Interesting question. I fitted a new matrix at the weekend, and the amount of airflow out of the windscreen vents is negligible low. I have 3 theories.
1. I have accidentally nicked a vacuum pipe whilst removing the old matrix.
2. The new matrix is smaller than the old one, in particular the height at the pipe connection end. Therefore the matrix does not seal the end of the box it sits in and some air escapes out the sides rather than passing through the matrix. A job for gaffer tape when the big thaw comes.
3. A flap somewhere in the heater is not flapping as it should. Can anyone tell me where the operating arm for the screen flap is - ie left or right, high or low.
On the plus side I am no longer getting all misty and temporarily at least I have a feint new car smell, which is preferable to the wet dog and coolant aroma I had before.
Steve.
ps. I have twisty knobs!
pps sorry to hijack the op.