Why A/C direction knobs don't have a face/windshield setting

Question

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?

Answer 1

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.

Answer 2

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.