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Most cars that I've seen in my life have a heat slider or dial, which goes from coldest to hottest, and also a button or switch that is labelled "AC". When the heat dial is set to cold, turning on the AC switch makes it even colder. I was under the impression that, when you turn on the AC switch, it basically adds a cold tube full of refrigerant in the path of the air, so that makes sense.

But that would also mean that the AC switch would make heating less effective. Often enough, though, I see people with the heat dial set to hot while the AC switch is on. Is there any benefit to it? Would competing heating and cooling inside the same system cause any damage?

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    Mr. Monk approves...
    – Harper - Reinstate Monica
    Dec 21, 2018 at 18:25
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    The AC will lower humidity to avoid foggy windows in cold weather. Some cars have symbols on the dashboard telling you what to select for best defog results. Usually that’s heat to max, AC on and air directed to the windshield.
    – Michael
    Dec 22, 2018 at 7:50
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    Data point: Commercial large scale air conditioning systems will in apposite conditions refrigerate air to below its dew point to drop out moisture and then heat it to the desired temperature. How cold it is taken depends on incoming humidity levels and target output humidity levels. Heat from the cooled air is used to heat the cold air after dehumidification (hopefully using a counterflow heat exchanger) to minimise overall energy use.
    – Russell McMahon
    Dec 22, 2018 at 10:27
  • The AC system of any car is already designed and set up so that the compressor isn't harmed by liquid refrigerant returning to its pumping chambers(what can possibly happen when the evaporator coil gets covered with frost) and too high pressures.The benefit to running both the AC and heater together is that hot and dry air is very effective at removing moisture from the interior and bringing it back to the evaporator coil where it will condense and be ejected from the car, and by using the AC frequently you keep the AC system seals lubricated and hence natural refrigerant leakage at a minimum.
    – Al_
    Dec 22, 2018 at 15:55

2 Answers 2

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You're meant to.

In fact, in older cars, it was automatic when redirecting air to the window defroster. On this GM control, the far right position would activate the A/C compressor.

Almost always, it is a cold, wet day, and you have both sliders all the way to the right. The pass across the activated A/C coils dries out the air, because of the way humidity works. So your window glass is immediately hit with dry air, and there is no need to wait for heat to become available.

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Later, as the engine warms up, the heater coil becomes effective, and now you are getting air that is pre-dried and then heated, which dries the air further. This dryness, moreso than the heat, defogs all your car's windows.

Humidity and the battle for your windows

The way humidity works is that warm air can hold quite a bit more water than cold air. That's why they talk about "relative humidity" instead of absolute humidity in terms of comfort. So if you have 10C/50F air, it won't have a ton of water to begin with, but if you chill it to 1c/34F, the water that's in it will condense out, and it will have almost no water.

To the 50F/10C windshield, it will seem like very dry air, and the condensate on the windshield will evaporate into it. When you also start heating the air to 21C/70F, it is much drier still, and is even more effective on the windshield you are blasting.

However, keeping condensation off the other windows is harder. Cars are poorly insulated and the temperature of the side windows will not rise much above outside temp. So if it's 50F outside, your air inside the car needs to be dry enough to be non-condensing (less than fully humid) at 50F. Outside air coming into the car is fully humid at that temperature, so it's no help at all. The A/C system is your secret weapon.

Having driven cars without A/C, it is much more of a battle to keep windows clear when you do not have A/C available. It is not realistically going to happen until the car warms up, and even then you need heat blasted at full all the time, as raising the window temperature above the condensing point is your only defense.

So if you have been struggling like that because it never occurred to you to click A/C on... Try it.

The A/C cannot function in too-cold conditions because the coils will freeze up. But if outside air is at freezing, it doesn't have much absolute humidity anyway, so drying it won't help much.

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  • It has occurred to me to use AC to dry out the condensation, however I've always decreased the temperature on the dial to cold or neutral for that, even when it's cold outside.
    – snetch
    Dec 21, 2018 at 20:02
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    Go ahead and blast it to hot. Assuming the automaker is remotely competent, the A/C condenser will be before the heater core. Specifically so you can do that. Ask any driver of an old GM, they know the one-thumb motion to slam em to the right on a cold wet day. Generally on that console you'd keep both sliders in about the same position and control intensity with the fan lever.
    – Harper - Reinstate Monica
    Dec 21, 2018 at 20:39
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    Wow, I knew the AC compressor kicked on when in "defog/defrost" mode but always thought it was to ensure some freon/oil circulation to keep the system lubed through a long cold season. Never thought of the humidity effect but it makes sense. Thanks, learned something new today...
    – Jimmy Fix-it
    Dec 22, 2018 at 3:00
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    This is a good answer explaining about absolute vs. relative humidity, but it's still pretty dumb to make it sound like AC+heat is always good. Especially on older cars, this does cost a considerable amount of energy. Don't do it when there's no humidity problem. And even when there is, we should be clear that this is in principle a pretty wasteful hack. The proper way to address fogging is to get a car with better heat insulation. (Sure a good new car will still use this hack, but only when it's really needed.)
    – leftaroundabout
    Dec 22, 2018 at 13:14
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    Quite right, I mean evaporator (of freon) which also happens to be a condenser of water.
    – Harper - Reinstate Monica
    Dec 23, 2018 at 0:06
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Yes, it is useful, okay and won't cause any damage.

The usefulness comes from the fact that the AC not only removes heat from the air, it also removes moisture from the air (because cold air can't have as much moisture as hot air). The removed moisture clears your windows quickly.

The only "damage" caused is the additional fuel consumption. On most new cars, the AC compressor can be adjusted and therefore, the extra fuel consumption will be minimal.

On my car, I always keep the AC on and trust on the AC computer to adjust the compressor to consume minimal extra fuel when AC is not so critical like on cold days. On very cold days, the computer actually automatically shuts off the AC compressor and thus the AC light on the dashboard is off.

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    One good "side effect" of running the AC when it's cold out is it gives the system a workout and keeps everything in good working order.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 21, 2018 at 17:45
  • I think I was told that when you have heating and AC fighting each other, the heat could heat up the AC refrigerant (maybe evaporate it, even?), and that could potentially damage the AC circuits. Is there any validity in that?
    – snetch
    Dec 21, 2018 at 17:58
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    @snetch Jaguar are very clear : have the AC running all yesr round and the system decides how much or little it has to work... they spent sufficient time modelling and testing to know...
    – Solar Mike
    Dec 21, 2018 at 18:10
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    @snetch Provided the high side is completely functional at rejecting heat, it's in fact completely the other way around. The short story is, not enough heat is what destroys compressors. The long story is, if the liquid refrigerant injected into the evaporator doesn't completely turn into vapour before entering the compressor's suction port, it does two awful things: 1)It effectively removes any oil from the compressor (since the oil is meant to dissolve in the refrigerant so that it can circulate) 2)The compressor reed valves get crushed into bits since they're not meant to withstand liquid.
    – Al_
    Dec 22, 2018 at 15:31
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    @juhist -- the refrigerant is supposed to all evaporate in the evaporator as it absorbs heat from the thing you're cooling (the air in the cabin in our case here). If it doesn't, you get slugging due to liquid refrigerant entering the compressor, which is the refrigeration equivalent of hydrolock in a car engine (translation: extremely bad for the continued life of an air conditioning compressor).
    – ThreePhaseEel
    Dec 23, 2018 at 5:55

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