[Botnik]

The concept you cannot seem to grasp is that when you open the windows when it's hot outside, the hot air doesn't go out, it comes in... I had thought this was not a particularly esoteric concept...

By opening the sunroof, in very short order the air is efficiently evacuated from the car interior.
This works especially well as the car moves, with lower pressure on top of the car.
Once that's done there is always that option you apparently have not considered of closing the sunroof.

As far as the sun beating down, that is irrelevant to the topic of this thread, which is related to the ability (or not) of a sunroof to open. The sun can still beat down through the glass whether the sunroof/moonroof opens or not.

The sun has to be more or less directly overhead to cause the discomfort you describe, and at least in the lower 48, it isn't always 12 noon..

The air in a car that has been sitting in the sun is hotter than the air outside. To quickly exchange this hot air, open all the windows you can and get the car moving. Also start the a/c to start cooling the parts of that system that need to be cool. So, yes, you do want outside air to come into the car, and you want it fast. It's hot air, but it's colder than the air inside the car. A quick google shows claims that the air inside the car can be 50 F hotter than the air outside. Limiting yourself to the sunroof limits how fast you can exchange air with the outside.

Once the a/c has started up and things are charged, you can close everything and use the a/c on recirc to cool the inside as quickly as it can. Once at desired temperature, you can switch to drawing in outside air if the a/c can maintain the temperature.

You're saying that the sunroof allows hot air out but doesn't allow air exchange with the outside? The sunroof is a huge hole in the top of the car, and if the car is moving, there will be some air exchange with outside. And you want this exchange, when the air inside is hotter than the air outside. When the air inside is the same temperature or colder than the air outside, you want the sunroof closed. In previous posts I do mention closing everything once the air is exchanged.

And because air is exchanged through the sunroof like it is exchanged through windows, the only way the sunroof would be beneficial would be if the air at that level is somehow cooler than the air coming in through the windows, only a couple inches lower.

Are you talking about only partially opening the sunroof? Or is the sunroof in your model year small?

I mentioned having the sun shade closed (the shade, not the glass) to reduce the sun beating down on the passengers and interior. The sun isn't always directly overhead, but even here, in the north, where the sun is often closer to the horizon than it is in the south, the sun does beat down on occupants and interior. Maybe your sunroof is a small hole directly above you, but in the cars we have, it's a giant hole. The post I replied to/about was about the use of the sunroof to cool the car, which obviously has to do with whether or not the sunroof opens.

 

[DragonSubie7]

@Botnik,
Getting into the weeds on this is not worth my time to reply further.
I'd suggest that it may be helpful if you buy a vowel.

 

[Vladder1]

Up til seeing the topic of this thread, I had not thought about the sunroof on my Forester for months. The only way to get the options I wanted was to move up to the Premium, which only comes with the sunroof. My last two vehicles were used purchases and both came with sunroof. Never used those either, aside from the novelty of tinkering with them early on.

 

[Black21Limited]

@Vladder1 Got to use it now and then while the warranty is still good, just in case. Nothing better than fixing something you don't use at the manufacture's expense. I occasionally put a CD in the player just to make sure it works. 🤣

 

[Kevin]

Topic Reminder "Why have a sunroof that opens - do you ever open yours?"

Also as per the posting guidelines in the forum rules:
"5. Similarly, do not use "Reply" or "Quote" when responding to an immediately preceding post and/or the original post of a thread as we do not want to read stuff twice and it just clogs up the thread."

Thanks

 

[345Sire]

Mr.Wilson, I appreciate the dilemma you endured,,,,,,
I open mine occasionally, BUT,,,,,,, I am very careful about remembering to close it, I have a next door neighbor who has borrowed my heavy duty Shop-vac TWICE, to vacuum out their car when they left it open, once in the SNOW! 😱🥶

 

[FozzieBalou]

Let's remember a basic tenet of science - heat (or hot air) rises, cool air sinks....

So by having the sunroof open, you're giving heat a natural path to escape the confines of the vehicle interior.

Sure, opening the windows can cause greater air flow, but it's often louder and more "violent" in air current than the open roof space.

Regardless of whether you remember to close it after opening, that's a different topic all together.

 

[DragonSubie7]

Good point Fozzie...

In addition to natural hot air rise, while a car is being driven, an open sunroof will actually suck hot air out of the car, because there is low pressure above the car in motion, as air flows up and over the car.

