Auto defogging windshield

[dakhlkt22]

I'm confused about how defogging the front windshield works on a 2023 M3 RWD.

I started out just trying to figure out if there is a way to just get unheated/uncooled fresh air through the "defogging" vents that point towards the front windshield.

However, it seems like Tesla does not allow fresh air per se. It only seems to allow you to select Auto if you click on the defogging button, which I guess would be fine except for the fact that I am actually still manually setting the temperature of the air that comes out. The fan speed will change depending on if the temperature I set is lower or higher than the ambient outdoor temperature. So my current hack is to check the outdoor air temperature and hope that Tesla is not using energy to heat or cool the air coming in. But it seems strange that Tesla does not simply have a non-recirculating fresh air option or even a recirculating air option for windshield defogging.

 

[Big Earl]

The concepts of recirculating and windshield defogging are not compatible.

The system will automatically defog based on information from the temp and humidity sensor located in the rear view mirror housing. Automatic mode will provide the best all-around performance with a modest energy consumption penalty.

If you want fresh air, with no heat or cooling, directed only at the windshield, configure your settings like this (see pic). It will likely be less effective than auto mode.

Note the windshield vent position, AC off, manual fan speed, recirculate off, and LO temp (no heat).

 

[dakhlkt22]

The concepts of recirculating and windshield defogging are not compatible.

The system will automatically defog based on information from the temp and humidity sensor located in the rear view mirror housing. Automatic mode will provide the best all-around performance with a modest energy consumption penalty.

If you want fresh air, with no heat or cooling, directed only at the windshield, configure your settings like this (see pic). It will likely be less effective than auto mode.

View attachment 978204

Note the windshield vent position, AC off, manual fan speed, recirculate off, and LO temp (no heat).

Thanks for the correction. Yes I guess with defogging I wouldn't want recirculating anyways.

My understanding is that LO would mean it is blasting the A/C.

https://www.reddit.com/r/TeslaModel3/comments/amlbmh

I am looking for no heat but also no A/C. I just want fresh air vented at the windshield.

 

[stopcrazypp]

Thanks for the correction. Yes I guess with defogging I wouldn't want recirculating anyways.

My understanding is that LO would mean it is blasting the A/C.

https://www.reddit.com/r/TeslaModel3/comments/amlbmh

I am looking for no heat but also no A/C. I just want fresh air vented at the windshield.

No, LO with AC off means fresh air only. It's the only way to ensure the system will blow fresh air with no AC or heat.

This is the exact same way an ICE car setting works (at least my old Camry). I turn off the AC light (which ensures AC is not on) and slide the temp to the lowest setting (otherwise heat may turn on if temperatures are cold enough, even one click above the lowest).

 

[Big Earl]

Thanks for the correction. Yes I guess with defogging I wouldn't want recirculating anyways.

My understanding is that LO would mean it is blasting the A/C.

https://www.reddit.com/r/TeslaModel3/comments/amlbmh

I am looking for no heat but also no A/C. I just want fresh air vented at the windshield.

LO will automatically enable air conditioning (AC button turns blue). You have to disable air conditioning after selecting LO. LO will try to cool the car off as quickly as possible within the constraings of your other settings. In auto mode, it will ramp the fan up to max and turn on the air conditioning. If you manually set AC off and a lower fan speed and make sure recirculate is off (gray), it will provide nothing but fresh air.

 

[dakhlkt22]

I noticed A/C turning on automatically. Clearly this is a way to hack the system that Tesla didn't anticipate as a legitimate use that people might want to use. It feels like I have to fight the system just to get some fresh defog air. Wish I could control the fan setting too. The fan setting is greyed out when it is on LO and the defog/defrost button is pressed.

 

[stopcrazypp]

I noticed A/C turning on automatically. Clearly this is a way to hack the system that Tesla didn't anticipate as a legitimate use that people might want to use. It feels like I have to fight the system just to get some fresh defog air. Wish I could control the fan setting too. The fan setting is greyed out when it is on LO and the defog/defrost button is pressed.

Yes AC will turn on immediately after setting to LO, just toggle it off and it'll stay off (as long as you are not using auto or defog/defrost). It's just a carry over from an ICE interface. Plenty of people have mentioned in the forums they would love to have a "fan-only" button similar to a home thermostat.

Defog/defrost uses AC and/or heat by default/definition (fresh air generally isn't effective at doing), so the car limits the control you have over it. You can always just set settings manually to replicate a fixed setting.

 

[dakhlkt22]

Defog/defrost uses AC and/or heat by default/definition (fresh air generally isn't effective at doing), so the car limits the control you have over it. You can always just set settings manually to replicate a fixed setting.

I always thought the reason opening windows works to defog the windshield is by equalizing the heat and humidity between outside and inside. Otherwise, the interior humidity from our breathing will condense on the windshield as a foggy layer.

