411 Heater Controls

[peter dowdle]

Hi All , I am after some advice on how my heater works on a 411 mk 2.
There are 2 controls on the dash.The demist defrost knob and the heat knob. Clearly the heat knob controls the hot water tap on the side of the heater box. does it do anything other than that?
The demist defrost knob in the demist position clearly sends air to the front screen but my question is what is meant to happen to the air in the defrost position ?
The reason i am inquiring is I get very little hot air in the cabin as I am driving along the road and get no heat out of the fresh air vents.
Strangely when the car is at idle in the driveway and the 2 speed fan is working I get lots of hot air in the cabin at the screen and the under dash trap door, but also through fresh air vents. The car becomes almost too hot which suggests I dont have a water flow problem.
My guess is that at speed the cold air is mixing with the hot air and lowering the overall temperature, which suggests a flap in the system isnt doing its job.
Should I at any time be getting hot air through the fresh air vents?

Any thoughts or suggestions much appreciated.