Clean your window.
Remember to pull your sun visors down. My AC doesn’t stay charged so I have to rely on heat alone to defog windows. The visors trap the warm air.
Uhhh the defrost button? Arr people that dumb?
The fog is moisture condensing on the cold windshield surface. Use heat directed at the windshield with AC on. The AC dries out the moisture and the heater warms up the surface preventing additional condensation.
If it’s already cold, you don’t need to turn the AC on. The air will be dry once it gets warmer.
This is wrong based on the assumption that it works the same in all areas of the world, but I’m not sure if this is -10 votes wrong.
Especially considering some of the other posts I’ve seen on here and how they get voted on
This can be correct in northern winters, with very high humidity this will only fog the windshield much worse.
You cant have high humidity when its cold, doesnt depend on the location.
100% humidity at 0°C is ~20% humidity at 25°, which is pretty dry.
Defrost setting uses the AC unit. Helps pull moisture out of the air
Air conditioners pull moisture out of the air by making it colder, reducing the amount of moisture the air can hold. Its not doing anything if the air is already colder than the air conditioner can go. The air conditioner will only help if it’s 55F/13C or more, where it will only fog up if you live somewhere very humid.
I know Google and the AI say otherwise. I am an HVAC engineer, and Google and the AI are wrong.
The moist air freezes to the coils of the ac unit, which is than heated by heated air and drips out of the vehicle through a condensate drain.
How do you not understand that basics of an air conditioner? It doesn’t just cool, it conditions the air.
That doesn’t even make any sense. Is it cold air depositing frost onto the coil, or hot air melting the frost? It can’t be both. You know that frost building up on coils is a huge problem that breaks air conditioners, right? Frost clogs the air stream, and creates a feedback loop where the coil gets way colder than it’s designed to because air can’t pass through.
I could have been more specific: the air conditioner can be useful in rainy or foggy warm conditions where the air conditioner can still cool the air further, and a car air conditioner goes colder than a home air conditioner does, around 38-45F (3-7C). It can also help if something in the car is adding moisture to the air, like if the occupants are sweating. However, when it’s just a cold day fogging the windows, the air conditioner does not help. It does not have a mechanism to dehumidify the air other than to cool it. Turn the temperature up to warm the interior surface of the windows.
Uhhh… why do you think they have defrost cycles?
If you turn off the ac for a short period of time, you run heat over the coils to remove all the accumulated frost, snd switch over again. This just used air instead of heating the coils with a reverse loop. It constantly cycles this loop to accumulate frost and remove it. It can be both, the coils are colder than the air, it accurate condensate, just like your glass. Why does your glass collect moisture if you claim this scenario is impossible….?
You say others are just using Google and ai, but clearly you’re the one using those tools and have no experience. Or did you just start school and it’s your second day?
Coil freeze over like you’re talking about is from an improperly sized unit, the defrost cycle can’t work properly.
But all acs collect moisture and have condensate lines, even home ones
Car defrost cycles are explicitly for the windows, not the coils. You still haven’t explained the magic mechanism by which an air conditioner dehumidifies air that’s colder than the coils, just pointed to the existence of condensate like that makes your point somehow. I already covered that the air conditioner can dehumidify air cycling through the car if humidity is building up from something inside it.
The coils are not designed to freeze. Ever. Having to manually defrost the coils by turning off the A/C (you have to do it manually because no system is designed to work this way, because that would be a stupid design) and run warm air through it means your system is malfunctioning.
Take the last word, if you want. Arguing on the internet is pointless.
Wouldn’t it be more efficient to pass the cold dry air back over the hot side heat exchanger to increase its moisture caring capacity before blowing it at the glass?
You know like a dehumidifier?
Probably, but the hot side of the car’s AC is outside of the car typically in front of the radiator.
I want to be able to blow hot air at my ice cold windshield in the -20°C Canadian winters, but American car designers are all like “oh you want to use the windshield vent? you must want the AC turned on!”
This isn’t just an American car thing, non American brands do this as well. That’s because it’s a good idea. The heater core has way more capacity than the AC, so the air coming out of the vent will still be warm. But running the AC is still a good thing, because it gets the moisture out of the air.
Without running the AC the humidity in the car shoots up and takes a while to clear, with the AC on the humidity stays controlled. This helps clear the windscreen sooner and helps with comfort in the car. It also prevents mold, which if you live in a cold but humid place, can be very annoying. It is very normal where I live to have the AC on in fall and winter, because otherwise the humidity inside the car is very uncomfortable and can easily cause mold all over the interior.
If it’s a cold winter day it’s dry outside. Just open your cabin circulator to bring in outside air, and you don’t need to use the extra gas to run the AC.
If you’re in an equatorial area, foggy windows will be because of unbelievably high humidity in hot to boiling weather, which WILL require AC because you can’t just suck in more wet air and make it hotter and expect that to work.
Is -20°C regular winter temps for Canadians? It’s colder than that here in Minnesota
I’ve tried that. It only made things worse.
American cars: “This obviously requires the AC.”
That’s when I just put the airflow on “feet”. Most cars will actually blow like 30% up on the windshield when set to the feet direction. This lets me avoid running the a/c if I don’t want to.
If your windows gets foggy you need to turn up the heat as that lowers the relative humidity and direct the air towards the windshield, also check below the floor mats for water. If your car is a swimming pool your windows will be foggy forever.
Counter intuitively, you want to turn the a/c on as that pulls the moisture out of the car.
During winter when a vehicle is defrosting, it runs a combination of ac and heat to defrost windows and condition the air to limit moisture condensing on the interior.
If your car has that, then yes.
Do they not make cars with Defrost buttons anymore?
