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In all cars I've ever ridden since I was a child, the air conditioning controls include a knob or slider to set where the air will blow, and it always has the following five settings:

  • Windshield
  • Feet + Windshield
  • Feet
  • Face + Feet
  • Face

Yet they never have a setting for Face + Windshield!

This is consistent across brands, and even across brand countries.

I've tried looking for an answer to this question for many years, yet I don't even know how to ask it correctly.

Why is this the case? Is there a mechanical, a legal, or any other reason why this is the case?

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    I agree I've never seen this either, but to what end would it serve? Why should an A/C system be directional towards both the face and the windshield? Usually, the only reason why it would be pointed at the windshield is for defrost or defogging capability. These usually happen when it's cold out. I don't know about you, but I'd not want it blowing on my face when it's cold out. Nov 27, 2018 at 21:20
  • @Pᴀᴜʟsᴛᴇʀ2: Under that logic, what would be the reason for feet + windshield then? I think it is more reasonable to have face+windshield rather than feet+windshield, but why simply eliminate one choice? I can imagine the mechanism to set the flow, and there is no reason I can think of why not all combinations should be allowed... Nov 27, 2018 at 22:59
  • I think if you answer my question of why you'd want it blowing on your face, you'd have answer to all your questions. Nov 27, 2018 at 23:02
  • I'd use such a setting when it's cold outside, I want to defog the windshield and want to feel warmer right away. The face is better than the feet to get warmer because in the feet I have pants and shoes that make warming much slower. I doubt that the reason is "because nobody would want to"... Aren't cars about freedom? Nov 27, 2018 at 23:13
  • Sure, but you're talking about air conditioned (cooled) air coming out ... don't want it on my face. It's just a suggestion as to why and not an answer. If you want cold dry air streaming out on your face, so be it ... not for me. Nov 28, 2018 at 0:13

2 Answers 2

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I can answer this for how it works on my 1970 Chevy C20 Pickup. It has changed over the years, but this might give some insight.

My truck has factory AC. Without factory AC, you did not get dash (face) vents, only foot and windshield.

There are 3 doors on the HVAC system. The first switches between the evaporator core and the heater core for hot or cold. The second door switches between the AC vents (face) and the other vents. The third door switches between the feet and windshield vents.

There is no AC switch, the AC turns on when the vents are switched to AC. This is a drawback as you can not direct the AC toward the windshield.

Most newer systems have the evaporator core, then the heater core, for defrost / defog. I wouldn't be surprised if the doors were set up similarly, but I don't see why they couldn't hold a door half open as I imagine they do for foot / windshield.

Makes me wish I would had looked into how the vents worked in my Integra when I did the heater core earlier this year.

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Two reasons I can see:

  1. Hot air rises - therefore, the best way to quickly warm up the whole car is to bring the hot air in as low as possible, i.e. at your feet. So if it's cold, you're better off using the foot setting than the face setting, which might quickly warm your face but not the rest of you.

  2. The hot air blown at the windscreen will also deflect off the glass towards your face, thus warming your face, and making extra air blown in that direction less necessary.

As for why the setting isn't available, that's actually fairly simple if you look at how the vents tend to work - or at least, how they used to work (I suspect more modern vehicles have electronic controls rather than a mechanical linkage).

On a mechanical system, you don't really have five settings at all, but three - windscreen, feet, face. The output is then controlled by a sliding input vent that lines up with these. To get the intermediate settings, you simply slide the vent halfway between two of the outlets. Because of this, you can't have a position that's halfway between the third and first options - so only two of the half and half options are available, and someone, historically, chose the two that were most useful. The reason it's still done now is, indeed, because that's how it's always been done.

-----------------------------                         |---------------
Air in ->       -> Windscreen           --------------    -> Windscreen
-----------------------------           Air in ->      ---------------
             |   -> Feet                --------------    -> Feet
             |----------------                        |---------------
             |   -> Face                              |   -> Face
             |----------------                        |---------------

If you'd like to see it in more detail, have a look at episodes 16 and 17 of "Project Binky" on Youtube, where they show the build of a custom HVAC system for the car in some detail.

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  • Thank you for your answer, but you're answering "why would people not want face+windshield", not "why is there not such a setting even available". I understand that many folks wouldn't be interested in having such a setting, but unless I'm misunderstanding something, I imagine the cost of adding such a setting is marginal, and if nothing else, it simply gives more choice to users. I suspect the real reason is "because every other car maker does it", but if there is an actual mechanical reason why doing this is not a good idea, I'm interested in learning about it. Nov 29, 2018 at 5:06
  • @PandaPajama I've added more information about how and why it's done like that
    – Nick C
    Nov 29, 2018 at 10:13
  • Thank you for the additional information. Your explanation was what I first thought of how the mechanism is internally designed. However, this problem is nonexistent if the three outputs are arranged in a circle, instead of linearly, which is by the way, how the knob is arranged. I can imagine it's simpler to arrange the outputs circularly, rather than linearly, and then have a mechanism to change the circular motion of the knob to a linear one, which achieves nothing else than preventing the windscreen+face setting from existing. Nov 30, 2018 at 2:30
  • I'll take a look at the video, but if you have a photo of the mechanism in a real car, or if you know the name of the mechanism so I can look it up online, it would be really useful. Also, if you can point at the timestamp in the video where they talk about this, I would really appreciate it. Nov 30, 2018 at 2:31

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