Piston Slap: Fresh Air on the Topic of Re-circulated Air?
TTAC Commentator sastexan writes:
Hi Sajeev,
Hope you are doing well. I haven’t seen a lot of activity from you on TTAC lately (but I haven’t been as religious a reader lately either between work and kiddos). I find it interesting what cars default to having their climate control (manual or automatic) on re-circulating versus fresh air. One of the things I like about our Odyssey is that it defaults to fresh air unless the cooling load called for is very heavy (in auto mode, in manual mode it won’t switch).
This is as opposed to most Toyota products (namely my mom’s 2010 Lexus RX350 and my mother in law’s 2015 Toyota Camry LE) that seem to rush to re-circulating all the time. I prefer fresh air, but understand that re-circulating is more efficient (I guess?) as the AC can work less changing the temperature and humidity of the inside air rather than the outside air. It probably doesn’t matter on short trips, but on longer highway trips fresh air with more oxygen seems like a much better idea.
What criteria do car manufacturers use for these settings? Why does Toyota force re-circulating on manual climate control? Am I just being over sensitive about the mix of fresh and re-circulating air?
Sajeev answers:
I write a minimum of two posts weekly ever since Neidermeyer ran the joint, and I gotta hear this? Oh, a shot thru my heart this is…but I digress.
Re-circulated air is, in theory, more efficient. Instead of working hard to condition outdoor air, the vehicle now works with a more manageable sample. Down in Houston, neck deep in a heatwave, the need for re-circulated air is real: why try to constantly condition 100+ degree air when you can recondition air inside the car? It’s more efficient; it can save fuel.
Hence when an owner asks for “MAX A/C” they get re-circulated air sucked from the cabin and forced through the A/C evaporator. Many vehicles push the blower motor to full speed on max A/C, provided there’s a separate recirculate button, like the BMW X5 pictured below.
But not all is perfect with re-circulated air. If, for a horrifying example, the front seat passenger takes off their shoes and their stank-nasty feet are (usually) right next to HVAC’s inlet for re-circulated air, in less than a minute you smell the stench through the dashboard. Yuck.
More to the point, rip one while running MAX A/C and not open the windows, I dare you!
Fresh air is nice, especially when the outside air isn’t significantly hotter than your desired in-cabin temperature. If you only need to knock down a few degrees and/or compensate for the greenhouse effect, go ahead and enjoy outside air.
All manufacturers should force re-circulated air as default when a user wants air conditioning. This ensures maximum efficiency and better fuel economy. There’s always (usually? maybe?) an on/off button for re-circulated air that’s easy to defeat for the stanky feet among us.
Bad idea? Well, it’s no less insane as BMW’s start-stop tech, or active grille shutters on a vehicle fascia with all the wind-cheating sleek-osity of a barn door.
[Image: Shutterstock user bogdanhoda]
Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.
More by Sajeev Mehta
Comments
Join the conversation
My sister's Journey, w/o tinted windows and no rear a/c vents needs to have the a/c in recirc all the time otherwise you get barely cool air here in steamy, hot So fla. Did it since new, so it's not a result of getting older.
Lots of comments and opinions here with no consensus. Also no mention of the air quality inside the vehicle. There have been studies (unfortunately I can\'t quote these) that show that air pollutants tend to build up inside the vehicle cabin unless outside air is continuously introduced. But, when a car is operated in heavy traffic at low speed the air outside becomes polluted from multiple exhaust emissions, especially heavy diesel trucks. On my Saabs, the ACC switches to recirculation when the car is stopped and then back to outside air when moving. This is to minimize the ingestion of more heavily polluted air. Regarding recirc on startup, I have never used the manual "MAX AIR" setting on any car because to me it didn't cool the car as quickly, as you were cooling much hotter air, as mentioned above. It seems to blow hot air out the vents very noisily. Another tangent - many people do not use manual HVAC controls properly or effectively. I have observed that they use only full hot in winter and full cold in summer with full fan speed, then switch the fan off when they get too cold or hot. It doesn't occur to them to modulate the temperature and fan speed to comfortable levels. I suppose something that complicated would require them to read the manual.