I’m sure everyone has noticed that in most cars (especially domestics) there is both a max and a norm AC (air conditioning). What’s the difference between the two?
Max AC recirculates the air in the passenger compartment, Norm draws air in from outside.
Max cools down faster, because it’s re-cooling air that’s already been cooled before. Under some conditions, though, humidity builds up faster.
I’ve always wondered this, and I just assumed what kunilou had mentioned. What I’d like to know is, why does it blow harder on max a/c if all it’s doing is drawing air from inside the car instead of outside?
Awesome answer. I had heard about the air recirculation before but not from a reliable source.
Not all vehicles do this. My two cars do, but my truck doesn’t. (All are Fords too.) It’s obviously something intentionally designed into the unit, not a mere consequence of something else, but the reason is beyond me.
Mine’s a Pontiac. I’ve had nothing but GMs and they were all that way, blowing harder on max.
Some systems have a max fan relay that may on come on in recirculation. (our air conditioners go to 11!) or in some case due to the different air path, it just sounds like more air is being moved, because you can hear the air intake as well as the discharge.
Humidity will not build up when using max A/C since A/C is a dehumidifying process.
I think you are confusing the fact that many systems do not allow recirculation when in defrost due to a potential of humidity buildup.
Are you sure it’s not the opposite (i.e. that defrost forces recirculation)? In my previous car, a Pontiac Sunfire, whenever I had defrost on the A/C light would also come on and could not be turned off, even though the air coming out was hot. I always figured it was because the A/C was running to dehumidify the air, and then it was heated before exiting the system.
Yes I am sure. Read what you just wrote, you answered your own question. In the interest of fighting ignorance, let me explain.
Look here is the deal, A/C dehumidifies air. When using defrost you want the driest air possible to clear the mist off of the windows. So the climate system forces A/C on whenever defrost is selected.
however there is a limitation in the system. When the outside temp is cold (say below about 40F) the A/C system does not work due to a lack of pressure in the A/C system. This is because the refrigerant pressure drops with temp, when the pressure gets too low, the system shuts down.
So let’s say it is 20F outside, and you get into the car and turn on the defrost, what happens?
Air is directed at windshield
A/C is commanded on (Little blue light on dash comes on)
A/C compressor does not come on due to low temp/pressure
Now at this point, the system still wants the driest air possible to go through the system and be blown at the windshield to keep it clear.
So what is drier? The air outside at 20F, or the 75F air inside the car with your breath, and the snow that has melted off your shoes?
The outside air is much drier under these conditions, and therefore recirculation is prevented when defrost is selected.
It is all about getting the driest air possible out of the system to the windscreen.
Just because the compressor is on, this does not mean that the air from the vents is cold. The air blows through the A/C part of the system (evaporator) before it gets to the heater core. You can get hot dry air with the A/C compressor on.
I had always assumed that the stronger fan for recirc air was because there is no ram air effect from the car’s movement, so the fan is set to (over)compensate.
Edit: On the cars we have here, the recirc/outside air is often an independant control that can be set whenever you like, however you like.
That is typical of the imported cars in the US (independent controls for recirc, A/C, and fan), and I figured the American cars were using the same approach. I haven’t owned an American-made car for a while but maybe they’re still using the old convention.
Do you know who invented car air conditioning in the US? Three guys named Norm, Hi, and Max. 
Might want to add to the discussion of the question in the OP that, when cooling down a really hot car (left to bake in the sun during the day, let’s say - yeah, I grew up in a desert, wanna make something of it? ), you don’t want to run the A/C on Max until AFTER you’ve cooled the interior of the car down to the outside temp first. Otherwise, you’ll take longer to get the job done, because you are cooling hotter air. Run the regular A/C with the window open at least a crack, to allow air flow from inside to out, then, close windows and put thrusters on Max. 
Actually it was a guy named Otto Tinke who invented it in 1928. I met his widow, once, who told me a wonderful story about how they went from St. Louis to Palm Springs in August to test the system. Apparently they got some very strange looks from the locals when they drove around Palm Springs with the windows up, and the temp above 110F.
It typically doesn’t blow harder. However, you have both the intake noise and exhaust noise from the blower when recirculating, so it SOUNDS like it is blowing harder.
Here in Houston, it’s quite possible the outside air is much more humid than that inside the car. But I see your point.
THANK YOU. You have finally answered a question that’s been bugging me since I bought my Ford Focus in 2000. And I don’t even have a/c, but I do have recirculate that doesn’t work when I’m defrosting the windshield. I’ve asked dozens of car-oriented people, and nobody’s had the answer yet until today.
Read closely, I said 20F, not 95F And I also mentioned snow on your shoes. As I understand it both of these conditions are fairly rare in Houston.
As they are in most tropical climates. 
My car, a Saturn, uses exactly this approach.
Winner, we have a winner!
I was discussing why you don’t get recirculation when the climate system is in defrost.
I gave the example of 20F outside, snow on your shoes, and 75F inside the cars to why you don’t get recirculation in defrost. yBeayf came back with a complete non sequitur about the weather in Houston. 
WTF over.
I don’t know about you or yBeayf, but when it is close to 100F and humid, I don’t use or need the defrost setting on my cars air conditioning. Frost is not very common under those conditions, so there is very little need to de it.
You can feel it blowing harder in my cars. It’s not simply extra noise, at least not in all vehicles.