Observation in heavy traffic this morning.

Yes, but I, and no one I know have had such mold problems in their car's vents. Now I personally don't run the A/C often in my current car, but I did in the gen2.

Cars that did develop a musty odor were due to carpets that got soaked, but not always. Our Sable had(hopefully now fixed) a leak that regularly soaked the driver's floor. Sometimes even getting the rear when parked on a slope. Since it is rarely used, it can go days before we notice the flood. Still no mold of mildew of note.

 

My system drains just fine. I find a puddle outside the car, not inside and I don't find condensation on any part of the interior.

Never had a problem like that.

 

The car is going to be 15 years old, and was at least 10 when we first noticed the problem. The internal down spout on the driver's side for the windshield gutter and moonroof had a hole corrode in a seam. It might even be clogged further down. So in a heavy rain, water can leak in behind the dash. Getting to it from the inside means removing the dash; from the outside doesn't appear possible without cutting the sheet metal. So I've block that gutter off with spray foam. Hopefully all the water will be diverted away from the leak now.

 

There is a fourth possibly more important reason - driving with the windows open reduces greatly the aerodynamics of the car and puts a drag and increased fuel consumption more than having A/C turned on....

You can program via OBD to have the FOB roll down the windows. It was standard on my Polo by inserting the key in the door and turning it to open it and it would work like charm. The one on the Prius, you really have to press the open button until they are all open, otherwise they stop at mid point (when you stop pressing the button, that is).

A/C cleans the air from pollen, dust and pollution as it is trapped by humidity which is then purged as described. Having AC on also means it can work as a dehumidifier by simply keeping the temp higher than the usual "LO" that everybody thinks it is necessary when having AC on.

I keep in summer AC at 24C // 75F, instead of 22C // 71F or less. I get dehumidified air, which feels cooler and then I let the AC in Auto and let it do his job. I am sure my fuel consumption hit is lower than keeping windows open or setting it at 18C // 64F full blast... (which also would cause me sinusitis)...

I also keep it on in winter (if the temps are below a certain min temp it will not work in any case) as it keeps the interior dry and removes the possible fog on the windows.

Moreover, in city traffic, AC enables me to have breathable air with windows closed and recirculation on rather than breathing directly the exhaust of all the diesels from the open windows. The filter (behind the glove compartment) gets really dirty here and better there than in my lungs.

In the end, as mentioned in other posts, to each its own.

The only thing I really don't understand and find most times idiotic in other people's car, is that they complain that the AC is too drastic, aggressive, cold etc., only to find out that the vents are all closed and they set it manual at "LO" or some other stupid super low temp. And even in AUTO, the system will never manage to reach the target temp and will continue to blast air in the effort...

When then you tell them how to do it properly, there is the "aaaah"... and you wonder why they have a driver's license....

 

I run my AC 95% of the time, for all the good reasons pakitt described. And I've been driving in the PWR mode 100% of the time with my current tank of petrol. (430 miles driven, so far.)

Would you believe my MPG has improved a bit? The display shows I'm getting 65. (All city driving miles.) For me, personally, I have found that there is NO significant difference in gas mileage, whether I'm using the Eco mode, Normal, or PWR.