I was thinking that Tesla set it up this way for user comfort. Like if it's really cold outside, you are still setting your cabin temperature at 70+. If you turn on defog with a setting of 70, is it warming up fresh air to 70 to blow over the windshield? Or is it a mix of recirculated and outside air? If it's warming up fresh air to the temperature setting, then that seems like it'd use a lot of energy. Can we run the defog air at LO or maybe a cooler temperature in the 60s and keep our bodies warm using heating seats and heated steering wheel?

 

[Big Earl]

I noticed A/C turning on automatically. Clearly this is a way to hack the system that Tesla didn't anticipate as a legitimate use that people might want to use. It feels like I have to fight the system just to get some fresh defog air. Wish I could control the fan setting too. The fan setting is greyed out when it is on LO and the defog/defrost button is pressed.

The defog/defrost button is supposed to be rapid, which is why it defaults to high fan and compressor on. You can send air to the windshield without engaging max defog/defrost by using the settings I posted above.

 

[Big Earl]

I always thought the reason opening windows works to defog the windshield is by equalizing the heat and humidity between outside and inside. Otherwise, the interior humidity from our breathing will condense on the windshield as a foggy layer.

I was thinking that Tesla set it up this way for user comfort. Like if it's really cold outside, you are still setting your cabin temperature at 70+. If you turn on defog with a setting of 70, is it warming up fresh air to 70 to blow over the windshield? Or is it a mix of recirculated and outside air? If it's warming up fresh air to the temperature setting, then that seems like it'd use a lot of energy. Can we run the defog air at LO or maybe a cooler temperature in the 60s and keep our bodies warm using heating seats and heated steering wheel?

To properly dehumidify and maintain occupant comfort, the system must dehumidify the air with evaporator coil (air conditioning), then reheat the air with the heat (electric if no heat pump, cabin condenser if heat pump) to have a reasonable vent temperature (above 50°F). If you don't reheat the conditioned air, you'll freeze yourself and also blow cold air on the windshield, which will lower its temperature and cause fogging on the outside due to dewpoint.

If you insist on micromanaging the system in order to use the least amount of energy possible, you will be trading off both comfort and humidity control. I see lots of threads where people fight the system, then complain about windshield and window fogging. Just put it in auto, select a comfortable temperature, and let it do its thing.

 

[dakhlkt22]

I guess it depends on what season we're talking about. In the shoulder seasons, the ambient temperature is not that far off from a comfortable cabin temperature. I mean it's slightly chilly to have the fan blowing 60 degree air at you, but maybe you'd feel fine if you're wearing a sweatshirt. If you could run the heated seats separately from the defogger, then you'd probably feel toasty even with 60 degree air coming in.

When the outdoor air reaches a certain chilliness, it does seem like this strategy might not make sense anymore. But in early fall and late spring, I can definitely see a point where it'd probably be a lot more efficient to follow this strategy for defogging.

 

[stopcrazypp]

I guess it depends on what season we're talking about. In the shoulder seasons, the ambient temperature is not that far off from a comfortable cabin temperature. I mean it's slightly chilly to have the fan blowing 60 degree air at you, but maybe you'd feel fine if you're wearing a sweatshirt. If you could run the heated seats separately from the defogger, then you'd probably feel toasty even with 60 degree air coming in.

When the outdoor air reaches a certain chilliness, it does seem like this strategy might not make sense anymore. But in early fall and late spring, I can definitely see a point where it'd probably be a lot more efficient to follow this strategy for defogging.

In those conditions you don't need a "defogger", only fresh air blowing toward windshield. As such you don't need to use that function at all. A defogger/defroster is supposed to clear out the front windshield very rapidly (that's why it turns on AC to dehumidify the air, heat to keep air temps reasonable, and fan at full blast).

 

[3sr+buyer]

I am looking for no heat but also no A/C. I just want fresh air vented at the windshield.

Or fresh air generally. It would help if they just added a heat on/off button (like the AC on/off button) to the climate controls. Then you can get fresh air by turning both the heat and AC off.

 

[Tronguy]

Putting on my Old Codger hat:

I've owned cars sans A/C; I've owned cars with A/C. With the non-AC cars, blasting the windshield with warmed air was the way to go. Which meant that if the engine was cold, one had to wait until said engine was warmed up before anything would happen. And if it happened to be warm out (the right kind of warm, humid day), occupants of the car would swelter. Such was life.

When cars with A/C started showing up (i.e., the A/C wasn't a massively expensive add-on), all these cars ran the A/C with the heat turned on. As a previous poster on this thread noted, the A/C chills the air which condenses on the cooling coil. The heat takes the dried air, warms it higher than the condensing point, and sends it to the window.

Now that I'm thinking about this: My guess is that the air gets warmed first, then air conditioned; otherwise, one might get icing in the air conditioner coil.

