You just have to pair "TO DEVICES *" and "TO EQUPMENT *" Families in you host and linked projects.
["TO EQUPMENT *" family connectors should be "System" or "Calculated", "TO DEVICES *" family connector should be "Preset"]
I use shared parameters for all involved connector families.
In the device's (hydronic radiators, fancoils) project you have to make the connector families to System Equipments:
In the other project, the connector families act like devices:
Pairing can be done by a custom parameter value for example in Comments parameter (GetParameterValueByName node), or they can be identified by common insertion point (Element.Location node).
After you have the pairs, it is easy to get Flow and other values from "TO EQUIPMENT *" family and set the parameters on the other.