GLK 2010. I was driving in cold weather and decided to turn the heater on. I can set the temperature on maximum and I can almost immediately feel like hot air is going out from the air vents. Everything is fine at this point.

Now I see there is also A/C button which is deactivated. So what difference will I notice if I press it?

It perfectly makes sense when I want to cool down the interior - I press AC button and cold air goes from the vents. But what difference do I get when I want to heat up the interior?

 

I think the AC turn on automatically when you have the air direct toward the windshield.

Other car manufactures have the same feature.

 

you are dehumidifying when you have A/C on & the heater.
A/C on @ any time will dehumidify.

Example is that your window fogs up during a winter rain/snow because of condensation due to humidity. Yet you want to be warm too. Turning the heat & activating the A/C will solve both the condensation (fogging) & keep you toasty.

 

Many people turn the AC off during winter , as among other things it keeps the compressor clutch deactivated and may give a slight improvement in mileage. However, as above, it does dehumidify and will probably keep the temp inside more exact if left on. It also sets up the vent motors in a slightly different way. If you decide to turn it off, turn it on for a short time once a month with the temp set to very low to circulate the refrigeration oil in the compressor and exercise the seals.

 

They did the A/C mileage test on Myth Busters. It was less then 1% difference in mpg.

 

Thanks, I'll leave mine on. I drive a lot in the Spring and Fall with the windows and roof open. That probably lowers the mileage more than anything else I could do except drive with a roof rack on.