# Auto Air Conditioning Logic?



## aadams1278 (Jul 8, 2018)

I've been trying to figure out the logic of the auto mode of the air conditioning. It seems that auto mode only enables recirculation mode rarely, if ever. A previous car I had would specifically come on with recirculation mode enabled by default because that's more efficient for the air conditioning to cool cabin air that has already been cooled rather than the hotter outside air. It seems the model 3 doesn't follow this logic, or at least not often, and this seems counter intuitive to me. 

Is there some other efficiency loss by running the recirculation mode i'm not aware of that Tesla is trying to avoid?

Also, I've come across what may be an omission in their logic for the rear seats with child car seats using the car seat anchors. I have two car seats in the rear of my model 3 and they aren't heavy enough to trigger the weight sensor in the seat. I haven't been able to confirm this 100% since I haven't taken the car seats out to test, but I suspect the rear air vents automatically come on if the weight sensors in the back seat detect a person. Because the car seats are not heavy enough to trigger a sensor, the rear air vents never come on when I have my kids in the back seat. I'm aware of it, and just manually turn on the rear vents when they are with me for now.

On that same topic, I have noticed with a different style car seat that we recently replaced, that it WOULD detect weight in that seat when accelerating or decelerating just right. This would trigger a "seatbelt unbuckled" alert on the screen for a second or two and then go away, rather randomly during a drive.

These two minor issues combined have made me think it would be nice if there was some sort of car seat mode which tells the car that something is there even if the seat sensor doesn't detect enough weight (for air conditioning purposes) and to ignore the unfastened seatbelt alert if it does detect weight since the car seat doesn't use that seatbelt.

Anyone else run into something like this with child car seats?


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## SoFlaModel3 (Apr 15, 2017)

aadams1278 said:


> I've been trying to figure out the logic of the auto mode of the air conditioning. It seems that auto mode only enables recirculation mode rarely, if ever. A previous car I had would specifically come on with recirculation mode enabled by default because that's more efficient for the air conditioning to cool cabin air that has already been cooled rather than the hotter outside air. It seems the model 3 doesn't follow this logic, or at least not often, and this seems counter intuitive to me.
> 
> Is there some other efficiency loss by running the recirculation mode i'm not aware of that Tesla is trying to avoid?
> 
> ...


I have the opposite. Rear air on without the kids on their seats. I think it's because I have tethered the seats really snug.

One giveaway for me was that the rear seat belt sensor used to trip for me. I had to run the seatbelt behind the car seat and connect it to avoid that warning.


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## Rich M (Jul 28, 2017)

My experience with the auto AC is that it runs in fresh air mode for the first few minutes of getting into a hot car, then switches to recirculate.


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## aadams1278 (Jul 8, 2018)

Rich M said:


> My experience with the auto AC is that it runs in fresh air mode for the first few minutes of getting into a hot car, then switches to recirculate.


Interesting, I'll have to keep a look out for that.


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## Rich M (Jul 28, 2017)

I typically turn off both AC and recirculate a few minutes before arriving home (long standing habit to prevent musty vents) and in auto mode recirculate always seems to be checked on hot days.


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## Guest (Aug 24, 2018)

Some cars switch recirculation ON if there is very high demand on cooling.
For example you start the vehicle and interior temperature is very high, exterior temperature is also high.
To cool the cabin fast, it's best to use interior air until fan speed drops way down.

If fan speed is high and exterior temperature is also high, cooled air might not be chilly at all.

Premium vehicles with premium HVAC system also have humidity sensor on the windscreen and in case
it's starting to fog up, recirculation is reduced, at least temporarily. 
AFAIK Tesla does NOT have advanced HVAC functionality on any vehicle.
AFAIK Teslas do not have VOC sensor (air pollutants) for recirculation control.


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## PNWmisty (Aug 19, 2017)

arnis said:


> For example you start the vehicle and interior temperature is very high, exterior temperature is also high.
> To cool the cabin fast, it's best to use interior air until fan speed drops way down.


