The recirculating setting does draw in some fresh air, otherwise there would be an unacceptable risk of carbon monoxide poisoning from an exhaust leak. And “negative air pressure” inside the cabin would make the possibility even greater.
Cooling air that is already cool is not as bad as cooling air that is already hot for any realistic scenario involving car ACs.
Remember: you soon reach the point where there’s no point in cooling the air anymore so the AC can shut off! You don’t do this when drawing outside air.
You should not be wasting energy cooling air to such a cold temperature that you have to worry in the least about thermodynamic efficiency.
The AC will always dehumidify the air. On recirculate, it’s going to extract even more moisture from already dry air. The only way it can add moisture is if there’s a problem with your AC system. You could have a clogged condensation drain. A dirty evaporator coil or some other restriction in the duct work as well as low refrigerant can cause the coils to ice over. You could have a leaking heater core and the condensation you’re seeing is actually engine coolant (but if that’s the case you’d smell it and have to clean it off your windows, and it wouldn’t turn into frost).
My WAG, is that wherever you’re seeing frost form on the windows, you have a vent blowing directly onto the window. The recirculating air has become cold enough that if it blows directly on a window, moisture will condense and freeze in that spot. That’s not that uncommon. If you’re seeing this frost in spots (as opposed to evenly across all the windows) that’s probably what’s happening.
If you turn the dial a little warmer, I bet it fixes the problem.
TL;DR it’s not that recirculating adds humidity, it’s that the air in the car is getting so cold, moisture is condensing on cabin surfaces.
Also, it should be kept in mind that your house, your refrigerator and most applications where we’re cooling something is done in ‘re-circulation mode’. Not that you really have an option, but for example, when you run your home AC, it’s not bringing outside air in, cooling it and then pushing already conditioned air to the outdoors. It just keeps recirculating the same air over and over.
The science still tracks, your interpretation of it is broken. The car’s AC on recycle is like a regular AC, it doesn’t change the pressure except in that cooling the air reduces the pressure. Large buildings with high occupancy and an air tight envelope will have an HVAC system that brings air from the outside. This is like the car’s regular AC setting.
There’s no setting equivalent to a portable AC for your car AC. The only way to create the same situation would be if you put a portable AC in the cabin, or if the car was horribly designed.
But in winter you’re running the heater rather than the AC, yes?
No, they’re not separate systems, with air conditioning being “cold” and heater being “hot”. It’s one integrated system, with air conditioning part of climate control, designed to remove humidity. In my Honda, when I set it to “de-frost”, to blow air on the windshield and side windows, the air conditioning automatically comes on and can’t be turned off, regardless how high the temperature control is set.
That wasn’t the question though. Yeah they’re not separate systems, but you can be running all-heat, all-a/c, or a blend of the two. “In Winter” means different things to someone in Los Angeles versus someone in Minneapolis. If your windows fog up when recirculating air “in winter” I assume you mean in temperatures near or below freezing outside. That’s the point at which most automotive air conditioners will not run no matter what button you press. So if the a/c is not running, and you’re recirculating the air, the system can’t remove the humidity from your breath that’s accumulating in the cabin. If the windows are cold, it doesn’t take much humidity to cause them to fog up compared to when it’s warmer outside.
I doubt very much that this is the case, since it’s happened with every vehicle I’ve had that has the “recirc” option, two of them bought new from the dealer. Unless every A/C I’ve had has been defective right from the start?
No, that’s not the behaviour I’m describing, and no, cranking up the heat doesn’t solve the problem: the heat already is cranked up. What solves the problem is turning off the “recirculation” function.
Here’s what I’m talking about.
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I come out to the car in the morning. I scrape all the condensed frost off the exterior of the windows, and then get in and turn on the car.
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I turn on the climate control to de-frost, cranked up to “High”, and scrape all the condensed frost off the interior of the windows.
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The blowers keep the windshield and side windows clear of frost. If I’ve not scraped all the frost all the way to the top of the inside of the windscreen, and completely off the side windows, the frost at the top and at the far edges of the side windows radually disappears, as the heater gradually warms up and the defrost blows ever-warmer air.
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I turn on the “recirc” button.
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Condensation starts forming on the windows and windscreen, farthest away from the areas where the de-frost is blowing. The area covered with condensation starts to expand, creeping down the windscreen and on the side windows towards the blower.
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I feel humidity building up in the car.
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I turn off the “re-circ” button.
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The condensation starts to disappear and the air feels less humid.
So, I never turn the “re-circ” on. I’ve never understood its function, frankly, since it just contributes to condensation in the winter. Nor have I never noticed any difference in the summer whether it’s turned on or off, although the air sometimes feels a bit stuffier with it on.
