By on August 20, 2015

recirculated air. shutterstock user bogdanhoda

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 [email protected]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.

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46 Comments on “Piston Slap: Fresh Air on the Topic of Re-circulated Air?...”


  • avatar
    wstarvingteacher

    Middle of target with the comment on Houston need to recirc. I was in the HVAC business in Houston for many years and there is something that reduces the disadvantages of recirc. That something is makeup air. On the lower end of the spectrum you will find that manufactured housing requires it by code . On the other end of the spectrum a pizza kitchen may have a huge swamp cooler on the roof dedicated to to feeding the kitchen exhaust with makeup. A home AC will lower the temp about 20 degrees every time it passes over the evaporator. It would never shut off in a Houston summer if it didn’t recirc.

    I have no idea if vehicles have a dedicated inlet somewhere on the suction side that allows filtered outside air into the process. If they did recirc would always be the way to go.

    • 0 avatar

      Judging by its location relative to the in-cabin intake, I reckon if the cabin air filter is replaced from behind the glove box (and not under the cowl) then recirculated air will also be filtered. Just from what I’ve seen on a few Honda and Toyota products that I’ve had the honor of working on.

    • 0 avatar
      Toad

      Don’t forget that one of the main functions of auto AC is to remove humidity; the re-circulation function does that much more quickly than using fresh outside air. That is why most automatic car HVAC systems default to recirculate mode at start up (the design engineers actually know what they are doing).

      The default to fresh air at start up in manual systems or once temp hits set point in automatic systems is because many people push the re-circulation button and never turn it off: that is why in the winter you occasionally see cars with all of the interior window glass completely fogged up from having no fresh air circulated in.

      Cars have to be idiot proof; for better or worse most car HVAC system are designed around this principle.

      • 0 avatar
        golden2husky

        Yes, removing humidity is the idea but surprise, using recirc may not be the answer. It is the best way to remove humidity if the source of that humidity is from outside, but what about from the inside? Consider: Rainy day, four people get in the car and if you leave the a/c on recirc, the windows will fog. Ever notice that when you engage defog, most cars default to fresh air? It’s true. You can override that in many, but not all cars. Another bummer of recirc is that when you have several passengers, the air gets stinky from breath, YUCK!. Recirc is for really hot, humid days only IMO.

  • avatar
    turf3

    When you first get in a heat-soaked car, you want to use outside air, so you will be cooling 100 deg. air rather than the 160 deg. air inside the car. For max. cooldown, change to recirc once the inside temp is cooler than outside. Unfortunately, the “Max A/C” button or knob position does not do this, it just defaults to recirc thus actually slowing the first part of the cooldown, which is the most critical part for comfort.

    Another point is that a system that defaults to recirc would overcool at highway speeds and moderate outside temps, thus (usually) resulting in hot air bleed into the system, which means the compressor runs longer and consumes more fuel (though it’s a small effect). Or, in a CCOT system, the compressor would short cycle increasing wear on the compressor and clutch. So for this condition, OSA is preferred, but a system that defaults would require operator intervention to move to recirc.

    If a system were to default to anything, I think it ought to default to OSA.

    Personally, I would be happy if the “Max A/C” were to go away, and we could just have a thermostat setting, blower speed setting, and recirc/OSA button. As described above, the “Max A/C” doesn’t necessarily give you the fastest cooldown; and getting into a 160 deg. heat-soaked car is when you want fastest cooldown, which unfortunately is not what the “Max A/C” setting delivers.

    • 0 avatar
      stevelovescars

      I was going to say the same thing. In your Houston heat wave, the last thing you want to do is run it on “recirc” when starting the AC in a car that has been sitting in the sun.

      I was pleased to find that the Ford Fusion I rented this past week would drop all of the windows when the unlock button on the remote was held for a few seconds. Getting the hot air out of the car quickly before running the AC would seem to be an efficient and comfortable thing to do.