That's incorrect. When you enter a vehicle sitting in the sun the interior temperature is much higher than the exterior temperature and it's better to use the cooler exterior air supply until the cabin is cooled below the outside air temperature. A car's interior can easily reach 120F even when the outside temperature is only 70 degrees. It's much better to draw the initial air from outside because it's already cooler.

Also, all cars have flow-through ventilation. The outside air that is drawn into the cabin is cooled through the A/C condensor and exits the rear of the vehicle through rear quarter panel vents with backflow prevention flaps. This front to back air movement flushes the hotter inside air out. Opening the windows the first minute or two of the drive can assist in removing the hotter inside air. When recirculation is enabled, most of the air travelling through the A/C condensor is drawn from the footwells and thus slows down this front to back air movement. So rear seat occupants are not cooled as effectively. Only when the hot air has been flushed from the cabin is it productive to switch to recirculate mode.


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## aadams1278 (Jul 8, 2018)

My Model 3 is currently in service so I haven't been able to test it, but the Model S P85D loaner I have doesn't seem to use this logic of recirculate off initially and then on after a while. I drove a 1 hour drive after being parked at the airport in 85 degree temperatures. I watched the auto mode during the drive and it never automatically switched recirculate mode on. 

While this doesn't necessarily prove anything toward the original question which was about the Model 3, it is still confusing as to why recirculate isn't being used.


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## garsh (Apr 4, 2016)

aadams1278 said:


> ...it is still confusing as to why recirculate isn't being used.


Recirculation is only desirable for maximum cooling. Generally, you want outside air to replace the oxygen being used in the cabin.


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## Guest (Aug 26, 2018)

PNWmisty said:


> That's incorrect. When you enter a vehicle sitting in the sun the interior temperature is much higher than the exterior temperature and it's better to use the cooler exterior air supply until the cabin is cooled below the outside air temperature. A car's interior can easily reach 120F even when the outside temperature is only 70 degrees. It's much better to draw the initial air from outside because it's already cooler.


Yes. In case one doesn't let hot air out. Though usually people get in through the doors and majority of hot air leaves very fast. Though objects inside the vehicle are still hot, that doesn't play significant role any more. Air outside might be somewhat cooler but not when it gets into HVAC inlet. Hood, windshield and plastics around there are very hot. and will stay hot for longer.
There is also a thing with humidity. Interior air is usually way dryer than exterior air and heating that dry air is faster.

I agree that in case there are people in the back, more chilled air gets to the back seat if recirculation is off.
And if it is not that hot outside. But when it is not hot outside, usually, people are smart enough to keep the doors open for 20 seconds.
Though as a thumb rule, recirculation should be switched on when fan is high. Air itself will get cold enough inside the vehicle
very fast. I mean like a minute. And before that first minute passes, recirculation air intake is already colder than hot exterior intake.
And cabin overheat protection doesn't allow inside air to get very hot (40C) anyway.


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## aadams1278 (Jul 8, 2018)

So let's ignore the first few minutes after getting in a hot car. Assume the car is already cooled off, regardless of the method. Now, from a strictly energy efficiency standpoint, wouldn't it be more efficient on the battery and deplete less range, if recirculation was used during a long drive? I am assuming that there is no additional energy requirement for the AC system to use recirculation mode than fresh air mode. No additional fans or anything, just a door that closes off exterior air of some kind?

I'm thinking (and I could be wrong since I am not an Air Conditioning specialist) that to cool the already cooled interior air, is much easier (less energy needed) than cooling hotter exterior air to the same temperature? If i'm thinking correctly, this should apply any time the outside air is hotter than the inside air by basically any amount.


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## Guest (Aug 26, 2018)

garsh said:


> Generally, you want outside air to replace the oxygen being used in the cabin.





aadams1278 said:


> Now, from a strictly energy efficiency standpoint, wouldn't it be more efficient on the battery and deplete less range, if recirculation was used during a long drive?
> 
> I'm thinking (and I could be wrong since I am not an Air Conditioning specialist) that to cool the already cooled interior air, is much easier (less energy needed) than cooling hotter exterior air to the same temperature? If i'm thinking correctly, this should apply any time the outside air is hotter than the inside air by basically any amount.


