I’ve noticed this in several cars: If I set the vents to aim at the inside of the windshield, it’s impossible to turn on recirculated air. If I have recirculated air on and switch to windshield venting, the recirculation automatically switches off.
This is true when vents are solely pointed at the windshield, and when they’re aimed both at the windshield and at face level.
I assume there’s some reason that multiple car manufacturers have agreed that recirculated air is bad for the inside of my windshield. What is that reason?
I don’t have the answer, but as I just noticed this on my six year old car last week, I look forward to the answer. (Grumble 6 days of falling snow. Grumble).
When you turn on the windshield vents, you want to make sure and use low moisture air, so you don’t fog them. Generally speaking, recirculated air is going to be higher moisture, so it’s more likely to fog and less efficient at removing fog from the windshield.
Especially when you’ve been running the heat for a while in winter, recirculated air can get very moist compared to fresh air. Not so much a problem when you’re running the A/C.
Recirculated air will have more moisture, whereas the air from outside will be drier. Some cars will automatically put on the air conditioner, which will suck the moisture out of the air as well. The best setting should be to have the AC on, heat on, and outside air, as the hot dry air will help the fog evaporate quickest.
Well, in real cars that are manufactured and sold in this world, it is generally physically impossible to turn on both the heat and the air conditioning (as they are typically mounted on the same switch/dial/lever on a continuum with one precluding the other). On the other hand, since the early 1980s, (at the latest), most cars that have been provided with air conditioning have been built so that the defroster setting of the heat system automatically turns on the a/c compressor.
Reciruclate is good - necessary, in fact - for the coldest air conditioning. Recooling the 70’ air that’s in the car works better than dealing with 85+’ air from outside.
Recirculate can also be helpful if you’re driving through something you don’t want coming into the cab, such a skunk smell or copious dust.
Otherwise, there’s really nothing to be gained from it. A properly working heater will put out more than sufficient heat in most climates. The whole point of vent is to get outside air. And recirculate works against you in defrost, because you are constantly pumping moisture into the cab (breath and insensible perspiration). There are some cars that allow recirculate with defrost, and it often tends to fog the windows. Your car is designed to prevent you from making that mistake.
Sure it is. You turn the AC on, but then turn the temp dial all the way into the red. AC + Heat = Dry Hot Air, great for defrosting/defogging your window. Also nice when it’s pouring out and your cold and wet. All though IME your friends will bug you over and over and over to turn off the AC, but then the windows will fog up from the excess humudity.
But if you could defrost (on most cars it automatically turns on the AC), with recirculate the air inside should become very dry in a few minutes thus helping to pull more moisture off the windows. Wouldn’t it?
I guess this is predicated on only Detroit making “real cars” because every Honda that I have owned or driven in in the past 15 years has had a separate button to control the A/C. Pretty sure my Dad’s Nissan had it as well. My '94 Ford Escort also had one but I am willing to concede that it wasn’t a real car. The button was necessary in case, God forbid, the compressor tried to kick in while you were “accelerating”.
The usual scenario in which this comes up: We have the air pointed to the windshield so that it’s not blowing right on us. We end up behind someone burning oil, within dead skunk radius, or otherwise nostrally assaulted. So we turn on recirc – but that either doesn’t work or pushes the air on us, depending on how that car’s manufacturer chose to work the incompatibility. Defogging isn’t usually an issue with how we want to use it.
So the rationale is that people would fog up their cars too much if they used recirc when it was pointed at the windshield? Is this a legally mandated safety issue, or something all car manufacturers thought was a good idea?
We’ve used the AC/hot air trick a number of times to defog the windshield, though – you can definitely run them both at the same time.
It would if you didn’t breathe or perspire. Humans emit a surprising amount of moisture.
It’s not a regulation. It’s something that some car manufacturers think is a good idea. They don’t all do it the same way. Many American cars don’t let you choose recirculate separately - it engages automatically in the “Max A/C” mode and cannot be engaged in any other mode. Basically, they’re trying to make things “idiotproof” for the average schlub who doesn’t really know - nor want to know - the fine points. This strategy keeps that person from getting frustrated by too many choices, in exchange for frustrating people like you who would like to have those options.
It happens, people put the thing on recirculate, forget it’s on and wonder 20 minutes later why all their windows are fogged up. So they put the defroster on, without switching off the recirc, and the windshield gets even foggier. If you don’t have these humidity concepts in the front of your mind, you may not even make the connection, and just get frustrated at the inconvience.
I spent one winter alternately driving a Honda Civic and a Nissan (can’t remeber the model but it was a luxury version) for my boss. In both cases, there was a control to “turn on” the a/c, but it only turned on the compressor while apparently not directing any air through the exchanger if the temperature setting was for “heat.” There was no way to tell the car to blow cold air out of the a/c while simultaneously blowing hot air out of the heater core–which had been the intended meaning of “impossible to turn on both the heat and the air conditioning.”
I’ve driven several air-conditioned Honda Civics from a 1982 model up to a late 90s model and they all had the blue A/C button and you could push that button and set the temperature to hot. You’d get hot dry air. In the Hondas, you had a separate control that directed the air either out of the vents, out of the floor or out of the dashboard with in-between settings to blend the two adjacent settings.
Come to think of it, every air-conditioned car I’ve ever driven could blow hot air-conditioned air. Even my '75 Chrysler. But in the '75 Chrysler, the defrost position doesn’t turn on the compressor. You can set it on A/C and turn the temperature up and it’ll keep the windows clear once you get moving.
My 2005 Ford Freestar has an A/C button and a recirc button and then the round knob that directs the air. It tries to be idiot proof…you can turn the knob to A/C and it turns on the A/C button automatically and when you turn it to Max A/C it turns on the recirc button automatically. But in the any position other than Max A/C, you can push the A/C button and Recirc button to set any combination of the two, including the defrost position. Then there’s a second knob that controls the temperature.
On the other hand, since the early 1980s, (at the latest), most cars that have been provided with air conditioning have been built so that the defroster setting of the heat system automatically turns on the a/c compressor.