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.
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.
I’ve never found a more confidently incorrect bunch than HVAC people. I’ve never understood what it is about that profession specifically that causes people to just turn their brains off. See also: the absolute disdain for heat pumps and the absolute insistence on massively over sizing furnaces everywhere by HVAC technicians.
I don’t have a side in this but both my ‘93 Ford Escort and my ‘01 Ford Crown Victoria would turn on the A/C compressor if you hit the Defrost button for the windshield.
Edit: Just searched it and I think you two are in a semantic misunderstanding.
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.
Air conditioners make it cold inside the unit at a condensation surface, collecting the water from the humid air there.
But that’s not how you defog a window, you heat the window until the air in contact with it can hold more water and so the water evaporates faster due to both water and air temperature. THEN this air can be pulled back into the AC to deposit it there with condensation.
Window / mirror defog doesn’t work like regular dehumidification that way. Especially not in cars / bathrooms where you might be having a too high stream of humid air coming in, then you focus on one surface (and since the windshield don’t tend to have heating wires, it uses the AC hot air)
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.
I would like to have the choice, because of fuel concerns.
i’ve never fiddled with an a/c system that denies you the choice, but i haven’t fiddled with them all yet.
Is -20°C regular winter temps for Canadians? It’s colder than that here in Minnesota
Where most Canadians live it doesn’t get particularly cold. Toronto is south of Minnesota. There are places that get really cold, but not many people live in most of them.
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.
I guess cold is a relative term. I’m from Wisconsin- we can have 100% humidity at 65 degrees sooooo
65°F so 18°C, and thats what you call cold? :D Then yeah, heater without AC will not clear your windshield up.
if that’s your high, it’s mildly cold. depends on the wind… if it’s your low, then dear god it’s hot
I’ve tried that. It only made things worse.
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.
When I press the defrost button in my car, it turns both the heat and AC on at the same time. I assume this is to remove water from the air with the AC, then warm up the air increasing its capacity to hold water with the heat.
Your assumption is correct Also your car automatically calculates the condensation point and keeps the temperature and air moisture low enough not to cause any fogging on your windshield. Source: I’ve been programming AC unit controls for cars for the last 10 years
How upset do you get when your buddy disables auto and proceeds to slam back and forth between full heat and full ac while claiming their shitty dial dj set is somehow waaaaay better than the computer?
That’s an instant un-friending offence
thank you for your service
They only do this with the automatic controls not basic knobs right? Apparently affected what filter I had to get.
Yes of course. The old school manual controls are usually mechanically connected directly to the flaps, so you cannot have any automated handling there
Thanks, just didn’t know how far we strayed, my first car had separate keys, apparently the new one can detect air quality and automatically close, new is also relative.
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in a lot of newer cars they can actually use the A/C to heat and cool the car, rather than using excess engine heat or an electric heater which is especially helpful when it’s cold and you’ve only just started the engine!
Fun fact (well, I suspect you know this already, but some didn’t, I had discussion with someone arguing with me that this is not true):
AC and a heat pump are basically the same thing, AC is just pumping the heat outside instead of inside.
Also when you say modern cars I am assuming you might be talking about EVs. Using a heat pump requires less energy than using resistive heating so it could be energy saving measure.
ICE cars on the other hand can get the heat from engine “for free” (not really, but since the heat will be produced anyway whether you use it or not, so why not use it?) so a heat pump might make a bit less sense in them.
You still need a heat pump for cooling in ICE cars
True, I mean just for heating using engine is cheaper.
I assume you mean EVs with heat pumps? I don’t know of any ICE vehicles that do this, seems inefficient to waste mechanical or electrical energy when you have perfectly good waste heat to use upon reaching operating temperature (though you may need to use an aux heater before reaching that, particularly on diesels)
Lots of new EVs have heat pumps, in cold climates they need it because the car doesn’t generate enough heat just from running.
It’s also an emergency safety heat bleed if your engine is overheating, redirecting some of the heat from that engine block can save your radiator some trouble and keep it going a few more miles
My car is an EV, so it definitely does this because there’s no excessive engine heat to use.
My car is a diesel so it takes a long time to warm up in the winter. They added an electric auxiliary heater to compensate. The air is really hot within seconds.
Yall have a defrost button?
It’s not about temperature, it’s about condensation and humidity. Turn on the AC regardless of temp (e.g. even if it’s cooler out and you’re blasting the heat) and the defog too.
Source: I fucking live in Seattle and it fucking rains all the time
but if I leave the AC on cold for too long I get mini fogs where the air blows
Always use heat. Anytime you have a fogged up window heat will cause that moisture to evaporate. Cold will always make the window cooler which causes more condensation.
Air conditioning will remove moisture from the air while cooling the cab, so the condensation can only form on the outside, where you have wind and windshield wipers to wipe off the condensation.
Nope
Trick question:
Dew point is the exact temperature at which the current level of humidity forms. Heat is good because it makes the air at the surface stop condensing. AC is good because it dries the air out forming the condensate in your air box. Cracking the window is generally enough warmth to interrupt the process. Most new cars have a defrost mode that turns on both heat and ac to dry the air out.
So the answer is do anything, except just turning the ac colder.
Clean your window.
Yup. The inside of the windshield gradually gets cloudy from all the plastics inside the vehicle slowly off-gassing. Soapy water, a squegee, and a microfiber do wonders.
Huh, that makes sense. I was wondering what caused that buildup slowly over time.
dead skin
I had kinda thought it was from my breath, but I never really looked into it. Could be all three?
Do they not make cars with Defrost buttons anymore?
That’s REAR defrost. Front defrost is on the left, and yes, all cars still have that.
Useful mnemonic: Rectangle for Rear defroster, Fan for Front defroster
here’s a useeful mnemonic. Fuck You For Giving Correct Mnemonics
it stands for the great lakes next year
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
ORANGE CIRCLES??? oh what a world
They make cars with buttons? I thought ever car made after 2020 had an tablet taped to the dashboard …
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.
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.
the fuck. that’s a white circle, not a red circle. i’m so confused.
crack a window
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.
How will it being hotter outside cause condensation?
same as vice versa - water vapor in the warm air cooling and becoming liquid as it contacts the cold windscreen where the ac inside the car has cooled the windscreen - you’ll see this too when you take a shower (hot air + cold mirror)
if your AC has too much water vapor or you have dampness inside the car (sweat, water, damp clothing) it can exacerbate it
So you mean water condensing on the outside of the car?
First I dunno what the AC or humidity inside the car has to do with that. Second, I dunno about yours but my car has windscreen wipers that will clear water off the outside of the windscreen. Third, any car is being heated by its occupants, drivetrain and air resistance, so the circumstances in which it is colder than the surrounding air by more than a tiny amount are virtually non-existent.
You’ve never had the ac on and see where the condensation blooms up from it?
No. I can imagine it cooling the glass and condensation forming on the outside, but it shouldn’t happen on the inside because the AC air will be drier and cooler than the air at the inside surface, so if it forms condensation, condensation would have already been forming just out of the rest of the air in the car.
Yeah, I mean I’m in the south it happens a lot to me here
Outside or inside though?
Humid and warm: use heat
Cold and dry: turn on a/c
Open the window.
Temp isn’t as important as ac
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.
Every time. And I’m halfway through life. I could do a web search, but who ever thinks of that except when this happens? Will I do a web search now? No. I have other stuff to think about. And so it goes.
