There is a coolant valve in the front heater pipe leading to the heater core. When fully closed, it blocks coolant flow from the core. When fully open, coolant flows through the heater core and produces heat, even when the climate control system is off. The valve should variably close or open depending on the temp setting on the CCU. Setting 60 is full closed and setting 90 is full open. The valve is controlled by a motor on the side of the climate unit under the dash. Either the motor is bad and just goes full open or closed no matter the input, or your climate control unit is bad and should be sent to BrianK to be fixed. If the heater valve were stuck, it would either be heat all the time or no heat all the time. That it does both makes me think it is the motor or CCU.
