Does the recirculate air option really do anything?
 

I have the circular arrow button that is supposed to recirculate the air. From memory, the manual says don't use it for more than 20 minutes, or maybe it shuts off after 20 minutes. It's supposed to be when you're coming up into a dusty area or where they're doing road work, so you don't get the dust inside the car while using the blower fan.

On a Jap car, when you do that, you can hear the air panels opening or closing, it helps the a/c cool down quicker, you can keep it on all the time, and when you put it in the recirculate mode, you can really hear and feel the fan increase. On my MB ... I don't hear or see any difference. No increased fan sound. No blower fan panels moving to open or block off air from the outside. Just ... nothing.

So, I have to ask, does this mode actually do anything on a MB?

1991 300d, 90k

I do believe that it does something. On the japanese car, the fans don't actually increase, all that happens is that air gets sucked in from inside the car instead of from outside of the car and you can hear the air getting sucked in. It's not exactly quiet.

On the mercedes the recirculate mode does work (at least in my car it does), but I would imagine that it's quiet because they designed it that way in an effort to make a luxury car.

The recirculate mode is only intended to be used for short periods of time. When you get in the car and it's very hot in the car then you might use the recirculate to try and cool the inside air of the car quickly for example. After that the A/C system would be working to try and cool already fairly cold air. This is no good. It's better for the A/C system to use the normal air mode as opposed to the recircuate.

It's also better not to use the recirculating mode when heating the car in the winter either, but for different reasons.

I disagree. Imagine inside car temp is 120F, if on recirc, the A/C must try and cool 120F air down. If not on recirc, the hot air will be expelled thru the vents, and the A/C will be cooling incoming cooler air at, say 90F (or whatever ambient is). Further more, A/C systems are equipped with a thermostat that reads the evaporator temp and adjusts A/C accordingly. Therefore, after the car has cooled down, it is easier for the A/C system to keep the car cool by recirc already cooled air. Naturally, some fresh air is bled in in most designs, regardless of recirc on or off.

Same logic applies here as does to A/C, but supplying heat is not as parasitical drain on engine power like the A/C compressor. The only caveat to using recirc all the time is if the system is soooo tight, that infiltration of "bad" air, like an exhaust leak, could pose a danger. Recirc of heat in the winter can allow a build-up of moisture from passengers' breath, etc but should not present a problem except in extreme cases (back seat at the drive-in?)...

I can't testify as to how the 300D should work, but I can tell you that you should be able to her the vents open/close if the system is working, although you will have to listen very closely. On the 560Sl, when the engine is not running, the cowl vents are normally closed. When the car is running, and the climate control system is on, the default is for vacuum to be applied to the cowl vent flaps, opening up for introduction of fresh air. When the recirc switch is turned on, the vacuum is bled off of the pods, and the flaps close under spring pressure, therefore closing off most of the outside air from entering the car thru the cowl vents. According to my manual, even when the cowl vents are closed (the system running in recirc mode), only 80% of the air is recirculated, with 20% still introduced thru the vents...ideally. Since spring pressure closes the vents, a leak or other failure in the recirc circuit results in the cowl vents being closed all the time (full-time recirculation). On a 560, you can remove the glovebox and see this in action very easily... Perhaps a 300 is similar. I imagine it so, but cannot say for sure.

also to add something. on most mercedes there is an ambient temp sensor, if it reads that the air entering the cabin is above a certain temp it will automatically close the recirc flap on it's own to allow for more efficient cooling

it works on my 350sdl. and it is a feature that i very much appreciate. i can imagine siturations in which it might not be a benefit, such as pointed out above when on a sunny day it is hotter inside than out. in that case i open the windows for a few minutes while i sstart driving to flush out the hot air.

i also use it on the highway when i come up behind an old van or pickup to avoid breathing the increased smelly polluted exhaust. or at a light when one is in front or beside me. works like a charm.

tom w

recirc air
 

your 91 is a 124 body and it has two fresh air flaps above the fan housing.it is common for the diaphrams that operate these doors to break and they are an almost impossible job to replace without removing dash.if you look in front of the windshield you will see the vents thru which the air is pulled.try closing these vents with duct tape[black so it won't show] it is also poss that another diaphram is leaking badly so there is no vacuum for the recirc doors to operate[eg.defrost]if you want to try to repair call me and i will talk you thru it.david poole dallas tx 214-9567007

That is the exact opposite of what you should do. If you re-circ on a hot day as you mentioned it actually cools faster than sucking in the hot air from outside the car. This works due to the fact that you are just moving the inside air and cooling it, instead of sucking in more air and then forcing the hot air out. Also in the manual I believe it recommends both re-circ and to roll the windows down when you first get in to let out the hot air. On every car I have ever driven they commonly call the re-circ button the MAX Air Button.

And since the re-circed air is cooling and then sucking in more air from inside the car it will actually cool the interior faster than normal mode would do. As the air cools more and more it is less strain on the AC System, where if the outside air is 95 and you are sucking it in the AC System is working at the same load no matter what the inside temp is until of course you achieve the proper temp and it allows the Compressor to cycle properly.

Jamie,

I realize it looked like I was suggesting using recirc full-time. That is not the case. I was trying to point out that re-circ should not be used initially when the car is soooooo hot, but that , after it cools, it is good to use recirc. Kinda got lost in the translation, I'm afraid... I see your point, but think the window idea is best of all... purge the excess hot air out quickly, let the car cool a bit, then switch to recirc.

When to use recirc
 

I think in short, the recirc would be used as follows:

1) If the car is sitting outside in the sun, the interior is 120, and it's 90 outside, you should let all the hot air out of the car, and when it's down to ambient temp of say 90 degrees, then you should use the recirc. The reason is, it's taking the same air and cooling it down from 90 to say 80, to 70, to 60 degrees, as opposed to taking it from the outside and cooling from 90 to 80, from 90 to 80, from 90 to 80.

2) If driving past bulldozers stirring up a lot of dust or something, where you get to avoid having at least some of that dust in the car cabin.

I'm just surprised they put a 20 minute max on that button. Any Jap car, you can have Max on all day.

1991 300d

I belive in most MBs if you press the recirc button you get 30 min of recirc if the compressor is engaged and 5 min if the compressor is not engaged. Something like that. The LED will go off when MB thinks you've had recirc on long enough :)

Sixto

i suspect it is a safety thing.

if you fall asleep it insures that you get fresh air after 30 minutes. if awake you can reset it.

tom w

this explains why mine seems to only work for a few minutes. i rarely use my AC and kept wondering why it worked but for such a short period. i thought it was faulty.

thanks

Since you're driving a kraut car, why make the comparison?

Actually, here in the midwest where it sometimes gets very cold, it's not uncommon to see drivers (of non-MB vehicles) driving in the winter with their windows completely fogged up from the inside simply because they've left their recirc button on all year. MB engineers very wisely designed the system to not allow the recirc setting to stay on > 10 min when ambient temperature is cold.

You'd be surprised how much water vapor each of us exhales; in the small space of a closed car, it dramatically raises the relative humidity if you don't bring in outside air in the winter.