Re: Ventilation System Question

["Andrei Broder" <broder@pa.dec.com>]

> Why does Audi disable the air conditioning compressor when the
> temperature drops below 40F? (I can't remember the exact temperature,
> but it was around that number).  

If the intake air is already at less than than a certain temp, a car
AC can cool the air it only very little further. The reason for using
the AC in relatively cool weather is to dry the air: the maximum
amount of water (as vapor) that can exists in the air decreases with
the temp, so by cooling the air first the water condenses and when
this air is heated back it is drier.  If the air is at less then 0 C
then the amount of water present is very low already and the whole
process ineffective and unnecessary.  I would think that at 40F ~ 4C
the AC would still dry the air somewhat -- the physical chemists among
us will soon tell us the truth ...

BTW, in many (most?) cars the AC is actually disabled at low temp even
though the AC light stays on ... 

In any case fogging (roughly) happens when the air inside the car
contains more water vapors than the air outside.  This means that you
are not getting enough outside air inside and/or lots of water vapors
are generated inside. The usual source for these vapors is respiration
and (invisible) perspiration, but wet clothes, snow, etc. are often
culprits.

A common problem when snowing is that the intake vents get covered
with snow.  This reduces the fresh air flow and the snow that makes it
through, evaporates in the warm air and adds humidity.  So make sure
that the vent intakes are free of snow.  This could have been your
problem but also it might be the case that the intake vanes were not
completely open due to some bug in the climate control and you were in
fact recirculating the air ... not unheard of in our beloved Audis.

> the Audi wipers just
> weren't up to the task, icing up so that I had to stop every 10-15 
> minutes to clear them so that I could see.  Any suggestion
> for dealing with that problem?

Fill your windshield reservoir with winter liquid, labeled good to
-20F.  (I once did an experiment, using my electric ice cream machine,
and stuff labelled 0 F (-18C) started freezing  at -6C!) Stores in
the mountains are a good source.    Spray the liquid now and then and
that should keep everything fine.  If you use higher temp liquid (that
contains less methanol) it only adds to the problem: since it gets
diluted by the melting snow/ice its freezing point gets higher and the
whole mess starts freezing again.   In any case keep a bottle of
de-icer on hand if you have to stop -- this makes cleaning easy.

Hope this helps,

- Andrei