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Re: A/C recirculation/warm air from dash ponderings...

   >I'm guessing that I'm not the only one to be slightly annoyed
   >by the presumption that I always want "fresh" air when I want 
   >heat/cooling, as well as no warm air from the dash vents....

   I always get a lot of hate mail when this thread comes up but from the
   prospective of people who design these things for a living, recirculation is
   a necessary evil.

   It is intended only to be used for maximum A/C cooldown in places like here
   in Texas. It is also useful for a short burst when driving through a cloud
   of dust or exhaust gas or the like. 

   Extended driving in recirc will do the following:
    1. Moisture will not escape from the car and the windows will fog.
    2. Odors from inside the car will recirculate through the evaporator core
   and sooner or later (generally sooner) will "poison" the core and it will
   begin to smell.
    3. The smell will be intensified because the evaporator will not be flushed
   with condensate water from the outside air.
    4. The body pressure will go negative and the air will be sucked in around
   door seals, through holes in the firewall, etc. The threat is that exhaust
   gas can be sucked into the car. Some consider this a safety problem.

   While it is true that some people understand all this and can make good use
   of manual control in certain situations, I guarantee that most people do not
   have a clue (members of the list excluded). Therefore most manufacturers
   provide recirc only in the max A/C position and many that once provided the
   button are going away from it.

   No warm air from the center dash vents is a design for cars with a heater
   but no A/C. It works well in winter where you drive with heated air on the
   floor and can manually open the center vents a little to allow cold air to
   the face.

   It is, however, a disaster in an A/C car. The air from the center vents can
   become bone chilling at moderate conditions.

   The SAE (Society of Automotive Engineers) has a proposal being considered to
   require 20% outside air in recirc mode. Use of 100% recirculation would
   require a timer to switch back to outside air in 10 minutes.

   So, that's the party line. I am now prepared for the onslaught of objections.

Damn, man, you make it hard to piss'n'moan by posting reasoned thoughts.

But I'll try...heh heh heh...

I agree that automatic systems can perform well. In theory. In RealWorld(tm)
practice, they often fall far short of that.

For example, Audi's suck (HVAC-wise).

My '83 UrQ's system is so TFU that it's best simply forgotten about . . .

My '98 A4TQ's system is a vast improvement, but it requires constant
diddling to make it "comfortable".

The problem with "smart" devices is that you have to work harder to
outsmart them and get what you want.

Case in point -- the sun sensor. As near as I can tell, it does
nothing.  (Hey, maybe it's busted?) Every morning as I go into work, I
have to crank the "smart system" down to 63 to keep from getting fried
out of my seat (I do tend to like it "cool", this is "winter"
time). Every night as I go home (sun is down) I have to crank it up to
68, 70, and even sometimes turn on the seat heat for awhile.  Consis-
tently.  Daily. Further, I have to manually force center-vents in
order to warm my pinkies (yes, I could wear gloves...), then -- once
the interior is more-or-less comfy -- I have to manually shut off the
center vents to keep the *(@%$#@&)(*!ing thing from blowing *COLD*
(er, excuse me, "fresh") air on those self-same pinkies.

Case in point -- manual fan speed. I select a fan speed (given
"breeze" factor). 2 minutes later, it has decided a new speed is
called for. 3 minutes later, it changes it again. I have to 
constantly "ride" the fan control.

Case in point -- recirc. IT WILL NOT LET ME SELECT RECIRC WHEN I
BLOODY WELL WANT IT. It's 37F outside, it will not let me have a
recirc mode to shut out that noxious odious truck (dead skunk,
whatever) for even a few seconds.

Case in point -- defog. Worse, it's 37F outside, snowing/raining,
humid as a tropical jungle, windows are dripping condensation, and it
refuses to allow the A/C (and recirc) on to dry it out. Hell, even ye
olde UrQe would allow me that option (granted, I had to run max A/C to
dry it out, then switch to max heat to make it not so cold [note: I
don't say "warm"], repeat; but at least it *could* be done).

Case in point -- fresh air. And God help your eardrums if you try to
open a window (or the sunroof) at highway speeds -- the
resonance/pressure-waves will suck the fillings out of your teeth,
just before pounding them into your brain. (OK, I grant you this is
nothing to do with HVAC per se, I'm just ranting and raving now.)

How they can do *SO* fine with 99% of the car, then so totally get
other stuff so assbackwards never ceases to amaze me.