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
this post was submitted on 02 Sep 2026
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Open the window.
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
They only do this with the automatic controls not basic knobs right? Apparently affected what filter I had to get.
thank you for your service
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?
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!
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)
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 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.
My car is an EV, so it definitely does this because there’s no excessive engine heat to use.
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.
Air-conditioners are in a way dehumidifiers, so that makes sense
crack a window
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.
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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.
Nope
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.
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.
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