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This is something that occurs on every vehicle I've driven in the last 15 years, so it's obviously not a malfunction but a design choice.

When you turn the selector to blow either on the floor or the defroster there is always some air blowing from the dashboard vents. If you don't want it to blow there you have to close the vents (which in some vehicles you can't really do or it whistles).

I find it annoying to have the heater blowing in my face in the winter. It's my feet that need the heat (I drive 300+ miles a day so it's kind of frustrating).

Why do the car makers do this? There must be some reason for it.

  • A bit of leakage will remain as usually flaps or deflectors are used to route or block the air and they are not completely sealed, but its minimal. More often than not if you select feet+defroster the air blowing in your face is the air rolling off the windshield. In cars where the windshield is a tad more vertical (older models or some SUVs) I cant feel any of that air, and because of a bit more headroom, while in some sportcars where the windshield is very inclined (and I had my head against the roof) it was like I had an hairdryer pointed at me. – Erik vanDoren Mar 16 '16 at 15:08
  • @ErikvanDoren It's not the defroster. I can feel it with my hand coming out of the dash vents when I have floor selected. I chalked it up to leakage for a long time, but I've driven a series of Mercedes Sprinters (my employer replaces them at about 200,000 miles, do I've been through about 10 of them) and every single one of them does it, brand new from the factory. It's not full-force but it's a significant amount of air that I have to close the vents. It just seems to be too prevalent to be leakage. Maybe it is, but I'd expect more from Mercedes. – Tom Mar 16 '16 at 15:17
  • Ive driven cars on both sides of the pond from more years I care to remember but I never had that particular problem, maybe its just because it doesnt bother me enough I guess. Rather, in some cars, I could put the selector between positions and get air from every vent which I found quite handy sometimes... If its just with the Sprinters then it might be simply the way the system is built but you say every vehicle in the last 15 years, which also cuts out the possibility of malfunction. Try the selector between positions, you might find a "sweet spot" as for reasons I dont think theres one. – Erik vanDoren Mar 16 '16 at 15:29
  • @ErikvanDoren on some vehicles it isn't enough air to bug me as long as the vents aren't pointed at my face. In the Mercedes it's enough that it detracts from the flow on my feet, so I just close the dash vents. It doesn't become a "problem" until one of the dash vents starts leaking and it whistles, as it is on the one I'm currently driving. I'll ask my mechanic about it tonight. Maybe he'll know. Or maybe even be able to fix it. – Tom Mar 16 '16 at 15:46
  • @ErikvanDoren this is just one of my long list of pet peeves in the way cars are designed. – Tom Mar 16 '16 at 15:49
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This hasn't happened with any of the cars I have owned over the last 22 years - which is a pity, because one of the things I would have liked is designed functionality to do what Erik vanDoren mentions - setting the selector between positions to try and get this to happen.

In my more recent cars (last 10 years or so) it is impossible to get this to happen as the vents/baffles are motor controlled so when I select one all the others close fully.

So I'm guessing the ones you have had are designed to allow it to happen, because a number of the customer base will be like me and want it to happen. And it's easier (probably) to allow it than prevent it unless you use fully closing vents.

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Use the Recirculate feature, without the fan blowing, and you should get complete sealing of the cabin, such as if you go past a feed lot for cattle in the U.S.

Heat/cold will still slowly leak from the evaporator/heater-core radiators under the dash, but no noticeable blowing air.

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