The heat pump must confirm that there is sufficient water circulation before operating under load.
Flow switch
Usually provides simple ON/OFF information on whether or not there is flow.
Flow sensor
Can provide an actual flow rate value, for example in l/min.
Why they are important
Low flow can cause a large ΔT, freezing in cooling mode, or overheating in heating mode.
Diagnosis
The sensor, filters, circulator pump, and actual water flow rate are checked.
