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Old 17th September 2021, 16:09   #1
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Default Rolling the windows down before turning on the A/C

I always had this doubt in mind. Different people had given me different answers. So, thought of posting this in this forum.

We all know the temperature inside a car will be much higher than the temperature prevailing outside. Let's take the case of Indian weather conditions for example, where

a) If the car is parked under a shade/garage, the temperature hovers around 30 - 33 degree celsius.

b) If the car is parked outside(directly under sun), then the temperature can go upto 38 - 44 degrees(or more) depending on the prevailing weather conditions.

There are people who religiously ensure that the windows are open before switching on the AC.

Below articles also advices us the same.

https://www.fleetnews.co.uk/news/car...in-hot-weather

The air circulating inside the car contains higher levels of benzene, due to the heat emanating from the hot plastic.

While that being the case, I have seen a few people who are allergic to rolling down their car windows(even for a minute) due to obvious reasons.

So, to sum it up, we have these questions.

a) Isn't it a good idea (or) isn't it advisable to open windows to expel trapped gas before turning on the AC, atleast for 5 minutes?

b) If it's for our own and our family's well being, why are people avoiding to do so?
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Old 17th September 2021, 16:13   #2
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Default re: Rolling the windows down before turning on the A/C

These days, my Polo is parked out in the open during the day, and I cannot thank enough VW, for being able to roll down windows from remote before I walk into the oven.

Once Inside, I turn on the AC and roll up front windows and let the rear ones open for sometime to let the hot air escape.

Last edited by akshay4587 : 17th September 2021 at 16:15.
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Old 17th September 2021, 17:24   #3
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Default re: Rolling the windows down before turning on the A/C

I have read about this too, and what I do is this:

Every morning, when I first start the car after it has been standing in the garage for 10 to 12 Hours, I roll the front windows down for anywhere between 5 to 7 minutes before switching on the AC. - This is a conscious behaviour to let the accumulated gases escape.

Over the course of the day, if I do need to drive, I roll the windows down based on how hot the car is. This is more to help the car cool down, rather than have anything to do with the fumes though.
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Old 17th September 2021, 20:00   #4
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Default re: Rolling the windows down before turning on the A/C

What I have religiously followed always is rolling down all windows for the first few minutes after starting the car and putting the fan on. I switch the AC on after approximately driving for 2-3 minutes. Have utilised this technique in all weather conditions. Also, during the summer, I open the sunroof for a couple of minutes, to allow the internal heat to dissipate faster.
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Old 17th September 2021, 20:24   #5
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Default re: Rolling the windows down before turning on the A/C

Quote:
Originally Posted by Anees_Offl View Post
So, to sum it up, we have these questions.

a) Isn't it a good idea (or) isn't it advisable to open windows to expel trapped gas before turning on the AC, atleast for 5 minutes?

b) If it's for our own and our family's well being, why are people avoiding to do so?
Regardless of the weather conditions I always turn on AC as soon as I start the engine. The temperature is set to 22.5C on auto mode. The blower speed is high initially but settles down within few minutes.
My car is parked in a rather open garage but is not under direct sunlight thereby not excessively heating up the interiors. What I do feel is the temperature difference between the inside and the outside. I religiously practice the 5 min rule. Roll down the windows for a few mins and drive until I leave my locality whilst the AC is ON. This ensures all the toxic air is out of the vehicle. The outside environment is very irritating with all the constant vehicle noise and a lot of drivers with their endless annoying honks So I roll up the windows and drive.

I do have a small query regarding this to the owners who have remote Start/Stop and remote AC functions. Is it a necessary feature or is it a nice to have feature? (never used one though)

Regards,
advaithraju
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Old 17th September 2021, 20:36   #6
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Default re: Rolling the windows down before turning on the A/C

Whenever and wherever from I drive the car, even at night or early in the morning from home out of our parking lot, I always go with the windows down and only the fan running. After driving for one KM, when the temp gauge reaches normal position, I will close the windows and switch on the Ac. Till then I will let the outside air circulate in the cabin. Always. Time of the day, hot sun or cool breeze does not matter.

The only exception is if it is raining.
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Old 17th September 2021, 20:45   #7
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Default re: Rolling the windows down before turning on the A/C

Quote:
Originally Posted by Anees_Offl View Post
So, to sum it up, we have these questions.

a) Isn't it a good idea (or) isn't it advisable to open windows to expel trapped gas before turning on the AC, atleast for 5 minutes?

b) If it's for our own and our family's well being, why are people avoiding to do so?
a) Yes, it is a good idea. It is always a good idea to expel out the hot air. However, it need not be strictly done before starting the AC. And also, it need not be for 5 minutes. Once your car is moving, even 30 seconds is enough to expel out all the trapped air and get in fresh air. This could easily be done even after you switch ON the AC. The main logic is not to burden the AC with cooling down the trapped hot air. That is the idea. There is no harm if you switch ON the AC first, and then a few seconds later roll down the windows.

b) There could be reasons to not roll down the windows, on some rare occasions. For example, if I parked in an area with foul smell or unclean air, or if it is raining heavily, I do not roll down the windows until I get out of that area. It is not like not rolling down the windows is going to cause any immediate or any noticeable harm to your AC. All it will do is make your AC work extra hard for a few minutes. Also, maybe a couple of minutes of breathing in that trapped air is not going to cause any danger to you.