Yes, it is true. If we look it from efficiency side, recirculation is way more efficient if exterior air is even slightly warmer than desired (except if there are many occupants, as one person is a "heater" of around 200W).
But, depending on HVAC logic, fresh air intake might be absolutely locked. And in this case, after around 10-15 minutes, air inside the cabin gets "old". With occupant's moist smelly exhaust (and CO2), it gets more and more uncomfortable. In that case, fresh air mode should be switched on for few minutes to purge "depleted" air. More advanced HVAC systems adjust recirculation mode according to time logic and/or exterior air quality and/or interior humidity data. For example, Nissan Leaf has very basic system of switching recirculation on in case of high AC load and to partial recirculation (30%) to save some juice later on. There is no need for 100% fresh mode in most weather scenarios. But if cooling/heating is switched OFF, fresh air mode is selected automatically. This is the most efficient way to drive. 
And when it is +14C up to +22...24C it is totally acceptable to drive without any heating/cooling. BMW has more complex system that adjusts recirculation mode with infinite steps according to many datapoints.

If cabin is already chilled down and we are talking about LOONG driving (30min or more), AC load is very light IF fan speed is slow. We talk like 10-15% of maximum. And in case of low air "purge" rate, it's "efficient enough" to keep fresh flap open (or partially if possible) for the whole trip (except first few minutes).
Though it requires occupants to be reasonable people and NOT CRANK AC TO "LO". Which, by my standards, is totally ridiculous behavior in any scenario.


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## PNWmisty (Aug 19, 2017)

arnis said:


> Air outside might be somewhat cooler but not when it gets into HVAC inlet. Hood, windshield and plastics around there are very hot. and will stay hot for longer.


I think the HVAC inlet heat is mostly an issue with ICE cars due to engine heat soak, not so much with EV's. Kepp in mind, the interior air inlet (recirculate inlet) will probably be as hot or hotter than the outside intake. Your other points are good and point to a relatively short period in which the fresh air intake should be used when the car has been in the sun. Typically 1-3 minutes. I don't use recirculate unless outside temperatures are over 90F or so and there is only one or two occupants. The more occupants, the less advantageous recirculate is (both from a fresh air standpoint and also cooling efficiency). People can put out surprising amounts of heat and humidity that needs to be carried away or cooled.


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## aadams1278 (Jul 8, 2018)

arnis said:


> ... But, depending on HVAC logic, fresh air intake might be absolutely locked. And in this case, after around 10-15 minutes, air inside the cabin gets "old". With occupant's moist smelly exhaust (and CO2), it gets more and more uncomfortable. In that case, fresh air mode should be switched on for few minutes to purge "depleted" air. More advanced HVAC systems adjust recirculation mode according to time logic and/or exterior air quality and/or interior humidity data. For example, Nissan Leaf has very basic system of switching recirculation on in case of high AC load and to partial recirculation (30%) to save some juice later on. There is no need for 100% fresh mode in most weather scenarios. But if cooling/heating is switched OFF, fresh air mode is selected automatically. This is the most efficient way to drive.
> And when it is +14C up to +22...24C it is totally acceptable to drive without any heating/cooling. BMW has more complex system that adjusts recirculation mode with infinite steps according to many datapoints. ...


Sounds like you know way more about different HVAC systems than I do. So based on everything you're saying here, where does the Tesla HVAC system fall on that list? "Advanced"? and are they the same on the models S/3/X? Or put another way, is Tesla HVAC doing a good job or does it have room to improve?


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## Guest (Aug 26, 2018)

aadams1278 said:


> Tesla HVAC system fall on that list? "Advanced"?


Unfortunately I've not seen wiring diagrams nor HVAC module live parameters to be absolutely sure, but, as far as I have witnessed, Tesla's HVAC system is very basic with LOTS of room to improve. There are some impressive features like Model 3 face vent and Model S/X HEPA filtration system, but both features lack autonomy, making them non-transparent and "manual" (exactly the opposite of what German luxury brands try to do).

