ravenuer

Because if you have a musty smell in the car, recirc will just keep recirculating the smell in the car. Outside mode will introduce fresh air, reducing the musty smell.

jimaizumi

Its more about moisture. AC creates a lot of condensation and when on recirculate, you're just passing it around and recycling it which only amplifies the condition. I believe some on this forum suggest using recirculate only up to the point that you get the interior to your desired temperature and then switching to fresh. If you want to keep the system dry, switch off the AC while keeping the fresh setting on. This would allow some time for your system to dry out before you turned the ignition off for the remainder of the day.

jcg0324

I normally use the fresh air setting for A/C in the summer but after the engine has been stopped then later started again the setting defaults to recirculate. I always then have to push on the fresh air button to reset. It doesn't matter if I am in "AUTO" mode or not. Maybe some very busy individuals are not aware they are always in recirc. mode. I know my wife doesn't.

AnotherBob

I have to pay some attention to that. I used to design automotive HVACs. Our ATC started in fresh to help push the hot humid air out in the summer, then to recirc when the air was adequately cooled. This for (the very very minor) fuel efficiency reasons (and improve pulldown test results ). I think (but aren't sure, it's been a few years) that it was other way 'round in winter to get it warm ASAP then prevent the occupants from raising the humidity and fogging the glass.

Lexura16

Someone earlier in this post mentioned the drain tube from the evaporator. This is the first thing to check to see if it is plugged which will result in the musty smell. To check for a free draining tube, take a drive with AC on. When you are finished with your drive, check for a pool of water on driveway/garage floor usually on passenger side at corner of engine bay and fire wall. This work best if outside temp is warm/summery. I cannot vouch if this will work when temps here are low as its now in central Ontario. A pool of water will indicate a free flowing drain tube and its absence the opposite , subsequently the resultant moldy smell. I have noticed this pool of water on the garage floor from my Lexus NX200T everytime this summer after a 40 minute round trip drive into town.

ravenuer

Good idea. I had the same problem on my Toyota RAV4. After clearing the drain and drying out the interior, every year or so I get underneath and, since I know where the drain is, I just connect another hose to it and blow thru it by mouth. Haven't had a problem since.