Car climate control sometimes starts on recirculated air?

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#1444430 9-Dec-2015 12:10

I dont give a crap about efficiancy of it. I dont like breathing exhaust and crap so don't open the windows when in urban traffic.

Hell, I even have the AC on in the convertable when the roof is down because it helps counter the sun. It uses so little that its not worth worrying about.

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Jeeves

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#1444529 9-Dec-2015 14:41

MadEngineer:
scuwp: Because it's easier to cool a carload of air faster than to cool all the air in the atmosphere. Initially it seems like a bad idea but it really does work once the initial cooling process starts.
this would make sense if a) the air con also came on with the recirculate function and b) if if was efficient to use ac, should I turn it on rather than open the windows. But generally it doesn't

You said yourself the AC comes on when it's in auto mode. If it's not in auto mode - then it's up to you to turn the AC and recirc on or off. (Hence the name, you know...automatic.).

Anyway, my guess is that they do this when they detect the cabin temp is hotter than the outside air. If AC is in full auto mode, it probably has the vents set to the windscreen, and recirc on. This would make sure cold air gets sent to the upper part of the cabin, and then sucked back out the recirc vents which are in the footwells. This will create a natural cycle to get the cabin air moving quickly.

While the first minute or so it might make more sense to use outside air as it's generally cooler - overall it will be quicker to cool the cabin air first and then introduce outside air.

There's also the fact that the fans will be able to pass air over the cooling coils quicker when on recirc, because it doesn't have to push against the positive pressure in the cabin like it does when it's trying to introduce outside air into a somewhat sealed cabin.
It's like trying to blow up a balloon vs blowing through a tube - the latter is much easier to do. (Caveat...I don't actually know if cars have an AC 'exhaust' vent for cabin air to exit the car out of... I am assuming they don't).

MadEngineer

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#1444584 9-Dec-2015 15:53

Not sure where i said the ac comes on - in the first post I said it only comes on when the temp is set to the lowest setting (and also when I switch the climate control from auto to windscreen)

AC and climate control are two different things. Climate control has an auto setting. AC is either on or off with a button and can get turned on automatically by the climate controls depending on the setting.

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MadEngineer

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#1444607 9-Dec-2015 16:24

http://www.manualslib.com/manual/987932/Mitsubishi-2013-Outlander.html?page=404#manual

It's pretty much the same as this. That page says holding down the ac button for 10 secs toggles the auto ac function. The manual for my car doesn't say this but I'll try it for the recirculated air button ...

You're not on Atlantis anymore, Duncan Idaho.

mattwnz

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#1444619 9-Dec-2015 16:29

joker97: Maybe you should buy a Ford. (the Non Aussie ones). THey all default to using outside air no matter what you do the last time you used the car.

Same with Mazda. Toyotas though always start on recirculated air no matter what the last setting was (unless you get one made by Mazda that has been rebadged), which is a pain.

MadEngineer

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#1444621 9-Dec-2015 16:31

joker97: Maybe you should buy a Ford. (the Non Aussie ones). THey all default to using outside air no matter what you do the last time you used the car.

hah yeah all our work fleet is ford, and no car seems to behave the same. Similar things occur - despite the car set to a cool temperature or the dial in the blue, the ac doesn't always come on and you wonder why things are getting toasty. Oh and the annoying radio that doesn't turn off with the ignition until you open a door. Just great when you pull over and key off for a phone call.

Re: above for Mazda - yeah I googled this and it's a common behaviour.

You're not on Atlantis anymore, Duncan Idaho.

Jeeves

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#1444622 9-Dec-2015 16:31

Weird - never seen a car that turns the AC on or off dependant on what temp you set. Must be a new thing to reduce compressor usage.

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MadEngineer

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#1444676 9-Dec-2015 18:28

The manual I linked to states:

Changing the settings Press the air selection switch for about 10 seconds or longer. When the setting has changed, the system will beep and the indicator light will flash. • When the setting has changed from enabled to disabled, the system will beep three times and the indicator will flash three times. • When the setting has changed from disabled to enabled, the system will beep two times and the indicator light will flash three times.

... while i didn't get any beeps or flashes, the AC now always comes on with the recirculate, which solves the problem, and adjusting the temperature up and down turns both the recirculate and AC off and on

You're not on Atlantis anymore, Duncan Idaho.

kiwirock

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#1446918 10-Dec-2015 08:57

Jeeves: Weird - never seen a car that turns the AC on or off dependent on what temp you set. Must be a new thing to reduce compressor usage.

My last car (Nissan Stagea) always had the compressor locked to the drive pulley when the A/C was on. No periodic idle adjustment surprises at extremely low speeds.

I've just replaced it with a Subaru Outback and the A/C only clicks in when needed. I did notice though yesterday if the outside air is the same or cooler than the cabin, the A/C switched completely off after about 10 minutes.

