On 14/04/2013 3:37 PM, R. Mark Clayton wrote:
> IIRC the manuals for the 5 BMW's I have had say the AC will not operate
> below 2.5C.

Again, that probably only refers to the cabin A/C which is used to cool
the cabin. But the A/C is also used to dry the air going into the
windshield defogger. Most car companies do that with their A/C.
Especially when the full-defogger is selected, i.e. where all of the air
is forced through the windshield rather than just partially through the
windshield and through the cabin.

> OTOH E23's had a feature whereby the system would switch to recirculate when
> demist was selected.

Recirculating the air inside to defog would actually cause more fog to
appear inside. In the snowy climates, as you enter a cabin, and after
awhile the snow melts off your boots and your clothes, more moisture
becomes available inside the cabin. When the air is being recirculated,
this moisture can't escape, and starts coating the windows and
windshield. The solution here is to make sure dry outside air comes in
and that it gets dried further by the A/C.

However, I can see it might the opposite situation in warm tropical
climates, where the outside air is moister than the inside air. Then you
might want to recirculate the inside air instead.

Yousuf Khan