On the linked schematic line Nr.5 coming from engine to water valves. Nr.6+7 from water valves to both sides of heater core. Nr.8 common return line.
The water valves are open in rest state so unless they are jammed closed there should be constant warm flow to heater core (slightly colder return flow depending how much of the heat is tranferred to cabin air) if you for test purposes unplug the electrical connetor on water valves. Depending on IHKA temp setting and core temperature sensor reading the water valves are closed with PWM voltage (by IHKA).
https://www.realoem.com/bmw/enUS/sho...diagId=64_1116