De-misting problem

[L John]

Please explain reason for your statement....

Try wearing glasses in a warm dry house, then go outside on a cold day when it's raining with an umbrella, do your glasses fog up?
No, even though it's likely around 97% humidity out there.

Now after some time, try going from the cold and rain into your house (possibly around 50% or lower humidity), your glasses will be pretty dry before you go in the house despite that high humidity outside in the rain, within seconds you won't be able to see because the glasses will fog up completely, and that's in the 'dry' house!

Dew point is what actually causes condensation to form. In this case it would be the combination of temperature of the air, temperature of the glass, humidity inside the car and dew point.

If the car has a good aircon system you could use recirculate to clear the windows but it's better to take cold air in from outside then heating it with the engine temperature even if it's just starting to warm up and drying it with the aircon.

So, recirculate is not a good option to clear the windows whatever the situation or weather.

 

[Stew2508]

The vents on that side seem to be working correctly, and heat is also coming out. I have tried with Aircon, with out Aircon, with Windows down, hot, cold etc... But nothing clears it. I have however seen something new which leaves me to think it could be something else. When I turn off the blowers, leave them for a little while and then turn them on again, what appears to be steam comes through the vents. I have also noticed that I am getting a top up coolant message on the dash. My theory is that maybe there is a small coolant leak that is venting through the fan. Are there any cooling pipes or similar around that area that could possibly cause this problem? I have noticed when trying to wipe the windscreen it appears very greasy indicating that it is something else other than water/moisture. Thanks

 

[LostKiwi]

You may have a leaking heater core. That would do it.....

 

[oigle]

Ahh ... but does aircon work as well when recirculating cabin air?
If so, it would continue to remove moisture from the air with all windows closed, resulting in desert-dry fug as opposed to jungle-humid fug.

Exactly. It will recirculate the air and continue to dry it out rather than having to attack a brand new lot of moisture laden air (if it is raining or foggy)

 

[EmilysDad]

I didn't think that recirc completely shut off the outside world

 

[umblecumbuz]

I didn't think that recirc completely shut off the outside world

Pity!

 

[Frosty149]

I didn't think that recirc completely shut off the outside world

Anyone know how to “shut off” the passenger seat area?

 

[EmilysDad]

Anyone know how to “shut off” the passenger seat area?

If only ....

 

[umblecumbuz]

Anyone know how to “shut off” the passenger seat area?

Gaffa tape?

 

[LostKiwi]

Exactly. It will recirculate the air and continue to dry it out rather than having to attack a brand new lot of moisture laden air (if it is raining or foggy)

In general humidity inside and outside a vehicle left overnight will be the same (air circulates through vents etc when left). Once you start to demist you draw moisture off the windscreen increasing the humidity inside the vehicle so now the air inside has more humidity than outside. If you run on recirc you now have to remove the high moisture content from the air inside (and at this point its higher than outside) before redirecting the 'dried' air back onto the windscreen. Of course given the air outside is at a lower moisture level than the inside it would be better to start with the outside air instead. and dry and heat that before directing it onto the windscreen.
Furthermore since the air outside is cold it can hold less moisture anyway so even just heating the outside air will increase its ability to absorb moisture off the windscreen. Drying it as well (through the aircon) is a bonus.

 

[umblecumbuz]

There is limited outside to inside air circulation when a car is parked overnight.

Air only circulates when there is either an independent power source (such as a car fan) or a temperature difference between two adjacent bodies of air. With the engine switched off, the temperature difference between internal and external air is the only energy source, and as a car rapidly cools once parked, this soon stops.

The only other way that any displacement can happen is if the external air is moving sufficiently to create a lee on one side of the vehicle - ie. gusty conditions and appropriate car position. It also needs an easy route both into and out of the vehicle, and the small fascia and footwell vents, coupled with the even smaller rear exit routes, do not provide such an easy path.

The car will also draw in a limited volume of air from outside during the cooling period, because as the internal heated air cools it contracts, but that is not the same thing as displacement of internal air.

Unfortunately, although the main beef is condensed water droplets on the windscreen, there is one aspect that is often overlooked. More moisture in the car Is absorbed by hygroscopic areas - such as the seats, carpet, roof lining, than the solid surfaces such as the glass. So as the car heats up the next morning, the system has a very large job to do. This helps to explain why it sometimes takes a long while to clear the windows. A fan set on its fastest speed with aircon operating and heat also on max will help release and expel more moisture from within the car, whatever the weather outside it.

 

[L John]

... If you run on recirc you now have to remove the high moisture content from the air inside (and at this point its higher than outside) before redirecting the 'dried' air back onto the windscreen. Of course given the air outside is at a lower moisture level than the inside it would be better to start with the outside air instead. and dry and heat that before directing it onto the windscreen.

Agreed.
Without aircon, the screen will clear FAR quicker on a cold rainy day if outside air is used instead of recirculate.
With aircon the same applies but it will be much quicker because it's also drying that air. Of course with aircon it will clear fairly quickly even using recirculate but it's not the best option.