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

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>    changed by going through the heater, so you heat it up, only to cool it down again, and
>    then you get nice frosty air in the cabin, right?
>
> No more so that cooling it before heating it. You are pumping the same
> amount of heat energy either way. It's "commutative", so to speak. I.e.,
> add 40F then subtract 40F is the same as subtract 40F then add 40F.
> Within reasonable limits.

(still disclaiming my own ignorance...)
right about absolute temp change, of course.
i believe wrong about removing moisture.
heating up air doesn't 'dry' or take moisture out, just expands the volume, so that by volume
there is less vapor. whereas lowering it down to an absolute dewpoint makes the vapor
condense, which then removes it.
air with a certain vapor content has to get to an absolute temp before the water condenses
out.  so if the air is say, 50% RH, and 40F its going to need to hit 25F or so before the
vapor condenses.
with your example, you pump it up to 80F, then down to 40F again, but never get near the 40F
dewpoint that will drive the water out of the air, you basically get the same air back again.

i think...?

I was thinking about that...and to an extent, I think you're right.
Certainly the absolute amount of water in the air doesn't change as
you warm the air up, although I think it's more a question of how
cold the A/C evaporator is (which rather tends to imply how cold the
exiting air is), so it may be a more-complicated issue of blowing
warm-enough air *into* the evaporator that it doesn't ice up (while
still condensing water out of the air), and then warming it up after
the evaporator to the desired comfort level.

I do know on my ole UrQ, and my Mitsubishi Mirage, I could engage the
A/C manually *whenever* the inside was fogging up, at whatever outside
air temp was prevailing, and immediately dry out the inside of my car.

Today's Modern Miracle Automatic Brains do not allow me to do this.
This makes an UNSAFE car.

The only time I've ever had an A/C unit freeze up was, coincidently,
on my UrQ, when the A/C thermostat broke, and locked the unit at max
freeze. This was lo-90's and humid summer time, and the evaporator
turned into a giant ice cube.

-RDH

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