No, the frost is already there because the air is cold when I start up. The frost disappears as the heater warms up. Without the “re-circ” on, the windows stay clear after being scraped and once the heater is warm enough to de-frost. If I turn the “re-circ” on, that’s when condensation starts forming, away from the de-frost blowers.
And, I’m not saying that the re-circ is adding humidity. I’m speculating that with “re-circ” on, the humidity from our breath starts to trigger condensation, going faster than the A/C can clear it, and with no outside dry air coming in.
^ Recirc is only for summer A/C, not for winter heating. That’s all you need to know.
For me, “winter” means at least -10 C, going down to -30 C, occasionally lower.
It’s not a matter of pushing buttons to turn on A/C. Turning “de-frost” on automatically turns on the A/C, or at least the light on the A/C button comes on. Are you meaning that even if the indicator light is on, the A/C function stops working? (I’m not a car guy. All I know is that with de-frost on, the A/C indicator is on and can’t be turned off.)
That’s my speculation as well.
Other way around, as described in previous post. When the windows are warmer than the cold air outside,the de-frost keeps them clear; turning on re-circ starts the condensation process, on the windows that are warmer than outside air.
What’s the purpose of re-circ?
Wasn’t that already answered in the thread’s first reply? Re-circ is so you don’t keep drawing in new hot, humid air from outside the car.
If that’s its purpose, i don’t need it. 
Yes, that’s what I’m saying. As the temperature gets close to freezing outside, the compressor will be constantly cycling on and off to prevent the evaporator coil from freezing up. When it’s that cold the cooling isn’t needed, and you can’t condense humidity out of the air effectively when it freezes either. Plus at low temperatures it’s hard to make sure the refrigerant actually evaporates before coming back to the compressor, and liquid refrigerant can damage or destroy a compressor.
I see what you’re saying, but I think what’s happening is that the warm but now more humid air blowing at the windows is now closer to the dew point, but since the car is already warmed up, you probably dialed back the heat and/or the fan speed a bit to keep from overheating the cabin. That means the window glass isn’t as warm as it was (especially if you’re now driving and cold air is blowing by the outside). I can drive with the A/C off and/or recirculation on in cool temperatures for a while, but once the cabin has warmed up to the point where I just need to maintain the temperature, that’s when moisture starts to become a problem.
As for the purpose, yes, it’s been explained, and if you live in a northern climate where you almost never need the A/C at all, then recirc isn’t really needed. When the temperature outside gets about 80-85ºF then you definitely want it.
Regarding why recirculation is louder, I think it’s increased turbulence, not necessarily higher fan speed. It may also simply be a case of the damper that opens to bring the air from the cabin back into the air system creates one more pathway for noise to travel.
Thanks for that explanation. Ignorance fought!
I rarely turn the heat down when it’s -30. 
It’s not the heat, it’s the humidity (lack of). I definitely have the A/C on when it’s +25 C or higher (summer here goes at least to +30 C, occasionally up to +40 C), but it’s a dry heat. Maybe that’s why I’ve never noticed any difference with re-circ on or off in the summer, other than it feels stuffier to me with it on.
Thanks for explaining all this stuff. I know little about cars other than how to turn them on and off and drive reasonably safely. 
Here in AZ, USA, it’s not the humidity, it’s the heat. The regular A/C setting is cooling 40-45 C outside air, and blowing it inside. The recirc setting is pulling in cabin air, which is usually cooler than 40 C, cooling it further, and blowing it back into the cabin. It’s noticeably cooler than when the non-recirc setting is used.
Thanks for explaining it. I will try it on one of our hot days.
There are ONLY TWO situations when it’s useful to have a car’s system on recirculate.
- To get the coldest air conditioning. This is by taking in air from inside the car that has already been cooled some and cooling it further, rather than taking in the hotter outside air.
Coldest A/C is what the recirculate feature was designed for.
- To avoid taking in really obnoxious stinky air when driving through certain industrial areas or skunk attack zones. The air intake should be reverted to outside (also called fresh) air as soon the malodorous stretch has been passed.
Recirc is NOT to be used with vent, heat, or defrost functions.
In recirc with the A/C off, the moisture from people’s breath builds up inside the cab so of course it fogs the windows. Don’t do that.
One further note: As mentioned above, as fresh air is forced in, some air will exit hidden vents. There’s never a build-up of pressure, positive or negative. With recirc there’s much less air exchange – after all, the whole idea is to recool the air inside – but generally there’s enough to prevent a problematic build-up of stale air (but not a build-up of moisture).
I once had a mid to late 80s Acura Integra with separate push-button controls for re-circ and AC.
One bitter-cold night I put on recirc but no AC and my windows fogged up; punching in AC and fresh cleared up the windows in minutes.