place (on the outflow side of the HPs) so that backwards flow/short circuiting when only one of the flow centers was running would be prevented through the non-running HP. Installer didn't understand it either, talked with Waterfurnace tech who told them they aren't needed, and decided my suggestion was the correct way to move forward. He removed the modulating valves and instead installed check valves on the output side of each heat pump. This seemingly cured everything: each circulator pump is only on when its respective HP is on and each circulator pump revs up and down according to its respective HP stage. Everything seemed okay. But I got this nagging feeling: why would Waterfurnace provide modulating valves (knowing I have 7 series units with variable speed flow centers on a closed loop) if they weren't necessary? In looking at the interior flow path through the HPs, it got me thinking too that if one HP/flow center is running, although the check valves on the HP outflow sides prevent backwards flow, what stops forward flow through the non running HP if only one is running? Would the combination of the pressure drop through the non-running HP and non-running flow center pump act as a blockage to essentially stop forward flow through the non-running circuit? Wondering if those modulating valves need to be placed on the inflow side of the HP to stop forward flow through a non-running HP? If so, were on the control board do they get wired as the installer showed me there didn't seem to be a place for them - only the variable speed pump signal wires. Curious as to what others have done/have installed for multi HP system each with own flow center on common ground loop. Thanks in advance.