Why is there no vent option for combined defrost/face?

Question

I've wondered about this one question for the better part of twenty years: why is there no option for you to set the air vent to split between face and defrost?

That is, the airflow options on (nearly?) every car I've ever driven are exactly the same: Defrost, Face, Feet, Face/Feet, Defrost/Feet. I appreciate that some newer models have half-points between some of these, but there has never been a face/defrost, which would seem to me to be the most appealing option, especially for those in a cold climate. (Defrost/Feet is the second best choice on a winter morning, clearly.)

Is it just on account of an arbitrary engineering decision that was made decades ago (perhaps to simplify some piping or electrics) and has just been perpetuated through inertia? Or is there an actual practical consideration why this does not exist? (I can understand how the vent system is not the most exciting feature to reengineer, but I would quite possibly buy a car with a face/defrost vent option.)

Or does this option exist even if I haven't ever run across it?

Thanks--

Answer 1

Yes, the option exists in some models. E.g. in my 2002 Opel Astra, I can enable/disable any of the air channels on climate control independently from others:

Usually, there's one ventilation duct going upwards and one going downwards, like here on Suzuki Vitara ('00-'06 model):

_{(source: autozone.com)}

The flap that controls whether the air from upper duct leads to defrost or passenger vents has no "center" position, it's pick one or the other.

But living in climate where most mornings start with demisting/defrosting, I rarely want to use the face vents at all. The warm air dries the eyes quickly and therefore causes drowsiness; heated seats are much preferable. 95% of the time my vents are directed to windows only; feet option gets added when feet feel really cold.

Answer 2

A number of Volvo models, if not all, permit air on face while defrosting. I run very hot, so it is a welcome feature to be able to have cool air on my face while clearing the windshield.

Answer 3

De-Frosting is just that, and its purpose is to de-frost the front screen, usually, on a cold morning. It should be regarded on a safety first basis, seeing where your going. I assume the assumption is made that the driver is dressed for cold weather before getting into the vehicle. In this case the de-frost would be maximised on defrosting the screen.

Answer 4

I recently discovered a scientific breakthrough regarding this topic. I live in Tennessee, and it's about to get miserably cold, already hitting 28. In February sometimes it gets below zero, when we get most of our snow. I found a good way to save gas when it gets time to warm your car up for defrosting etc. every morning. Start your car, let it run for about 5 minutes with the heat on defrost and feet. Turn it off and wait 5-10 minutes. I did this the other day and my windows were completely clear, as if the heat transferred that quickly. As for your question I've always just thought that since heat rises, you'd get less heat dissipation if the heat starts at your feet. It may help to warm up the entire interior faster.

Answer 5

I don't know, but I always just assumed it has something to do with air volume going through the vents.

The basic three are Defrost, Feet, and Face. Feet is usually a tube with only two vents. So, adding Feet to either Face or Defrost doesn't affect the volume and speed of air coming from the vents.

However, Face usually has 4 or 6 vents. And Defrost is across the full windshield, plus usually two additional vents, which defrost the front side windows by the mirrors.

Because Face and Defrost, comparatively have more vents than Feet, if Face and Defrost were combined, it could cause a noticeable change in the volume and speed of the air coming from each individual vent.

If the air volume and speed is affected too much, perhaps the effect would be that the Defrost wouldn't be able to keep the windshield clear. Again, I don't know that, but it seems logical to me.

I suppose you could increase the output of the blower to compensate for the change in required air volume and speed. In that case, you might also have to increase the size of the heater core.