this post was submitted on 02 Sep 2026
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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.
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 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.
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.
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.
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.
Does the HVAC engineer seriously need the mechanism of condensation explained to them…?
Coils frost, glass can frost, you’re ignoring reality for your textbook numbers.
It can also freeze, because it’s -20c outside lmfao.
i mean, the guy who ran one of the local hvac companies lived down the street for a couple years. steve had his head wedged so far up his pucker that i'm pretty sure he didn't even understand the repair manuals. inherited the company from his dad and had the attitude that goes with it.
I'm inclined to believe him over you or the other guy 🤷. I'm not an HVAC engineer, just a rocket scientist turned data analyst.
Using quotes from your textbook, please explain why it’s called deFROST. If what you’re saying is true, it would be decondensation or something along those lines.
Spoken like someone talking out of their ass and making shit up, if what you’re saying was true, we wouldn’t be “arguing”. You would be providing facts, you could also provide sources to back up your asinine claims as well….
They already explained that, because it's intended to defrost frosted windows, not because it defrosts frosted condenser coils. How would you even know when to use it if that was the case?
Spoken like someone arguing for the sake of arguing, aka, a troll. Have fun under your bridge!
The deFROST button that looks like this?
It is quite obviously defrosting the windshield. Its other use is removing condensation from the windshield.
That’s the front and rear defrosts mate.
Ones curved to show the front windshield, the others flat to show the back window.
Yes, that's what the thread was about. You claimed that cars defrost AC coils, not windows.
So I don't know anything about the math or physics of the situation, but I do actually work on cars. A car has both and evaporator and heater core. Meaning it is capable of using the evaporator core to act as a dehumidifier before the fan from the blower motor pushes it through the heater core and up into the cabin. The heater core gets it's heat from the engine coolant. When the heat is activated a little internal door opens and runs the coolant through it. I'm assuming you'd understand the rest. But thats why it takes time for your hot air to blow. The engine hasn't heated up enough to transfer it to the coolant.
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.
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.
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
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.