~ long fogging and AC (was: Re: 1st Snow in My A4 (1/2 long))

[marriott@Summa4.COM]

Hi Ivan,

Disclaimer:  I'm entirely unfamiliar with the A4 CC system. My comments are of a general 
nature on the function of HVAC in autos and air/water vapor mixtures.

> I don't know what exactly the thermodynamics were, but the car got
> fogged up. Running the defogger didn't help much, because either the fan
> was recirculating the warm air or it was the outside air, but heated by
> the car. At any rate, air that the fan was putting out was _moist_.

Maybe the system is malfuntioning or too complicated to set right for this condition. 
Make sure "vent/fresh" is selected and _operating_. The BMW 316 I rented in Germany in 
June had CC, and more buttons than my stereo receiver. Took three days to get the AC 
working . . . I was looking for "kaltesluft" or such in the manual.

> Had
> I been allowed (by Hans und Klaus) to use the AC, the same air would've
> been dry. 

If the outside temp was below ~40F and you were heating the air before you blew it into 
the car, the air _was_ dry.

> In every air-conditioned car that I ever owned - a Volvo, a Nissan, an
> Isuzu and  my father's Oldsmobile - I could run the AC at any
> temperature.

I'll bet you could _select_ AC at any temperature, but the compressor wouldn't run at 5 
F. That's the way the AC cars I've had (two non-CC Audis, Subaru, Mercury) worked.

> None of the coils ever froze. And you know why? Because I
> don't like air-conditioning in freezing temperatures. Do you?

Ummmm, not usually.

> >Below 32F/0C, air holds virtually _no_ moisture. Below about 40F/4C
> >it's darn close to
> >nothing. You can still have 100% humidity, but the amount of water
> >required for that is
> >nearly zero. Best results come from just running the heat with
> >"fresh/vent" selected, if
> >that's an option, and the fan on medium speed. Keep blowing the wet air
> >out and sucking
> >in nice dry air from the outside (Bob D', NO!). 
> 
> 
> This is exactly how it happened: the climate control ran the heat at
> medium speed.  But as the air inside  got warmer, the moisture started
> to settle on the windows.

Somehow I think your system was recirculating.

> Once the "fog" turns to
> >"frost" on the
> >inside of the glass, you're
> >pretty well done for. The heating system doesn't have the oomph to warm
> >the windows above
> >the freezing point, so you're basically waiting for the frost to
> >sublimate. Try to be
> >proactive:  when picking up passengers, kick the rear defrost and fan
> >on _before_ they
> >get in. That way the system never gets behind--it just has to keep up.
> 
> 
> Unfortunately, we all got in the car at the same time...

Ouch. Worst case. Fogs every car I've ever had. Try driving with the windows cracked and 
the fan on "max" until the heater starts working. Or have your passengers hold their 
breath? ;-)

> Look, the point is this: If I'm allowed with my manual transmission to
> select say the first gear at any speed,

Even with the "manual" gearbox this isn't true. Try selecting first gear or reverse while 
going 100 mph. It won't work, because the system (synchros) _knows_ better.

> why am I not allowed with the
> manual settings of my climate control to select say air-conditioning at
> any temperature?

Look, my point is this:  If the air is below ~40F there's no water in it anyway. Running 
the compressor to "dry" this air is useless. Also, the car system is not designed to run 
at these low temperatures.

The _mechanical_ system of the AC knows better than to run the compressor at low temps. 
HOWEVER, the _electronics_ may be closing off the outside air and running the car on 
RECIRC ==> BAD BAD BAD. 

> Ivan
> fogged in in Boston...

hth,
James
overcast in Manchester (NH)...