Hi TC,
That is an interesting and very possible theory.
I wonder though.....if the flow is retricted enough for the coolant to cool down in the heater matrix when the fan is on and likewise would heat up more than usual around the engine...would the engine not run a lot hotter than normal?
Ive been giving this some thought. My heater definately improved when I had it vacuum evacuated and vacuum refilled. Maybe the evacuation sucked c**p out thats not possible by draining alone. However, as Ive said elsewhere, whilst its much improved it still don't burn. before I had it done I could only get heat on 27 degrees and not much then. I am now ok on 21 or 24 if its very cold but then have to turn it down.
This is obviously as frustrating as my temp gauge and radio (progress has been made with it).