This increases the effective airflow, as the cooling system doesn't need to overcome the internal pressure in the car.
When I lived in Phoenix, outside temps of 115+F meant you could use all the help you could get!

Thankfully, I no longer live there.

 

[Kevin]

Where I find the moonroof really useful is when I have the rooftop camper mounted I can park the car in the heat and/or rain and leave the moonroof open. The camper shields from the sun and rain (unless the rain is torrential of course.)

 

[Mckillio]

Up til seeing the topic of this thread, I had not thought about the sunroof on my Forester for months. The only way to get the options I wanted was to move up to the Premium, which only comes with the sunroof. My last two vehicles were used purchases and both came with sunroof. Never used those either, aside from the novelty of tinkering with them early on.

If mine/roof is ever damaged, I'll definitely have it repaired without the sunroof.

 

[Jersey Shore]

I heard that a vehicle with a sunroof is reinforced more than one without. That was the case with my Dodge Magnum.

Well, a vehicle with a sunroof may be more reinforced than one without, but I doubt the structure is stronger and stiffer. Cut a large hole in a unibody and it’s not going to be as stiff.

 

[PMRsForester]

Use mine as often as I can. Favorite is sunny, 60’s & heater on. Love the Fresh air on a long drive. Live a little and open it up.

 

[Halcyonix]

I also open mine quite a bit. Cooler night drives in late Summer and Autumn are amazing.
Also, I kinda go out of my way to not use A/C as much as possible due to the amount of power it sucks from the engine. I only have an Fb25b, so... not a lot to spare. Lol.
Plus with the ginormous sunroof you get with the Fozzy... open it, AND all the windows... It's practically a Wrangler! 😂

 

[Black21Limited]

@Jersey Shore The roof panel is likely the only difference. There is still plenty of roof left to do what it needs to do.

If you had the skills and tools, you could remove the structural adhesive, and swap roof panels, to either gain an opening, or remove one. It has been done on various makes/models. Usually to gain a factory sunroof. The donor car is usually a salvage unit and the A/B/C pillars are cut off, then the roof panel removed in the shop. The recipient car is then carefully taken apart, and the replacement roof glued back in place.

There may be some spot welds, but roofs are primarily glued in place. The structural adhesive is stronger, provides more area of bonding, and seals better than welding. And less chance of rust.

But probably 99% will never attempt this. Probably more like 99.7%. The ones I have seen were mostly done due to hail damage or rust, I don't think I have seen anyone do it just because they wanted to, with a perfect roof panel.

 

[Botnik]

In addition to natural hot air rise, while a car is being driven, an open sunroof will actually suck hot air out of the car, because there is low pressure above the car in motion, as air flows up and over the car.

This increases the effective airflow, as the cooling system doesn't need to overcome the internal pressure in the car.

If you simply think about it, with only the sunroof open and lower pressure above the sunroof, and no other windows open, the airflow out of the car is limited to the airflow into the car, which is through the environmental system. With more windows open, and the sunroof open and the lower pressure above the sunroof, there will be greater airflow through the passenger compartment, and if the air inside the passenger compartment is hotter than the air outside, then having more windows open means the hotter air inside the car is evacuated more quickly and the car is cooled faster. Once the car is cooled, close it all. But, with the car in motion, having all the windows open is going to dwarf the effect of the sunroof as air simply flows through the car, and all the internal air will be exchanged in seconds.

 

[skody54]

I've owned several different vehicles over the years and have never understood why sunroofs were designed to open. I do occasionally enjoy the extra light from them, but have never felt the need for them to open. All of the associated hardware, electronics, motors and gaskets necessary to open and close, for me are wasted. They also create failure points. Most likely, never opening one that opens might also cause future problems. For me, I can either pass or wish they were just like a front windshield and couldn't open.

Minority here. Oops. I use the sun/moon roof every time I can. Granted the opportunity to do so is really limited living in the Northeast. I have a 2023 Forester and a 2016 Outback both have sun roofs. And I also have three convertibles, a 2017 Beetle, a 1964 Pontiac Catalina, and lastly a 1990 Geo Metro.
What's wrong with me? LOL

 

[DragonSubie7]

...with the car in motion, having all the windows open is going to dwarf the effect of the sunroof as air simply flows through the car, and all the internal air will be exchanged in seconds.

The point you miss is that the hot car, open sunroof venting technique is used for a hot car on a hot day.
Apparently, you never lived in a desert environment.