Tesla is a newcomer and they can't invent everything in 7 days. Though I'm disappointed that they don't learn from what others have done. "There is no need to invent a bicycle philosophy" is what I would expect.
*Basic HVAC features that should be available on all Teslas:*
Air quality sensor - this sensor should be mounted in the fresh air intake duct. It measures pollutants that are not healthy for occupants. Also recirculation flap motor that is "high speed" if not fast enough at the moment.
Automatically heated side mirrors with PWM control according to exterior temp data - I've not confirmed this function is available on Teslas. Can anybody confirm that mist on exterior mirrors clears up within half a minute after vehicle switched on and rear window defrost has NOT been activated? This is a basic safety feature.
Face vent temperature adjustment - this is extremely unacceptable on S/X - practically every slightly premium vehicle has the ability to blow chilly air to face and warmer air to windscreen and feet. You know that red-blue knob next to face vents. Well.. Tesla's don't have that.
Driver only mode - many EV's have that to maximize efficiency. As the name states, all flaps are closed except for driver.
Adjustable face vents -AFAIK, Tesla is the only vehicle that doesn't have that... Adjustment for air speed for every face vent.
Heat pump - this is the biggest drawback. So big that I actually consider skipping Model 3 due to that. It's not just about the range. It's about the fact how much electricity is wasted during winter period after years of vehicle ownership.
Steering wheel heating - most EV's have that. Not Tesla Model 3.
AP hardware maintenance - this is not interior but it applies to this category. No Tesla can clean cameras. This is totally unacceptable in bad weather. If AP 3.0 doesn't add that functionality, this will be a big problem as soon as it moves out of sunny California to anywhere it snows. I'm afraid it will even dip the stock as it is actually something that incapacitates Tesla's AP hardware from ever reaching, by definition, Level 4 autonomy. Ever. As this is so complex problem, retrofit will not be offered. Also problems with radar and snow buildup. Latest problem is with Model 3 and missing rain sensor. It appears camera is unable to reliably measure windscreen transparency and Tesla appears not to be able to fix it with software. I would also say that rain sensors on S/X are not predictable enough.

*Features that Model S and X should have (at least with premium pack):*

Solar radiation sensor - compensation for sunny weather - more chilly airflow for front passenger upper region in AUTO mode.
Steering wheel and seat heating with compensation and memory function - totally software driven feature. I've dreamed about this for like 5 years and just lately found out that this has been "invented". But not by Tesla.




What's the idea: no need to click those buttons for half a year when it is very hot/cold outside. Just set it and forget. With profile memory. Also notice option to adjust how much heat goes to the back vs bottom of the seat.
Actual 3-4 zone climate control - There appears to be somewhat 3-rd zone on some Model X's, but it is uncommon for Model S type vehicle to have near to nothing for rear passengers. That includes temp sensors and also no adjustment panel if it even had some adjustment. This is a problem of minimalistic Tesla design - nothing in the back.
Mist sensor on windscreen - this adjusts how much air is required for windscreen to keep it clear while keeping it as low as possible.
Electric windscreen defrost - many manufacturers cheap out on this. This is not nice for Model X's awesome windscreen.
Sunblinds everywhere - Yes I know that most owners in California are happy. But it's still way too non-premium to have NOTHING for privacy and radiation problem (most go with aftermarket tinting for rear of the vehicle). Model S lacks blinds on the rear window (electrically actuated) and also lacks blinds on the rear doors (manual or electric) and also lacks blinds on the panoramic roof (electric). Not just for sun, also thermal insulation from cold. There is a massive thermal leak through all these surfaces, especially roof. Model X also lacks blind on windscreen extension. All these things skyrocket climate draw that all adds up when there is a million Teslas on the roads all around the world.

Most of these things boil down to vehicle simplification - which is good. But many things, like I said, basic features, went too far and are not "OK" for electric vehicle and lifestyle it brings with it.

