Blend Door-How SHOULD it Function
 

The blend door is only fully closed (heat) when I set the temp to HOT, regardless of selector for defrost, center or floor.

When I drop the temp one notch to 85, the blend door moves and too much cool air starts to come through. If I manually select the diverter to the CENTER vents, door closes and I get full heat. Setting the diverter to defrost or feet, the blend door moves and lets to much cool air through.

In other words, I get full hot heat, but only when I select the center vents or temp at hot.

Both blend door and divertor door (don't know if those are the right terms) fully move. I previously replaced all the foam on the doors and they are fully taped up and have full movement.

Not 100% sure (and car is not around to check) but would bet the computer is reading the interior and exterior temps when in auto mode and adjusting the door to get to set temp efficiently. When you push the actual center button you are putting it into a "manual" mode and it gives full heat (until you reach over and turn it down)

I thought the same. However, while in "manual" mode, when I select air to floor, the blend door opens adding cool air. Seems the blend door should stay put and only the divertor should move to direct air to the floor instead of center.

The material on the doors has probably deteriorated to the point that it can't properly regulate the flow of cold air. This is basically how mine performs now that it's done blowing foam in my face. My gf hates it in the winter. I'm toasty hot and she's cold.

Getting heat in the winter frustrates me when drivng the Boxster. I have replaced the foam on the center door. I will at least try your work around this winter.

Blend and center doors should work independently, one for temp control and the other to block or unblock flow from middle vent.

At least this was my first-hand observation testing them during my surgery there to tape-shut holes on those doors found completely naked without original foam.

And Boxster heater is so powerful and very quick to warm-up, even with low temp thermostat.