Tukukkk

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?

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.

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.

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.