windscreen Condensation

[skyblues11]

I am no big expert on car aircons and such like and something had been bugging me lately but i could not work it out why.

When the car had been parked in the rain, and after driving it for about 10 secs, condensation starts to build up where the dash and the foot of the screen meets. Within 20secs the whole screen area is covered, and the side window had misted up as well!

so bad i had to stop the car and use the air-con to dry the air. This is soo unnecesary and burns fuel. I never had this experience before from other cars. Perhaps only when driving home from a gym, with showered wet hair ( and a hard 6 pack ).

I have read the manual and it did have a little paragraph on this, but not very useful at explaining why.

Surely this cannot be normal . I know its made in Portugal but they get more rain than spain!

i might consider using an anti-fog liquid to see if there is any improvement.

[RW1]

Yes its normal. Keeping the inside windscreen surface clean helps it clear quickly. I have other cars that do it, ie Mk2 Scirocco. Its more pronouced on cars where the windscreen is more horizontal. Stick the defrost on for a minute and let the aircon dry it off.

Why: The incoming air hits the windscreen more square on and the air flow is slowed sufficiently to dump some of its moisture. Where as a car with a more vertical windscreen tends to have vents that send the air at a glancing angle, so it is kept moving much faster and therefore retains its moisture carrying capacity.

C.

[Andy-GTI]

Are you using the recirc function on the aircon?

[Sanders83]

Give the inside of the windscreen a couple of good polishes. Really makes a massive difference. Had to do mine 3 times cos it was so greasy when I picked it up from the dealers. Did it another 2 times yesterday and it now only steams up on the bits that I missed so I can tell it works well.

[skyblues11]

Are you using the recirc function on the aircon?

no Andy, just the normal climatic controls, with non-conditioned air coming sucked in.

I only put the A/C on ( non-recirc.) when it gets really bad.

re-circ or non-recirc., it does clear the screen well to be honest.

the trouble is, once you got the A/C on, you cannot switch it off, especially on wet but hot days ( not many of them left!), because the condensation would spread like wildfire.

[RW1]

Maybe you have a small leak somewhere! Had a new windscreen at all?

C.

[skyblues11]

nothing been done to the windscreen. It could be just a case of moisture in the air sucked in. With AC off, the incoming air is wet whenit hits the already cold dry air.

i need to do a course on air temp!

[micke_011]

Even with the aircon turned on you'll get the initial condensation as the compressor won't be on instantly and you'll the the moist air hitting the windscreen causing it to fog up.

[ZephyR]

Yes this can happen on all cars in cool damp conditions but not as spectacularly as it can appear in the Roc.
You can set off driving and within a minute the whole screen can be misted up. Quick use of the fan and air-con soon clears it and its gone. But I've never had another car as bad as this.
From observation I reckon this only happens when you've been using the air-con earlier on during the day. Its almost as if there is residual condensation left in the ventilation system which is blown out on to the screen as soon as you start moving and push air through the vents. It doesn't mist up until you start to drive off.
Try turning the vent control away from the screen before you set of.

[maddogdave]

It's all to do with the way the air con / heating system works. The A/C cooling side will always create some condensation intermally as it cools (and dries) the air and this normally just falls off into in internal drip tray and then exits a small hose under the car. (Hence the pool of water under the car if you're parked up on a hot day and had the A/C running). If you turn off the A/C but leave the vent fan running, you will get damp / musty air out of the vents for 30-45 seconds as the residual condensation is no longer cold enough to drip away so gets mixed with the air. Similarly, if you shut the system off completely / stop the car with the A/C on and it's a warm / humid day then it will do a similar thing when you re-start. If the vents are set to windcreen then you will get the visible fogging. Can get the situation on a wet day where once you have turned on the A/C to clear the windows you can't turn it off again without misting up, so you either leave the A/C on or don't use it at all. To purge the damp air from the A/C when you have been using it you need to run the fan at medium / high speed with the A/C off, but direct the air to the dashboard vents and open the windows (as long as it's not raining). This will prevent the mist up when you next start the car if you do this as you park up.
The defrost (windscreen) button will put the vents to the screen,turn on the A/C and also heating as this is the quickest way to clear the screen with warm / dry air. (As long as your engine is warm enough to give out some heat). If you switch to auto after using this function (don't leave the air going to the screen) then it shouldn't mist up again when you turn off the A/C.

It's all to do with the dewpoint of the air and the amount of moisture that the air can hold at different temperatures. Hope the above explains but if you want to know the (really boring) technical stuff then look up Psychrometric Chart.

[skyblues11]

thanks Maddogdave. i think your explaination is comprehensive enough. i guess i have to improve my own understanding of the AC system as a whole. Its becoming a standard item in modern cars but yet little is written about them, compared to Sat-navs etc.

[bounge]

Had this issue on a Mk5 golf so wasn't surprised when the same happened on the roc!