Like a lot of foreign vehicles their manual heat and air has an A/C button and that button actually turns the compressor on and off, aside from the compressor cycling that is done to prevent freeze up or other bad things from happening to the system. However in those manual systems the A/C button does turn it on and off.

The Prius autoclimate system is totally automatic and the A/C switch isn't an on/off switch like the manual systems, but really an available/not available switch. If the A/C button is lit, it means that the compressor is available to the system for use, not that it is on. In fact the only way to tell if the compressor is running is to listen for it.

Now, the reason that you want to lave that A/C light on is because during the winter vehicles use the A/C system as a dehumidifier, and it is very effective. The vehicle will usually only use it when you have the defroster on, but in many instances if your vehicle was not equipped with A/C on defrost it would take much longer to clear humidity from the interior glass.

There are also mechanical benefits to running A/C during the winter. A very knowledgable service manager from 20 years ago at a very nice Mercury dealership where I live (I'm sure he is long departed now) once said that you should run your A/C during the winter at least a couple times for about 10 minutes. The reason is that refirgerant and oil are circulated through the A/C system when operating and circulating that oil through the system keeps o-rings and various joints and seals lubricated and sealed. Thus keeping refrigerant in the system and preserving the system from premature wear and premature or unnecessary charge services. This advice has kept all the A/C equipped vehicles in my family running very well for many years, including a piston based A/C compressor on a 1978 Mercury Cougar that will freeze your behind off during the summer.

However, it is a lot easier now because cars built for the last 25 years or so that had A/C would run the compressor when the defroster was on. And the Prius is the same way.

Auto climate on the Prius, like almost any auto climate equipped car is like a home with thermostatically controlled heat and air. Set the thermostat at the temperature you want the cabin to be at and let it do its job. It will adjust air temperature, air outlet and fan speed to acomplish your desires. This doesn't mean that you can't go into the climate control and direct the air where you want or the fan speed, however if you are in auto, as indicated by the light on the dash, and then choose to direct air specifically to your feet, the climate control will no longer automatically adjust that setting. So, whatever settings you manually set will not be set by the system, until you press the auto button on the screen or the a/c button on the wheel.

In the winter I tend to set the system at 68 degress. During the summer I tend to move it up to 74, the A/C is so effective in the summer that 68 is downright cold. Just for reference the temp range you can set the system too looks like this.

MAX COLD, 65........85, MAX HOT

I hope this helps out.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)