When it's 115F outside, the last thing you want to do is open the car windows to let that refreshing hair dryer air into the car. When it's hot, it is MUCH better to let the hot air out the top, where heat likes to go, rather than through the windows, where the cool air from the AC mixes with the hot air blowing through the windows and heats up the car rather than cooling it.

With the windows open on a hot day, the car will never cool off, and the window air exchange is the last thing you want to do.

With the sunroof open on a hot day, the hot air finds its way out without the outside air coming in except through the AC, so using the open sunroof on a hot day works considerably better than an open window.

If it is cold or mild outside, it's probably not that hot in the car, so sure, open the windows, and you probably don't even need the AC, or maybe you just enjoy the warm air in the car and don't open anything.

The best technique for cooling a car in Canada, versus the US desert southwest or the outback in Australia in summertime is not the same.

 

[Subie Sojourner]

A beautiful day, nice temps, yepper that sunroof is opened.

 

[Botnik]

The point you miss

Let’s look at your points: Hot air rises, there is suction at the top due to car movement and external aerodynamics, opening windows lets hot air in. And this one, “hot air coming in through the windows heats the car up.”

As previously referenced, a car left in the sun is hotter on the inside than on the outside, and can reach 50 F hotter on the inside. Therefore, upon returning to the car, to cool the inside of the car in the fastest way possible, we need to exchange the inside air with the outside air. Once the inside has cooled down to the same temperature as the outside, we close all openings and use the a/c to cool the inside further.

Your method: Open the sunroof, leave the windows closed. Car stationary: Hot air goes up, is exchanged with air from outside through the sunroof. Uh oh, “hot air coming in heats the car up.” Remember that the air outside can be 50 F cooler. Unfortunately this air exchange is less than it would be having all windows open because of the smaller size of the sunroof compared to all the windows. Now, open the windows instead of the sunroof. Lots more air exchange happens, due to opening size, especially if there is horizontal air movement. And you still get the same “hot air rises” convection. Now, open sunroof and windows. Now you get lots of air exchange, with the convection combined with maximum opening size, combined with benefits from any horizontal air movement present.

Car in motion, forwards: Open the sunroof, leave windows closed. Now there is suction up there. Nothing changes, but there might be some more air exchange due to the motion of the car. Uh oh, hot air coming in….

Car in motion, forwards: Now turn the a/c on. Set it to draw air in from outside. Uh oh, hot air coming in…. Air exchange with outside is faster if the car is in motion if you use the windows. Now set a/c to recirc. If you haven’t exchanged the inside air with outside air, you are simply trying to cool air that is 50 F hotter than it is outside. Also, since the car has been sitting, the a/c system isn’t going to be charged for several minutes after turning it on.

In all cases, exchanging inside air with outside air is faster using the windows, or adding the windows. In all cases, having the sunroof open allows the sun to beat down on your head and the inside of the car.

If you think the sunroof somehow sucks air out without allowing air exchange with outside, let’s consider: Open the sunroof but it’s sealed, except you have your vacuum cleaner hooked up there so that there is only suction, and no air is coming in through the sunroof. Windows closed: You get lower pressure inside the car, no air exchange, so you are sitting in air that is 50 F hotter than it is outside. You get a little reduction in temperature from the reduced pressure due to ideal gas law (PV=nRT, solve for T) but it’s a small benefit when you consider the 50 F you have to overcome. Open the windows and now the sunroof vacuum cleaner can exchange the air. Draw air in through the a/c and now the vacuum cleaner can exchange the air. With the a/c set to recirc, you’re cooling the 50 F hotter air instead of simply exchanging it. The a/c isn’t going to be helping much for the first few minutes.

If you think any of this works differently depending on geographical location, please explain how.

Let’s look at this other point: Using the windows instead of the sunroof creates turbulence inside the car.

This is kind of trivial. You can open the sunroof any amount, creating different amounts of turbulence inside the car depending on the opening size and speed of the car. Same with the windows.

Is there some kind of benefit having air moving over your body so that sweat can evaporate faster? Well, that would require air movement, which you would get more of with the windows open.

 

[Botnik]

"5. Similarly, do not use "Reply" or "Quote" when responding to an immediately preceding post and/or the original post of a thread as we do not want to read stuff twice and it just clogs up the thread."

Maybe it's better to simply trim the quotes and include only the quoted material necessary to provide context, because there are both nested and chronological orders when displaying threads, and posts may not immediately succeed the post they are a reply to.