Air Exchange
July 7, 2004 4:12 AM   Subscribe

Another air conditioner question. In both my car and my window units at home, there is a feature that I will generically call "air exchange." I can turn it on or off. It seems to somehow bring in air from the outside for some reason. What is this for, and when is it useful?
posted by stupidsexyFlanders to Home & Garden (13 answers total)
There are two modes on every a/c I've ever seen - although it used to be called "regular" and "max a/c" on GM cars. When you're re-using the air that's already been chilled, it tends to get a bit stale. When fresh air from outside is used, it's fresher, but starts out at a higher temp, so it's not as cool. It's useful if someone in the car smokes (since air is being forced in, air is also forced *out* the window) or if you fart. One last reason: when the air is being recirculated, it's going through the drying/cooling process multiple times. That ultra-dry air is irritating to some people.
posted by notsnot at 5:08 AM on July 7, 2004

On a long trip to Tennessee, on a really hot summer day, we had our car air conditioner fail on us because we had it set on cooling the air from outside, instead of recirculating the air. The cooling mechanism was covered with ice when we finally pulled over to take a look. Since then, when the air conditioner is on, it's always in recirculate mode.
posted by crunchland at 5:32 AM on July 7, 2004

Alternately, I've had the interior of the car fog up so badly as to be dangerous when my car ventilation was on recirc. I can't recall if it happens when recirculating both heated and cooled air, but if you suddenly can't see out of your window, check the recirc button.
posted by blueshammer at 5:46 AM on July 7, 2004

I don't think the inside of your windows would fog up when you've got the AC on -- that usually dries out the air pretty well. In fact, I use my car AC as a defogger, with the temp slid over to the hot side.
posted by luser at 6:25 AM on July 7, 2004

Yup I've always thought of it being as the difference between breathing 'fresh' and 'used' (eeeww) air. I've always had problems with irritated sinuses after being too long in car with AC, I just wind the windows down, much to the annoyance of any fellow travellers.
posted by carter at 6:30 AM on July 7, 2004

The window actually fogs up on the outside, because the air blowing on the windows is so cold and it is so humid outside. It can usually only get that cold if it's on recirc.
posted by smackfu at 6:40 AM on July 7, 2004

I know someone who years ago couldn't figure out why the windows of her otherwise wonderful Prelude fogged up. Sure enough, the HVAC was set to recirc.
posted by pmurray63 at 7:29 AM on July 7, 2004

As for what it is for, it is for decreasing malodorous stankifications.

If you or the dog have let a stanky-ass fart, you want fresh air. Now.

If there is Sqonque de Pew outside, you want to keep fresh air out at all costs.

It's also there for when the temperature is mild and you're not actually running the a/c, just using it as a fan.
posted by ROU_Xenophobe at 7:48 AM on July 7, 2004

We always follow a pretty basic rule about this on our AC units--they're usually on "fresh air", unless we need to either cool the house down quickly or keep it much cooler than the outside.

If you've just come home or gotten into a car on a hot day, and the AC's been off, the inside is going to cool down much more quickly if you keep re-routing the same body of air through the unit. Similarly, if it's really hot outside, you don't want to be injecting that warm, humid air into the overall mix.

Outside those two conditions, though, keeping the unit on "fresh" does keep the inside space a lot less stale (unless there's a dead skunk or a hog farm out there).
posted by LairBob at 7:56 AM on July 7, 2004

As others have mentioned, the recirculate setting (on import cars) and max a/c (typically domestics) helps cool the car faster when it's been baking in the sun or something. And it might help keep the dead skunk odor from getting in, if you see it coming in time. But other than that you should leave it in normal mode and let some fresh air circulate through the car.

PetPeeveFilter: seeing people driving nearly blind in the winter, with a layer of frost on the inside of their windows, scraping pathetically with a credit card to create peepholes... and knowing they have their system set on "recirculate" because they've never read an owner's manual.
posted by Tubes at 8:09 AM on July 7, 2004

Wait -- your house A/C has an indoor/outdoor option? Mine only takes air from inside, through the intake vents, one upstairs, one downstairs. At least here (TX), taking air from outside would be homicide on your A/C.

My wife prefers the A/C in her car to come from outside, I prefer mine to come from inside, unless it's not hot out. She is always puzzled as to why my car is always cooler. I've explained it to her before, she simply does not believe me. She also believes that A/C works better if you're moving, for the same reason (moving the car will force air from outside through your car faster -- if you're not getting the A/C air from outside, this isn't true). She's wacky but I love her.
posted by RustyBrooks at 8:15 AM on July 7, 2004

No, rusty, not a central air unit, I was referring to window units.
posted by stupidsexyFlanders at 8:30 AM on July 7, 2004

And as a semi-related side trip, if you *don't* have central air, you wouldn't *believe* how useful a whole-house exhaust fan can be.

I've seen one pull a house down from 94 to 74 degrees in about an hour, run just after dusk.

Running it for that hour before you leave for work, too, will cool the attic air, making the house heat up less during the day. And they take a damn sight less power than A/C does.

So if it really *is* the heat, not the humidity... ;-)

Though, if you live next to aforementioned hogfarm, this will likely not be too useful.
posted by baylink at 9:44 AM on July 8, 2004

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