      • 0 avatar

        Unless I’m parked in a garage, I always open the windows for 10-20 seconds to get the hotter air out. I’ll usually start MAX A/C at the same time to get the hot air out of the dashboard plumbing too.

        • 0 avatar
          dolorean

          Sajeev, one hopes you open the garage door before you do this ;)

        • 0 avatar
          Fordson

          My GTI will drop the windows with the key fob, too. I wish it would open the sunroof, too (it WILL close both the sunroof and windows with the fob…), as this will vent hot air even faster.

          The remote will do this from over 100 yards away, so as soon as I clear the exit door of home or office, or get within 100 yards of the car, I hit that unlock. The 30 seconds it takes me to get to the car probably bleeds it almost down to ambient.

    • 0 avatar
      dal20402

      “Personally, I would be happy if the “Max A/C” were to go away”

      Agreed. It’s dain bramaged. In a manual system, it just creates confusion, because no one knows what it actually does. In an automatic system, it’s redundant with setting the temperature to LO.

  • avatar
    John

    All autos allow some fresh air circulation all the time, although the vents may be hidden. I presume this is to prevent carbon monoxide poisoning.

    • 0 avatar
      Scoutdude

      I have never seen a car that lets some fresh air in all of the time. Yes there are vents but they have flapper valves that let air out of the car not into it. The fresh/recirc doors seal at near 100% so if you have recirc on you are recirculating something like 99.9% of the air.

  • avatar
    mike1dog

    I prefer recirculated air whenever possible. In high humidity situations using strictly outside air will not remove all the moisture and the car ends up feeling swampy inside. I used to know someone who disliked using recirculated air and it would get so humid in his car that condensation would form on the outside of the evaporator case and drip onto the floor. It also seems to mute the stench of outside odors. I guess modern cars have filters which help this, but I seem to drive by a dead skunk or the rotting corpse of some animal at least once a day.

  • avatar
    hawox

    remember to change air con filters!

  • avatar
    PrincipalDan

    I dislike Toyota’s switch to re circulation when the temp knob is cranked all the way cold. This is mostly because if my wife is with me she jumps into the car that has been sitting in the sun all day and cranks the knob all the way over.

    • 0 avatar
      Trichobezoar

      Yep, our ’07 Mazda misbehaves the same way, which is a shame because it’s literally the only gripe I have with the car. In the literal sense, even: “http://trumblings.blogspot.com/2013/06/100kmi.html?m=1”

  • avatar
    CoreyDL

    I let my car choose for me. When I have the AC on, it will switch to and fro as it pleases, usually keeping it on recirc after a couple of minutes. But it also has to do with outside temps as well. I haven’t seen a car (IIRC) that did -not- have a manual fresh/recirc button. It’ll just kick the car out of auto mode.

    I never use auto mode anyway, it really bothers me for some reason. I want AC, recirc, at the lowest temp possible for my short trips.

    Also, inquiry!

    The car chooses on its own whether to use the wafty vent which is located inside the center speaker housing on the dash (vent to keep air from going directly at your face). Does this have to do with outside temp and/or humidity, so it doesn’t fog the glass in the summer?

  • avatar
    sirwired

    DOES re-circ save energy? I always thought that A/C compressors used the same amount of power no matter the temperatures involved, and all the temp regulation was accomplished by bleeding in heat. (If it’s hot enough that you need Re-Circ at all, it’s probably too hot to shut off the A/C entirely.)

    • 0 avatar
      CoreyDL

      My dad has always stated this idea too. It doesn’t matter the temp you are using, if the pump is running it’s sapping your mileage. All else is just blend of cold and warm air.

    • 0 avatar
      dal20402

      The difference is that if the system is starting with cooler air the compressor has to run for less time to achieve the desired temperature.

      • 0 avatar
        CoreyDL

        But the compressor is always on! Whether it’s doing a lot or no, it’s still working.

        • 0 avatar
          dal20402

          Not in any car I’ve ever owned. With the AC on, the compressor switches on and off as necessary to produce the desired temperature at the air outlets.

