Re: 1st Snow in My A4 (1/2 long)

[Ivan Sever <sever@husc.harvard.edu>]

On Tue, 10 Dec 1996 11:50:43 -0500 (EST) marriott@Summa4.COM wrote:

>> But that's not all, folks. The heavy and moist air fogged up all the
>> windows within 2 minutes of when the auto-heat came on. "No
>problemo," I
>> though and pushed the AC button hoping to dehumidify the air. But
>noooo:
>> Hanz und Klaus back in Bayerwaldenburgenstrasse decided I 'vil not
>need
>> aircondition im Vinter!' Completely fogged up, I almost went off the
>> road.
>> Wow, what a safety feature!
>
>Couple of problems here.
>
>Problem 1.
>Humidity in car (due to breathing, sweat, drying of wet clothing)
>
>Solution 1.
>Dehumidify air. Run compressor. ==> Problem 2
>
>Problem 2.
>Compressor won't run below ~35F/2C.
>
>(Reasons:  Air holds little moisture below ~40F, and virtually none
>below 32F/0C. No
>water ==> nothing to dehumidify ==> no need for compressor. Also, if
>the evaporator, in
>the car, gets below 32F/0C, any moisture around will _freeze_ to the
>coils, negating
>their operation and possibly damaging the coils via expanding ice.)


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_. Had
I been allowed (by Hans und Klaus) to use the AC, the same air would've
been 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. None of the coils ever froze. And you know why? Because I
don't like air-conditioning in freezing temperatures. Do you? Does
anyone? And that rare short blast of dry air on the windshield was never
enough to freeze the coils.


>Solution 2.
>Disconnect outside temperature sensor. Allow compressor to run all the
>time. ==> Problem
>3.
>
>Problem 3.
>AC system won't work well as the temperature drops. This is just a
>function of the
>refrigerant, the laws of thermodynamics, and system design. System may
>ice up or be
>damaged.
>
>Solution 3.
>Make sure you're not on recirc. On my '87 4kcsq (no climate control and
>about 47
>generations from an A4Q) the recirc flap closes anytime the AC switch
>is actuated,
>_whether the temp is too cold for the compressor or not_. So, if it's,
>say, 38F outside
>and the compressor won't run, and you (or at least I) hit "defrost/AC,"
>you can actually
>make the problem
>worse--the flap closes and puts you into recirc, so the inside air gets
>wetter and wetter
>. . . but no compressor and dehumidification.
>
>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.


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...

>hth,
>James


Look, the point is this: If I'm allowed with my manual transmission to
select say the first gear at any speed, why am I not allowed with the
manual settings of my climate control to select say air-conditioning at
any temperature?

Ivan
fogged in in Boston...