Been playing around with the spare heater box and checking the flaps on the one in the car. I think this is just the way it is, annoyingly.
More cold air ends up going to the face vents in the dash because of the way the blend flaps are arranged. This effect is made worse by the fixed speed air conditioning. Say you have the air set to face/feet, interior is warm, the blend motors have backed off slightly. Firstly, more cold air is ending up at the dash vents, and then the A/C compressor engages again - the cold air suddenly gets quite a bit colder again, and the air from the dash vents feels quite a lot cooler. Meanwhile, the air still going to the footwells is still warm, because where it exits in the heater box has encouraged more mixing... Set it to feet/screen position and this is less noticeable, because the screen outlet is near the footwell outlet.
I suppose this is just a bit of a 'me' problem as I like the air at the face/feet position all year round once things are demisted, and while in Summer it can't be cold enough, in winter its a bit annoying as on hi/hi, it does become an oven pretty quickly. I want to be warm, not on fire.
For anyone that hasn't seen inside one, here is half of a heater box. To the left you have the footwell outlet, then the screen outlet, and the dash/face vents on the right. Blend flap right in the middle, where you can sort of see how the cold air meets the heated air coming around the core, and I'm presuming they sort of blow against each other. The probe stuck in the box in various places seems to agree. It just doesn't mix well, but I can't think of how I'd encourage it to without adding something that would likely obstruct either the blend or distribution flaps.
Southampton with a Nanocom for Thor/GEMS P38s
1996 4.6 HSE, then became a 4.0, now cubed.
1997-8 4.6 HSE Motronic/Wabco prototype vehicle. Now M57 powered. Still auto.