          If the compressor ran all the time, condensation could freeze on the condenser, disabling the system. This actually happened to me on one long drive in my 2006 Civic in horrible August East Coast conditions — 98 degrees and extremely humid. I thought my compressor had gone south. I couldn’t take the heat and pulled over to go into a plaza for half an hour to cool off. When I came back, there was a giant puddle of water under the car and the AC worked perfectly, and the lightbulb went off in my head.

          • 0 avatar
            CoreyDL

            Hmm. So the thing you hear when you kick on the AC is of course the compressor, then after a while it’s just the additional pull on the alternator?

            What I’m trying to express is that when you shut the AC off with the car running, there’s always a difference – and you wouldn’t think there would be if the compressor only ran when needed.

          • 0 avatar
            dal20402

            The evaporator stays cold for a bit after the compressor turns off, so the change in the temperature of air the system is blowing isn’t immediate. If you turn the AC off when you can hear the compressor running, you’ll notice it takes a bit of time for the air to warm up, maybe 30 seconds.

            It’s hard to hear the compressor in current cars, but easy in older cars. There’s also noise from the compressor clutch, which can be ludicrously loud — it was loud enough to worry passengers in my first car, an ’87 Taurus. My LS is the first car, though, where I truly can’t hear evidence of the compressor at all.

          • 0 avatar
            CoreyDL

            The only thing I can notice in my current car is a change in the idle, and maybe a click when I turn it off. I always make sure to shut off the AC before turning off the car.

          • 0 avatar
            wstarvingteacher

            Actually the pull on the alternator is only from the electromagnets in the clutch and the inside motor which mostly just runs all the time. I do not mind if the AC is on recirc when the car starts. You lose a lot of the heat just opening doors to get in and the recirc takes it down very quickly if the AC is working.

            It actually does consume more energy if the car is hot because heat equals higher pressure. That pressure reduces quickly on recirc and not so quickly on outside air on a hot day. That energy is mechanical not electrical because up front it is not electrical AC. The real answer is to use a sun shade and let the AC just do it’s job. For the past couple new cars I have been unaware when the AC is working. The compressor is just turned on and off by the clutch and the inside fan keeps circulating.

            I could seldom argue for outside air in a high humidity area like the gulf coast. Removing the water is big and (as mentioned by someone) if you set the temp too low it can turn the evap into a block of ice. Mostly, I am just happy that the AC has progressed so far since I put that JC Penney AC into my 68 Nova all those years ago.

          • 0 avatar
            Toad

            Look at the face of the compressor pulley: you can see the center of the pulley spin when the clutch engages the compressor. The center of the pulley will stop spinning when the clutch disengages.

            BTW, that is how you can tell if your AC compressor is actually working.

            The AC system in a car is binary: it is either on or off (just like the HVAC in your home). The ventilation fan in the car keeps running, but the AC cycle on to lower temperature to a set point, then cycles off until the temperature rises to a pre-determined level above that set point.

          • 0 avatar
            golden2husky

            Actually, the A/C compressor on some new cars is not binary at all. Instead of a clutch, there is what is called a swash plate inside the compressor. By varying the angle of the swash plate, the displacement of the compressor varies, and with it the cooling capacity.

        • 0 avatar
          Scoutdude

          No the compressor is not always on in all cars. There are 3 basic systems; Clutch Cycling Fixed Orifice, Thermostatic Expansion Valve and Variable Displacement Compressor. All of them vary the load on the engine to some degree or another.

          Clutch cycling system. (common on Ford, GM and some other brands) This turns on the compressor when the pressure in the system is high enough and once it gets low enough the compressor disengages. This results in an average pressure around 30-35 psi which means that the refrigerant is boiling at around that same temp. With this system the flow rate through the orifice exceeds the cooling demand in most cases. So the evaporator floods and that liquid refrigerant continues to boil and thus absorb heat. Once all the refrigerant has boiled off the pressure rises the clutch kicks back in and the cycle starts all over again. Using recirc greatly drops the compressor on time and improves mpg.