Point about this, either way it runs: the air blown at the inside of the window is dry. The temperature is variable, since the amount of heating and/or air conditioning is variable, so this'll work on warm, muggy days as well as frigid days when warm, humid air in the cabin hits a cold windshield or, alternately, one is trying to melt freezing stuff hitting the outside of the windshield.

All of the A/C cars I had stated that the defrost worked best with the A/C on. Which is what I did.

So, with the OP in this thread: Is it really necessary to micro-manipulate the HVAC to clear the windows by force turning off the A/C? Or is just a wish for non-A/C, fresh air?

 

[rpiotro]

Thanks for the correction. Yes I guess with defogging I wouldn't want recirculating anyways.

My understanding is that LO would mean it is blasting the A/C.

https://www.reddit.com/r/TeslaModel3/comments/amlbmh

I am looking for no heat but also no A/C. I just want fresh air vented at the windshield.

That may not clear your windshield depending on dewpoint/RH. That is why the AC is operating. To remove the moisture.

 

[Big Earl]

Putting on my Old Codger hat:

I've owned cars sans A/C; I've owned cars with A/C. With the non-AC cars, blasting the windshield with warmed air was the way to go. Which meant that if the engine was cold, one had to wait until said engine was warmed up before anything would happen. And if it happened to be warm out (the right kind of warm, humid day), occupants of the car would swelter. Such was life.

When cars with A/C started showing up (i.e., the A/C wasn't a massively expensive add-on), all these cars ran the A/C with the heat turned on. As a previous poster on this thread noted, the A/C chills the air which condenses on the cooling coil. The heat takes the dried air, warms it higher than the condensing point, and sends it to the window.

Now that I'm thinking about this: My guess is that the air gets warmed first, then air conditioned; otherwise, one might get icing in the air conditioner coil.

Point about this, either way it runs: the air blown at the inside of the window is dry. The temperature is variable, since the amount of heating and/or air conditioning is variable, so this'll work on warm, muggy days as well as frigid days when warm, humid air in the cabin hits a cold windshield or, alternately, one is trying to melt freezing stuff hitting the outside of the windshield.

All of the A/C cars I had stated that the defrost worked best with the A/C on. Which is what I did.

So, with the OP in this thread: Is it really necessary to micro-manipulate the HVAC to clear the windows by force turning off the A/C? Or is just a wish for non-A/C, fresh air?

Spot on, although it's definitely cool first to dehumidify, then reheat. There is no concern with water freezing on the cooling coil, because the compressor output is variable, so the system runs only as hard as necessary to maintain the cooling coil above freezing.

 

[dakhlkt22]

Well I was able to figure out my workaround. I think the "defogger" is a completely automated system. For what I want to do, I can use one of the icons on the left for the air direction with the air pointing up. That allows you to manually send air through the windshield vents. Then, by setting it to LO, I'm getting fresh air through the vents. This is a better solution because then I won't be fighting with the logic of the defogger button.

I also tested and it seems like heated seats and heated steering wheel can be turned on manually. Doesn't seem like those are controlled by a temperature setting. I guess I'll just have to wait for cooler weather and see how long I can stand fresh air blowing into the car.

Maybe it's just me, but I definitely remember a time in my childhood when the solution to fogged up windshields was to roll down the windows. After some shivering by the passengers, the windshield would defog. Did everyone do this, or was this a "frugal" solution?

Later, when I became a driver, there was always a "defog" button in all my cars but I never actually knew how it worked and that it was using A/C to dehumidify and then reheating the air. I only learned that on this thread.

 

[Tronguy]

Interestingly, I've recently gotten a 2023 M3 as a trade-in for a 2018 M3. Being the whacko I am, I've been Reading the Manual.

So, the defogger/defroster on this car has three, count them, three! settings.

Off.

Defog.

Defrost.

Off is colorless; defog lights the indicator green; defrost lights the indicator red. Hitting it again goes back to colorless.

Is this standard for all Teslas, or just the ones with a heat pump?

 

[stopcrazypp]

Interestingly, I've recently gotten a 2023 M3 as a trade-in for a 2018 M3. Being the whacko I am, I've been Reading the Manual.

So, the defogger/defroster on this car has three, count them, three! settings.

Off.

Defog.

Defrost.

Off is colorless; defog lights the indicator green; defrost lights the indicator red. Hitting it again goes back to colorless.

Is this standard for all Teslas, or just the ones with a heat pump?

It existed prior to heat pump. Defog runs AC to dehumidify the air and enough heat to keep cabin at reasonable temps (depending on your target temp setting). Defrost runs AC to dehumidify the air and in addition, the heat setting is set to "HI" to blast the windshield with maximum available heat to melt the ice.
What's the difference between the Hot and cold defrost options?

The effectiveness of defog can vary depending on what your target cabin temperature is relative to the outside air/glass temperature.

 

[father_of_6]

defog lights the indicator green

Blue?