This past weekend a caller mentioned that her Subaru Impreza manual specified turning the air flow to outside air while using the air conditioning. I was as unbelieving as Click and Clack when she announced this. But when I looked at my own Subara Forester manual, there it was in print - set the air flow to outside air. Is this possible? It seems to work both ways (recirculating air or outside flow) but now I am wondering if there is any damage that could be done or perhaps it uses more fuel in one setting than the other? Does anyone know more about this? They promised to find out more about this, but so far nothing on the web site. Thanks.
You will start choking on the recycled air after a while. Or you will wonder what’s wrong with your air after a short time. Fresher air will prevent headaches, sleepiness and catching colds. The airlines use recycled air and the people have been complaining about it for years. Recycled is to help the inside of the car cool off faster when you have just started the car. You may want to use it if the temp is in the nineties.
You can use it either way without damaging anything. Using the RECIRCULATE feature just means that no fresh air will be mixed with the air in the car and the vent outlet temps will be a bit colder than in the other position.
In a Subaru service school once, it was stated that the REC. should be used when in heavy traffic, industrial areas, etc. when one wants to cut off outside exhaust fumes, smells, etc. It’s a coin flip as to how you want to do it.
Many years ago, one could not even cut off the outside air at all on a Subaru if not equipped with A/C. Some goofy Federal law requiring that outside fresh air be mixed at all times with the air in the car.
Works pretty nice on one of those blustery, 3 degrees above zero days in which you’re praying for the heater core to override the incoming air temps! 
You can use the AC in either setting. The “recycle” setting allows the AC to work less because the air inside the car is already somewhat cooled and dehumidified, but I seriously doubt you could ever measure a gas mileage difference.
Set it to whichever position seems more comfortable to you, and don’t worry about it. Using the “recycle” setting will not make you sick or give you a headache. It’s not 100% recycled air. There is still some fresh air coming into the car.
The system in my Legacy automatically selects “outside” air whenever I set the system to “defrost,” regardless of the position of the recycle button.
I have a 2005 Outback and use recirculate when it’s very hot for colder air and to avoid diesel fumes if I get in traffic near a truck. I’ve never had any problems.
It just occurred to me, my car has the automatic air and will go to recirculate on it’s own when I first start driving the car in hot weather. It switches back to fresh air once the cabin is cooled down.
Could it be the limits of a bad design? Maybe it would get too cold and freeze up if set to recirculate.
But yes, in theory, it would be more efficient to recirculate. The compressor would run less to regulate temperature and use less power.
I wonder if there’s a heat exchanger to precool the incoming air with outgoing air. It certainly sounds like a good idea!
Lots of automatic “climate control” systems do this. It cools the interior more quickly if you recirculate the air, then the system switches to outside air.
You can still over-ride this depending on the vehicle, but if I select “defrost” on my Subaur, I CANNOT recirculate the air.
This really is a non-issue, and mostly comes down to personal preference. Drawing fresh air into the car helps prevent the windows from fogging on the inside, but this is usually only a problem if you have several people in the vehicle.
Do what you want.
I have a WRX (speedy impreza) and I use the recirc intially then once interior is cooler move over the recirc to outside air partially.
However if you are defroster always turn on AC button AND use outside air.
Thank you all! I can rest easy now knowing I have AC use options that will not hurt my wonderful Forester – thanks for taking the time to respond to my question.