Of course, case b) is a rare occasion, and almost always, I follow the practice mentioned in case a).

Last edited by Dr.AD : 17th September 2021 at 21:07.
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Old 17th September 2021, 20:46   #8
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Default re: Rolling the windows down before turning on the A/C

Always desirable especially during hot weather and post "non use" spells when it is very necessary.

When parked in heat during sunny days (also summers) the interior temperature touches 50 degrees Celsius or even more at times. The hot air needs to be cleared and the windows are needed to be rolled down. Only after we can see a reduction in interior temperature should we roll up the windows and switch on the AC.

Similarly during many days of "non use" the stale air inside the cabin needs to be freshened up.

However, on dusty of smoke/smog filled stretches, our own judgment needs to be used while rolling down the windows. It's better to allow such hostile stretches to pass over.

But apart from the other factors, it is also very important to get rid of the toxic benzene formed inside the cabin due to heating up of the plastics and the upholstery. This finding was reported by researchers I believe about a decade ago.

Last edited by anjan_c2007 : 17th September 2021 at 20:50.
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Old 17th September 2021, 20:54   #9
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Default re: Rolling the windows down before turning on the A/C

On rare occasions my car is parked in open, not under any shade, I just pop up the sunroof and let AC (rather physics) do it's job. I find this more effective than rolling down windows.
I hate rolling down windows as the dust and heat waves (vant think of a correct word ) it's unbearable.

Last edited by Akshay6988 : 17th September 2021 at 20:58.
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Old 17th September 2021, 20:54   #10
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Default re: Rolling the windows down before turning on the A/C

Isn't this what the fresh air mode is for? I just use fresh air mode for a few mins when I start as well as every 2 hours or so on a drive. I don't really bother to roll down windows much. More than the smell or dust it is the wind noise that irritates me.
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Old 17th September 2021, 21:27   #11
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Default re: Rolling the windows down before turning on the A/C

Quote:
Originally Posted by anandhsub View Post
Isn't this what the fresh air mode is for? I just use fresh air mode for a few mins when I start as well as every 2 hours or so on a drive. I don't really bother to roll down windows much. More than the smell or dust it is the wind noise that irritates me.
Yes, fresh air mode will do that job of swapping the air, but way too slowly. This is not the intended use of fresh air mode. The intended use of fresh air model is to get in fresh air as and when needed (or sometimes in continuous mode), at a small rate, once the internal temperature is already somewhat stabilized to desired levels.

However, when you get in a parked car, there is a large volume of air which is hot and stale. It is best to expel this out as quickly as possible, and not burden the AC system with the job of cooling down this large static mass of hot air. Rolling down the windows even for 30 seconds will do this job and do this quite well.

On the other hand, if you solely depend on the fresh air mode to do this job, it might take 15 minutes (or even more) of overloaded AC operation under fresh air mode to completely swap the air mass trapped inside with fresh air while simultaneously cooling this hot air. The AC will be doing a lot of unnecessary work of cooling down the air which is much hotter than the ambient temperature.

Last edited by Dr.AD : 17th September 2021 at 21:39.
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Old 19th September 2021, 08:49   #12
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Default re: Rolling the windows down before turning on the A/C

I drive around for a few km at decent speeds to let the air circulate and lose the hot air. During this time AC runs in fresh air mode.

In about 5 min, I roll up, air recirculation - On, AC - On.

If it's too hot, then I also circulate the air manually by opening and slamming a door while one of the other windows is cracked open about an inch.

This is probably the only place where I think a sunroof would help. With the sunroof, it should be easier to lose all the hot air. Just send all cold air to your feet for a while with the sunroof cracked open. Or turn on fresh air with the roof open. This should help bring the temperature down rapidly. TBH, I haven't tried this; it's strictly theory for now.
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Old 19th September 2021, 09:39   #13
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Default re: Rolling the windows down before turning on the A/C

Own diesel cars which need 1-2 mins of idling so I take these 1-2 mins to work on the trapped air and circulate it with fresh air.

1. Start

Roll down all 4 windows
Turn on the car
Turn on the AC
Turn off re circulation mode
Idle for 1-2 mins
Drive the car till outside the society
Roll up the windows

2. Finish

Roll down the windows
Switch off the AC
Fan at high speed with foot duct flow mode
Idling for 1-2 mins
Turn off the fan
Turn off the engine
Roll up the windows along with car lock with key(VW)

Last edited by VWAllstar : 19th September 2021 at 09:40.
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Old 19th September 2021, 09:40   #14
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Default re: Rolling the windows down before turning on the A/C

If I park car for long hours or parked in Sun, I follow the practice of rolling down the windows, put Ac in fresh, run it in full blast.

As mentioned, it helps to bring down the temperature immediately
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Old 19th September 2021, 12:59   #15
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Default re: Rolling the windows down before turning on the A/C

When I leave the car parked in hot sun, I roll down one of the windows by just enough gap to insert a finger (power windows make it a bit difficult to get it right). Usually rear one on the side of parking is chosen, as it helps to keep it subtle.

The experience when you get back into the car is far more comforting.
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