If you have a fancy new car without one just yell at your on board AI system until you start crying and then the tears are more blurring than the fog and you just don’t care anymore.
They make cars with buttons? I thought ever car made after 2020 had an tablet taped to the dashboard …
That’s REAR defrost. Front defrost is on the left, and yes, all cars still have that.
Umm, no shit Sherlock? It’s literally in the picture I lifted from wikihow - just happened the rear one is circled.
Here, let me provide a version suitable for your comprehension level:
No need to be a dick, YOU posted a picture with rear defrost circled. How are we to know you meant the other one?
No need to be an idiotic pedant. I said defrost, not front or rear, and posted an easy picture because I figured that anyone who had the capacity to read would understand what I was talking about. Clearly I was mistaken.
How were they supposed to know you didn’t circle it? Don’t be a knob, easy mistake
Defrost turns on the AC and blows cold air. That temporarily removes the fog and keeps the cabin cold. The alternative is to use max heat and let the hot air raise the cabin temperature which will help clear the fog.
And it stays gone too. Using cold causes the fogging to return if hot air or the heater is used in the cold car. The defrost button is a myopic solution which ignores user comfort.
People in comments assume the heat or air conditioning even work.
Air conditioning also rapidly reduces humidity, so that tends to be the ideal option.
But if neither heat nor air conditioner work, well tough it out and roll the windows down…
If neither the heat or A/C work, you shouldn’t be driving that POS on the road.
Just because the ac/heat don’t work it’s automatically a POS that’s not road worthy?
Depends on the part of the world and season. It wouldn’t be roadworthy to drive in freezing temps without heat on the windshield
Yes.
Our heater does work, just no blower motor, so if we need heat, just open the vents and drive for airflow.
Our air conditioner unfortunately does not work, but we have manual windows that roll down by hand for airflow.
And today I just happened to install a fan on the dashboard.
Yes it’s a piece of shit truck, but ignoring the ‘luxury’ features that cost more than the truck itself to fix, it’s pretty darn reliable.
See my previous posts regarding Kermit the 96 Sonoma for reference. More reliable than new model Ford trucks that brick themselves because rainwater got into the taillight assembly…
Your blower motor probably does work, but the resistor assembly hooked up to the knob that controls its speed is most likely blown. This was the case with my truck when I got it and is rather common. Thirty four bucks will solve that problem for you, and it just plugs right in.
That would not be street legal here. Must have a functional defroster if the vehicle has a windshield.
“Here” is an important word! I assume you are Canada based. I live where we rarely or ever get freezing temperatures, and only decently started having warm/hot summers. Why would defrosting or ac be mandatory in such climate?
It never rains there?
We have wiper blades?
Humid and warm: use heat
Cold and dry: turn on a/c
I think you got the right idea, just backwards…
Assuming your heater, air conditioner, and blower motor all work correctly, you should use your heater defroster in cold weather, and your air conditioner defogger in warm weather.
Note the difference in environmental control terms…
- DEFROSTER > Used for cold climate, heater, to melt the frost off your windshield.
- DEFOGGER > Used for warm humid climate, air conditioner, to dehumidify the air.
Failing either heater or air conditioner, welp, just do what we do and roll the windows down.
Both of those settings turn on the A/C to remove moisture.
Yeah… no… that’s only the newer generation of vehicles, the enshittified ones that assume everything for you.
Back in the 90’s and 2000’s, the engineers had common sense and if the temperature dial was set anywhere above 50%, the air conditioner would never turn on, as it’s a waste of energy at that point.
Uhhh my 96 civic had an air conditioner button, and it also turned on when you turned the defroster on.
You’re making shit up, it’s a simple interconnection, and the physics of air conditioning removing moisture has been known for a long time.
During winter snow constantly gets inside of a vehicle, so without an interconnected ac with defrost, the interior would always fog up. It just may not light up the button, but defrost turns the ac on.
My 78 Chrysler was designed to use the AC in defrost mode even in a winter blizzard and was much more effective and drying the windshield because of it. Most heating/cooling system in cars were combined much earlier than that to take advantage of dehumidifying the air. It also has the benefit to run the compressor to keep lubrication and stop seals from drying out in the AC system that would otherwise not be used to months at a time.
There are some vehicles with separate heating and AC systems, (mainly cheap or aftermarket units) but the vast majority run together. (At least in North American cars.)
My 2015 vehicle does it that way (a/c on for both), so it’s not exactly a recent change in design philosophy…
I said 90’s through 2000’s, I said nothing about 2010’s or beyond.
I do not have much of any experience in working on anything past 2005.
Anything past 2014 is what I call new generation enshittified, as that’s around the year they basically started tracking everyone, refusing to offer printed service manuals (Hayne’s/Chilton), stop offering parts and requiring ‘owners’ to take the vehicle back to the dealership for repair.
I’m all about the right to repair. Your 2015 is a new generation enshittified thing, sorry.
So why are you discounting what people who do use modern cars are saying as incorrect if you don’t know much about it? I’d wager most people probably aren’t driving 25-35 year old vehicles anyway so at best you’re just stating outdated and irrelevant knowledge.
My 04 turns on the AC with the defrost, so did the 01 I used to drive, both base-level economy boxes so actually wait what the hell have you been driving?
In my old '90s car, heat only just made the fog worse. You had to have AC on too. Though it did trust you to know that and turn it on yourself.
match the temperature outside.
if it’s cold out you need to make it colder inside
if it’s hot out you need to make it hotter inside
the condensation is caused by difference in temperature.
yes, it may be inconvenient or uncomfortable
If you have a defogger/defrost setting the temp doesn’t matter since it will just heat or cool the windshield on both sides, eliminating the differential that causes condensation.
