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BMW X3 Windows Fog Instantly

16 messages,  Last post on Feb 02, 2010 at 11:58 AM

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What is this discussion about? BMW X3, BMW X5, Safe Driving, Safety Technology, Heating / Cooling, SUV

Messages Page 3 of 3

#14 of 16 Re: BMW X3 Windows Fog Instantly [pp2009pp] by wwest

Jan 21, 2010 (5:53 pm)

Replying to: pp2009pp (Jan 20, 2010 8:22 pm)
"..the windows fogged up instantly.."
 
I assume you are now aware that this was simply the result of...
 
"..Turned the snowflake button off.."
 
Turning the A/C compressor off after previous operation solely for purposes of dehumidication will almost ALWAYS result in a sudden rise in the passenger cabin HVAC system airflow humidity. Add cold, or even sometimes just cool, interior window/windshield surfaces to the equation and yes, the windows/windshield will fog over, sometimes virtually instantly.
 
What you have to worry about moreso than manual intervention(***1), turning it off yourself, is the system itself disabling the A/C (***2) with no obvious indication/ALARM to the driver.
 
***1 You can become aware of the results of turning it off manually.
 
***2 OAT declined below ~35F, engine coolant is on the verge of overheating, and last, when you parked the car for 10-15 minutes, an hour, or even yesterday.
 
"..I now drive with the recirc off and the snowflake button on..."
 
Dangerous, that, you would be much more safe via disabling the A/C compressor entirely durirng the cold season. We, you, have no way of knowing if the A/C will be an effective dehumidifier, mother nature has complete and total control of that issue.
 
Whereas HEAT, heating the incoming cabin airflow, or more especially, heating the airflow for prevention or removal of windshield condensation, Will ALWAYS be effective at lowering the relative humidity.

#15 of 16 Re: BMW X3 Windows Fog Instantly [wwest] by pp2009pp

Jan 28, 2010 (4:17 pm)

Replying to: wwest (Jan 21, 2010 5:53 pm)
If heating the cabin reduces humidity, why is there more fogging with a cabin full of people exhaling warm air?
 
From what I have read:
 
- cold air is drier which is why cracking a front window to pass colder outside air across the inside of the windshield reduces the moisture of fogging.
 
- A/C compressor apparently quits around freezing but above freezing removes moisture from the HVAC system.
 
- I thought the AC dumps moisture out of the car - there is a pool of water - when it is turned off. It doesn't get stuck in the system.
 
I will try it with the snowflake button off and the recirc off and see if that snowflake makes much of a difference. Since the front windshield defogger is a big blast of warm air, I see what you are saying. I find the entire thing confusing.
 

#16 of 16 Re: BMW X3 Windows Fog Instantly [pp2009pp] by wwest

Feb 02, 2010 (11:58 am)

Replying to: pp2009pp (Jan 28, 2010 4:17 pm)
In a "static" situation heating the atmosphere will result in lowering the Rh. Put a steaming teakettle (four humans) in the mix and you will have to continually raise the air temperature to keep the Rh constant.
 
Yes, cold air is consistently drier air. That's what makes this situation even MORE dangerous. That COLD and DRY FRESH airflow coming from the outside and flowing through an A/C evaporator covered with moisture due to just previous dehumidification capability will often result in SUPER-SATURATION of the cabin.
 
Dehumidification of the system airflow can be efficiently done ONLY if chilling that airflow to ~35F will bring it to dewpoint. If the incoming airflow is both warm and humid then well and good. If the incoming air is fairly humid but already very near that 35F temperature then the efficiency of the A/C for dehumidification might be nil.
 
"..It doesn't get stuck in the system.."
 
But yes, it does..!
 
Assuming climatic conditions allow, and the A/C system is operating, as more and more condensate accumulates on the roughly 10,000 square inches of evaporator vane surface area it will begin to form into droplets to the point wherein gravity will overcome viscosity and those droplets will now flow down the evaporator and eventually exit via the drain hose provided.
 
As long as the evaporator is being cooled down to the level wherein dehumidification will occur this is a continuous, ongoing process.
 
 But.
 
The instant you switch off the A/C compressor, or it is switched off automatically, the continual gathering of additional moisture soon stops. And now you might well have those 10,000 square inches of evaporator vane surface area thoroughly coated with condensate, WATER.
 
Now it might well remain there until climatic conditions change to the point wherein the evaporation of the condensate will begin. Climatic conditions solely within the A/C plenum, within the plenum AND passenger cabin, or if you leave the windows down inclusive of outside climatic conditions.
 
The 2010 RX350 has a new feature added to the climate control system. If climatic conditions warrant (I assume sub-45F OAT) it will automatically switch the system into combined footwell and windshield outflow. That's presumably such that the interior windshield surface gets continually warmed, hopefully keeping that surface well above the dewpoint of the cabin atmosphere.

Messages Page 3 of 3

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