          Thermostatic expansion valve. (common on Chrysler and Japanese) This system regulates the flow of refrigerant to regulate the temp. In this system the compressor runs almost all of the time but the load does vary based on flow. This is the least efficient system and it is actually more efficient the greater the amount of heat that needs to be removed. Which is why it is totally stupid to force recirc on a system like this. However almost all of the systems like this also have a freeze sensor which will disengage the clutch if the TXV cannot prevent the evap temp from dropping too low.

          Variable displacement compressor. This changes the volume of the compressor to meet demand. This is efficient but makes for a much more expensive and troublesome compressor. It is combined with either a fixed orifice or occasionally and thermal expansion valve. Using recirc will increase mpg on one of these systems.

  • avatar
    Jeff Weimer

    What really irritates me is cars that put recirc on without any indication. Case in point – Chevy Cruze (and probably all chevys) with manual A/C. If you select full cold on the dial, it will engage recirc but leave the recirc button indicator off and the message center icon will still say fresh air. Take it down one notch, and fresh will be engaged and everything else works normally.

    • 0 avatar

      I noticed that too on a Cruze rental (of which I’ve had six or seven, at this point). That’s one of my few complaints about the car, because otherwise it’s great. And a couple of times, I’ve had the LTZ, which comes equipped with the automatic A/C.

  • avatar
    skor

    I’ve got an unrelated air re-circulation story, so I guess this is a hijack.

    My geezer neighbor….86 years old at the time….. leased a new Caddy SRX. He’s been buying or leasing Cadillacs for years. He called me over one day, opened the door to the SRX, pointed to the dash and said, “See that?” I was like, “What the touch screen?” “No, that.”, he replied. I had no idea what he was on about. After a while I realized he was pointing to the air re-circulation button. I said, “What about it?” He went on, “This car has the ability to draw in fresh air from the outside, or re-circulate cabin air, blah, blah…”, like he was reading some Cadillac ad copy. I walked him over across the street and opened the door to my 20 year old Mazda, pointed to the air re-circulation button and said, “This car has the ability to to draw in fresh air from the outside…..”

    The geezer honestly believed that air re-circulation was some bit of high-tech that was exclusive to Cadillac, “The Standard of The World”.

  • avatar
    dal20402

    My G8 would pretty much never use recirc in auto mode. If the car was hot it would just crank the fans to max (causing an irritating buzz, consistent with G8 interior build quality) and keep them going until the car cooled down.

    Both of my current Japanese cars will switch to recirc when the air outside is very hot. The Lexus does it intelligently, not using recirc if the air inside is even hotter. The Subaru is more aggressive about using recirc and I occasionally find myself calling for fresh air manually.

  • avatar
    MarkZ06

    I prefer using manual mode in my Z06 since I find that the system usually chooses a faster fan speed than necessary, but I find it annoying that on start-up it forces recirc mode most of the time. My Infiniti M35 had a similar default but I was happy to find out that it could be defeated by entering a special programming mode… No such capability with the Corvette so the drill is start the car…press the recirc off and go.

  • avatar

    Amen on the “it’s on recirc, but won’t admit it”. It was annoying and surprising to learn that my climate-control MINI does this. I smoke a pipe occasionally, and have the window down a bit and extra fan speed to let most of the smoke/smell escape the car. I noticed that if the a/c was engaged, recirc was on, period, now recycling wafts of English Latakia throughout the HVAC system. Not good.

    It *has* a recirc button, presumably for other seasons when it isn’t trying to cool and there’s a stinky diesel about. But it’s ineffective when a/c is on (since, it’s already recirculating).

    My car is “programmed” to allow a few minutes of fresh air, every so often. You can feel and smell the difference after 30-35 minutes of highway operation; there will be a brief interlude (5 mins or so) of moister, fresh-smelling air while it take a break from recirc. Then: it reverts for another 30-35 minute spell.

    I liked it better when I had control of this.