It's getting late here. I could go on for some more, but I think I made it clear, that HVAC is far from good. If all these features.... ALL of them, would be offered at least on Model X/S with premium pack, I would consider it as a premium HVAC system with no drawbacks. But even then it would not be groundbreaking (most features I listed are something I took from BMW). For that, Model 3 should follow occupants position with interior camera and automatically direct airflow of facevent. Model X should adjust windscreen blind position automatically according to sun position so it would shade eyes if driven towards the sun. Model S should defrost panoramic roof


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## aadams1278 (Jul 8, 2018)

arnis said:


> AP hardware maintenance - this is not interior but it applies to this category. No Tesla can clean cameras. This is totally unacceptable in bad weather. If AP 3.0 doesn't add that functionality, this will be a big problem as soon as it moves out of sunny California to anywhere it snows.


I came across this recently. Looks like it may address this concern above.
https://teslaownersonline.com/threads/heated-camera-picture.5418/


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## Guest (Aug 26, 2018)

I mean clean as in clean from pollutants that are left after snow melts. Imagine driving with windscreen defrost but no wipers. It's insufficient for anything besides fogging up due to condensation.


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## aadams1278 (Jul 8, 2018)

Back to my original post, I've noticed something very interesting. My Model 3 has been in service for the past 2 weeks, upon getting it back a few days ago, there were two new software updates since I dropped it off. Now I've been driving it for a few days and it has remained on recirculation mode constantly during every drive. It's been high humidity and in the mid 90s. I'm wondering if one of those updates changed something in the AC logic? 

Either way, I'm liking what i'm seeing now...


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## TrevP (Oct 20, 2015)

The fogging issue is something I struggled with when I initially got my car last winter but I found the right settings that prevent it. Basically forget about "auto" mode in the winter or moist days, it doesn't work.


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## Guest (Dec 12, 2018)

TrevP said:


> The fogging issue is something I struggled with when I initially got my car last winter but I found the right settings that prevent it. Basically forget about "auto" mode in the winter or moist days, it doesn't work.


Emmm. It's actually a bug.
ALL BMW's since at least 1998 (with automatic climate control) direct airflow to windscreen in auto mode if it's cold outside and warm air is requested.
Also applies to Nissan Leaf - feet and defrost, no air through face vents. 
So I would recommend to report a bug.

PS: slightly more advanced HVAC systems (like BMW since 2004 and 5-series or higher) have windscreen mist sensor. If it senses condensation, it will signal HVAC AUTO algorithm to add more airflow to windscreen.

BTW: Again, mirrors should be defrosted automatically (and separately from rear window) like ALL bimmers since 1999.
So many convenience features are missing on these high tech Teslas


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## PaulK (Oct 2, 2017)

I think the “auto” behavior has changed a few times over the past six months of firmware updates. 

Presently on v9 if you manually turn off A/C, that disables auto mode. Meaning, if it gets colder it won’t direct warmer air to the floor, and if it gets colder it won’t increase the fan speed. 

Most cars will remain in AUTO mode even if A/C is commanded OFF. And if it’s warm the fan will start blasting, as a hint to turn the A/C back on so it can catch up. 

For efficiency, I want to be able to decide if the humidity is low enough, and I don’t need cooling, to save the power. I also prefer the old graphic which showed red when the car was expending energy to heat the cabin. 

I’ll email Tesla with my wish list, please do the same if you agree.


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## Defjukie (Sep 28, 2017)

I agree. I'd really like to know, at a glance, whether the heat is on at all in my car.


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## G78M3 (Sep 5, 2018)

PNWmisty said:


> Also, all cars have flow-through ventilation. The outside air that is drawn into the cabin is cooled through the A/C condensor and exits the rear of the vehicle through rear quarter panel vents with backflow prevention flaps.


Does anyone know where are these rear quarter back panel vents in the model 3?


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## Guest (Feb 3, 2019)

G78M3 said:


> Does anyone know where are these rear quarter back panel vents in the model 3?


Huge rectangular hole between rear wheel and bumper.


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