It may seem counter-intuitive to run your mobile A/C when the weather’s cool, or even freezing, however running the A/C in the winter not only does not do any harm to your heating/cooling system, it may in fact, offer benefits.
Simply put, running the A/C and heater will dehumidify and remove moisture from the air.
To explain further, the relative humidity is the percentage to the dew point for droplets to form. This point increases as temperatures rise. Therefore, the heater alone will heat the air, but to actually remove moisture, you have to cool the air below the dew point with the A/C.
If you only use the heater core, and not the A/C condenser also, the air will be hot, but likely moist, and it won’t defrost or defog the interior of the car. So the car must use the A/C condenser to first remove the moisture. This will help to slow rust and other water damage (such as upholstery glue coming undone) and prevent fogging windows. Therefore, the AC should always be on in most cars (even in the winter).
Moisture,clima
Most cars are actually DESIGNED for the A/C and heater to run together. When you run the A/C and heater simultaneously the A/C compressor is energized and the evaporator coil (under the dash) removes moisture from the air. The dry air is then routed and mixed with warm air and sent up to the dash top windshield and side window vents. As stated above, dry warm air is highly effective at quickly defogging the windshield and side windows—much more so than just heated air.
In addition, regularly running the compressor (or running it all the time) helps keep the compressor seals lubricated, and that is a very good thing.
The A/C button turns on the compressor, that is all it does. The compressor is now ready to come on if it is required to cool down the interior of the car. In addition, every time you select the defrost button you are turning on the A/C. A/C button or not, the compressor is now ready It’s interesting to note that this is the normal mode for most modern automotive climate controls when the Defrost mode is engaged. Most systems are designed so that running defrost runs the A/C compressor and blows the conditioned air at the windshield.
So, in closing, the A/C is not for cooling only. It is an integral part of the Climate Control system and works in conjunction with the heater to control temperature and humidity, the “climate” of the vehicle cabin.
LMK 2020
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.
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.