try tapping on the heater valve with a small hammer and see if it starts to behave.

my W124 did that too and the tapping fixed it. It gets stuck with wrong and neglected coolant.

Also make sure the thermistor next to the dome lamp is not covered in dirt/lint. The pushbutton unit measures cabin temperature from it.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)