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Comment by CobsterLock | Hacker News Reader

Maybe I am asking for too much but I want physical knobs for: Volume Control, HVAC Fan Speed, HVAC Temp.

A tangential question: Does anyone know why there is an AC button on older cars? I can turn the temp all the way down but that just does outside air temp. If i want real AC I need to hit a button to turn it on. In my ideal car turning the knob to the lowest temp would get me cold air no matter what.

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Because the button is connected to a relay that turns the AC compressor on or off. The compressor actually draws quite a bit of power.

The hot/cold adjustment is adjusting a baffle/mixer that can either let in "cold" outside air, or air that's been heated by the engine compartment into the car. Think of it like driving around on a sunny day in the fall - sun is heating up the inside of the car, but the outside air is cold enough it would be silly and a waste of gas to turn on the A/C compressor.

If you want to defrost your windows, you want hot, dry air. Air conditioning dries your air. If you combine A/C and heat, you get the best defrost performance.

A/C also consumes some amount of power and fuel efficiency- you may want the coolest air you could possibly have without A/C.

Two reasons: 1) The compressor uses some power from the engine, reducing gas mileage. 2) You can actually use it in conjunction with the heater to get dehumidified hot air.

I had a 1996 (iirc) Buick that had AC/Heat knob that did that. It was red on the right, blue on the left. If you left it at the detente position it was outside air. Turn in left, AC; turn it right, heat. Then there was just a separate knob for fan speed. Now that you mention it, I’ve never seen that again.

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