Lancia Motor Club

Finally getting around to repairing the interior heater on the PN coupe,  . the matrix was completely corroded , the motor not working, and the shut off valve missing. I have bought a replacement matrix ( about the same size ) got the motor working and replaced the valve . The controls seem simple enough with the lever on the side of the unit operating the hot water valve mounted on the bulkhead either on or off, the Blue coloured twist knob operates the flap on the air intake , either cold air from the front of the car or in the opposite setting i assume slightly warmer air from the engine bay. The red coloured twist knob simply moves a damper which covers the back of the matrix and stops air pushing through it . What i am  struggling to understand is the purpose of that flap as it restricts air flow over the matrix as with it closed and the fan on all the fan would be doing is recirculating air from the interior which would rapidly cool down, am i missing something here ?.

It's a while since I repaired the matrix and fan on my car, but from memory the red knob enables you to close the flap in such a way that cold air from outside does not pass through the heater matrix. The benefit of this is that you can switch from hot to cold much quicker than you could if you switched off the valve to the heater matrix which would then need to cool down as hot water would still be in it.

Thanks AG that seems to make sense . Tricky job refurbing the heater especially as i used a non standard matrix which was very slightly larger and the flow and returns from the matrix came out of the end rather than the top as original. Had to form two new holes through the bulkhead and form quite a tight turn with the heater hose . The fan is the original and single speed which isn't ideal but lets not be too picky. Difficult connecting the cable that operates the flap as the clutch pedal was only 5mm away so had to remove the pedal to allow to tighten a single self tapping screw, all whilst upside down with my head under the dash . Next job, retractable seat belts.