Aredwood

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#1448450 11-Dec-2015 00:30

MikeAqua: I have read (but can't find again) a report on research undertaken in Europe where the authors concluded that people are more like to crash if they have their A/C set to recirc. The author's believed that reduced oxygen levels affected concentration and exacerbated fatigue. They recommended that A/C be designed to switch to fresh air after 15 minutes (or something like that) on recirc.

My Mercedes Vito always defaults to outside air whenever you start it. And if you switch it to recirc, it will switch back to outside air after 1/2 an hour. Which is really annoying when you are in rush hour traffic. A/C on or off makes no difference to this behaviour. Friends VW Passat was the same.

Have managed to locate the wiring going to the recirc control solenoid. So a little project this weekend will be connecting it to a switch. Instead of the climate control unit. Have already done this for the heat control solenoid. As it would start pulsing the heat solenoid valve at a low duty cycle after the aircon had been on for 5 min. Which defeats the purpose of aircon. Merc climate control has clearly been designed by a lawyer instead of an engineer.

mdooher

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#1448514 11-Dec-2015 08:38

Aredwood:
MikeAqua: I have read (but can't find again) a report on research undertaken in Europe where the authors concluded that people are more like to crash if they have their A/C set to recirc. The author's believed that reduced oxygen levels affected concentration and exacerbated fatigue. They recommended that A/C be designed to switch to fresh air after 15 minutes (or something like that) on recirc.

My Mercedes Vito always defaults to outside air whenever you start it. And if you switch it to recirc, it will switch back to outside air after 1/2 an hour. Which is really annoying when you are in rush hour traffic. A/C on or off makes no difference to this behaviour. Friends VW Passat was the same.

Have managed to locate the wiring going to the recirc control solenoid. So a little project this weekend will be connecting it to a switch. Instead of the climate control unit. Have already done this for the heat control solenoid. As it would start pulsing the heat solenoid valve at a low duty cycle after the aircon had been on for 5 min. Which defeats the purpose of aircon. Merc climate control has clearly been designed by a lawyer instead of an engineer.

Does it not have an air quality sensor? perhaps it needs the filter cleaning?

edit: It appears not, (that I can find) the only Mercedes that doesn't have one?

Matthew

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#1448569 11-Dec-2015 09:32

It is merely because it is easier to rapidly cool/heat the fixed volume of air in the car in most circumstances.

Obviously humans have brains and can determine whether to override that default...

Some systems are equipped with better/more sensors and brains. I had a BMW that could detect air pollutants and automatically recirculate to prevent them entering the car.

Rolls Royce specify that you should be able to drive from the Arctic to the Equator and experience no need to alter the climate control at all.

MadEngineer

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#1448625 11-Dec-2015 10:44

Yes, I should note that the air con in this car (2005 vrx diamante) is awesome and quickly deals to a hot cabin.

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#1448710 11-Dec-2015 13:10

Yay - got the AC re-gassed in my '88 Safari today. I thought it might have been borked as the AC compressor clutch didn't seem to be engaging, but the tech informed me that it might have an electrical cut-out pressure switch so the compressor can't run dry - so may just work when it had been re-gassed - and it did. $120 well spent. Their system checks for leaks, so hopefully good for another few years.
A/C in cars ain't what it used to be. Pre Montreal Protocol when they gassed them with CFCs and people cared less about fuel economy - those were the days. I had Falcon/Fairmonts in Aus that would pump cloudy streams of ice-cold freezing vapour from the vents like a blast-freezer - phenomenally good on 40 deg C days, and very fast cooling down the car left parked in the sun at midday. Since the late '80s, A/C was not the same in any new cars I've had.

mdooher

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#1448855 11-Dec-2015 19:17

Fred99: Yay - got the AC re-gassed in my '88 Safari today. I thought it might have been borked as the AC compressor clutch didn't seem to be engaging, but the tech informed me that it might have an electrical cut-out pressure switch so the compressor can't run dry - so may just work when it had been re-gassed - and it did. $120 well spent. Their system checks for leaks, so hopefully good for another few years.
A/C in cars ain't what it used to be. Pre Montreal Protocol when they gassed them with CFCs and people cared less about fuel economy - those were the days. I had Falcon/Fairmonts in Aus that would pump cloudy streams of ice-cold freezing vapour from the vents like a blast-freezer - phenomenally good on 40 deg C days, and very fast cooling down the car left parked in the sun at midday. Since the late '80s, A/C was not the same in any new cars I've had.

In 1989 I had a Mazda 626 Capella jap import .It had a vent under the steering wheel that when opened on full cold wearing shorts resulted in instantaneous hypothermia. It was f__king awesome.

Matthew

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