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).
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.