My wife drives a 2019 VW Tiguan. When I occasionally drive it in the winter, I notice she has both the heater and the AC on.
She claims the owner’s manual says to have the AC button on to remove humidity. But I can’t figure out why you’d have the AC compressor running at the same time as the heater. Why? — Richard
Like your wife said, Richard, it’s to remove humidity. You should listen to her. In fact, maybe I’ll write to her next time I get a question that stumps me.
The way air conditioning works is by removing humidity from the air. That’s the “conditioning” part of air conditioning. It cools the air, too, but removing humidity is a big part of what makes you feel cooler and more comfortable.
Why would you want that in the winter? Let’s say there’s humidity in the air, which there usually is, and let’s say the outside of your windows are cold because it’s, what? Winter!
Now, when you heat up the inside of the car, you have cold glass on the outside and warm glass on the inside. That causes condensation or fog to form on the inside of the windows (see also: beer glass). The result? You can’t see!
But if you remove the humidity from the air inside the car, there’s nothing to condense on the inside of your windows and fog them up. That’s why you run the AC. In fact, many cars automatically turn on the AC when you activate your defroster.
In terms of whether you’re “wasting” something by running both the AC compressor and the heater at the same time, you are, a little bit, but it’s small potatoes.
While the AC compressor does use energy, and reduces your mileage a bit, your heat is really free. With an internal combustion engine, your cabin heat is excess heat that’s being thrown off by the engine. And it’s being produced whether you’re sending it to the passenger compartment, or letting it dissipate into the atmosphere.
I think most people would agree that it’s better to spend a few cents more on gasoline — and use your AC compressor when you need it — than to let your windows fog up and drive into the back of taco truck.
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