  • avatar

    Funny you mention the X5, and BMWs in general. You see those letters “A” and “M” above the re-circulate button? BMWs have a two-mode re-circulate function. In the automatic mode, the car will draw in outside air until it senses a pollutant in the air, and then it will switch to re-circulated cabin air. In the manual mode, the car re-circulates the cabin air at all times. My (slightly older) X5 does this, but I haven’t tried it yet.

  • avatar
    onyxtape

    How many more years until we can get sensible climate control with a home-thermostat-style temperature switch in our cars? It doesn’t matter at what price point the car is – you set the temperature a couple degrees cooler than the ambient temperature, and the car feels you need to be blasted with Level 5 AC for 40 minutes.

    • 0 avatar
      SoCalMikester

      cadillacs have had that since the 70s or earlier

    • 0 avatar
      dal20402

      My Forester does this sometimes, but none of the other vehicles I have/had with auto climate control did it. The only time they blast you with high fan settings is when they are really trying to cool down a lot of hot air.

      Also, auto temperature mode still works even if you turn down the fan setting. My wife hates high fan settings so that’s how she uses it.

  • avatar
    Roberto G.

    Well, in my experience with my present car the re-circulated AC stinks… at least the first breath that comes out, should you forget the recirculation on when you turn off the ignition. And mind that I’ve removed since long time the cabin filter, because once it let me without AC on a hot August day on the highway to Rome, being it totally full of water and dark like coal, notwithstanding that (according to the bill I paid) it should have been replaced at my first warranty check. Then I understood that the cabin (or pollen) filter is just something to allow a dishonest mechanic to steal from you. Just think a little: what about the pollen when you drive with the open windows? Mind that I had seven cars before this one and all were without AC and cabin filter, and I’m still alive.. moreover I do not suffer from any pollen allergy and that filter is hard as heck to replace, since it goes into a very narrow space inch by inch and I should remove the glove box lid and a crosspiece to reach it. So bye to the damned cabin filter. But nevertheless (cabin filter yes or not), the re-circulated AC stinks of mold and old tennis shoes, so I try to use it the least as possible and only when outer temperature is higher than 100°F.

  • avatar
    sgeffe

    Automatic Climate Control FTW! (Even if I turn off the A/C most of the time.)

    Does the job perfectly! I set it at 68 degrees during the winter, 70 degrees when the normal high temperature in my area hits 55 degress (around the time I can shut the heat off in the house), and 72 degrees when the normal temperature hits 70 degrees, so the fan doesn’t run as hard. OCD? You betcha! (No flames, please — my Nomex underwear is at the cleaners!) In the summer, I’ll run full “AUTO,” which runs the A/C most of the time. (Unless the temperature and humidity are pleasant enough that I can get by with the sunroof and/or windows open.)

    Without the ACC, I fiddle with all the temperature and fan settings CONSTANTLY, especially in spring and fall; in summer, I just leave the temperature at full cold. (The mode setting is usually dash and feet.)

    ACC takes the guesswork out of stuff for me — mostly dash vents in summer, bi-level spring and fall, feet in the winter. My first two Accords (and two cars with ACC) would switch over to fresh air a bit sooner than my 2013, which seems to put a bigger emphasis on fuel economy; you can practically hold the entire A/C unit of that car in the palm of your hand, so capacity plays a factor! It stays in Recirc most of the time, and lowering the fan speed as needed, to obtain the set temperature; only if it’s been running a good while (45 minutes on a sunny day with no direct sun on the upper-dash sensor, or on a cooler, cloudy, but humid morning), will it go to fresh air. When I come up to the car as I walk to my parking lot, I can lower the windows and open the sunroof with the remote, then once I close everything up again and start the car, I set the system to OSA to cool things down for a minute, then hit “AUTO,” and touch nothing until I shut the car down. (My parents always taught me to turn any A/C off before shutting down, in order to lessen the strain on the engine when starting up later; I think my Accord will wait for an ignition signal before it activates the compressor, so that may be a moot point.)

  • avatar
    Volt 230

    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.

  • avatar
    claytori

    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.

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