I am experiencing exactly the same issue. Just wondering if there was solution to this.
My setup is as follows; OS 3.0, DC version purchased about 6 months ago. Flow meter is a new (quality) device purchased at the same time. It registers 1 pulse per 10 litres and connected with shielded cable. Flow was reading accurately up until a few months after installation. Readings are now ridiculously high , i.e thousands of l/min when the system is in idle and when in use. Flow meter cable has since been removed from controller and flow is still registering flow albeit much lower i.e. range 0-300 l/min.
Just wondering how this issue was resolved with Marco83.?