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

>Date: Wed, 10 Mar 1999 16:22:47 -0500 (EST)
>From: "G. Benedikt Rochow" <rochow@segosf.hlo.dec.com>
>Subject: A/C recirculation/warm air from dash ponderings...
>I've got an '84 5kS, but these thoughts and questions probably 
>apply to a number of models.
>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.

Bob Cummings
87 Coupe GT