On my 2001 I noticed on the vent options the picture with the heater blowing out front vents with feet vents has the blue line, so I opened up the hood to see if the AC compressor would also be on. I assumed it would since this vent option is what I use for the AC, but I was surprised to see that compressor spining with any setting. I always assumed that the heat was seperate. So How exactly does the whole system work? I doubt that AC freon is being used while I have the heat on, and if that compressor is spinning doesn't that mean its just like its being used in the summer, which would use more gas mileage? Why does the compressor need to spin, isn't there some fan right next to the heater core that pushes the hot air?

 

Short answer. The ac compressor is used to dry the air so there is less moisture to keep the windows from fogging up.

 

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That's the bi-level setting. I rarely to never use it. It's only good for the summer when you have hot feet. But it doesn't seem to blow as much cold air into the car as A/C or MAX A/C.