Someone explain this aircon & demisting windows wizardry pls

[All that jazz]

So there's been some discussion about this in the past but I still can't work it out or I'm doing it wrong.

Scenario: cold morning, heavy condensation on outer windows, medium film of mist on inside windows after getting in.

So I start the engine in my diesel Vauxhall and focus the blowers on the windscreen. What should I be doing with the aircon button, temp dial and recirc button to get the inside mist cleared quickest without physically touching the windows with a cloth etc? Bear in mind also that you don't have any sort of heat whatsoever for 15 mins in diesel VXs.

I've tried it with aircon on cold, aircon on hot, ditto with recirc button on etc and nothing seems to make any difference. Pollen filter thingy was changed last summer so would assume is still fine. Aircon unit itself is in good working order.

[Mave]

Have you cleaned the inside of your windscreen? I find mine demists more easily when its clean.

[All that jazz]

@ Mave, yeah did it a while back with some proper glass cleaner. It doesn't get too foggy but it's enough to need something on it to see out of.

@ dibbers, I thought the whole thing about aircon is it draws the moisture out of the air, so that should demist the windows then, regardless of temp, no?

[SSBB]

Does the air-con chill when there is no engine heat? I assumed without a re-heater, the air-con would remain off to avoid overcooling of the incoming air. The air requires cooling then heating to dry it.

[Gafferjim]

I find that if set to recirculate it make internal misting worse, leaving the setting to external air, blower at the windscreen on full clears it in a few minutes.

[skyrover]

Use cold outside air to demist.

Once clear switch to recirculating air to warm cabin faster

[Dodsy]

A few pointers. The AC compressor in most (maybe all) cars doesnt run below 5 Deg C so you wont gain the usual demisting benefit from the AC when its cold out.

If you are misted up and put on recirculate it will just get worse, when demisting you want to blow fresh air over the moisture to evaporate it then have it leave the car. If you have recirculate on the now mositure laden air stays in the car and recondenses on the windows

The theory with AC is that it cools the air and the moisture is removed before that air enters the car. So even if the car engine is cold you are blowing very dry air onto the windscreen that will demist it quicker than without AC.

[All that jazz]

Seems to be a lot of conflicting info already.



I do however concur that using recirc seems to make the condensation/misting up problem worse rather than helping. Pretty sure I can hear the aircon clutch engage on my Vx even at low temps but whether that means it's actually doing anything useful in the cabin I have no idea.

[Dodsy]

All that jazz said:

Seems to be a lot of conflicting info already.



I do however concur that using recirc seems to make the condensation/misting up problem worse rather than helping. Pretty sure I can hear the aircon clutch engage on my Vx even at low temps but whether that means it's actually doing anything useful in the cabin I have no idea.

AC relies on cooling the incoming air by x amount before blowing into the car and if necessary warming it up a bit on the way. If the air is already cold the AC unit cant work effectively and will ice up so it shouldnt cut in.

[All that jazz]

You say that AC is for cooling the air, but you can also run the AC on hot (assuming engine temp sufficient) if required so it's not just for cold air. I seem to recall reading an old post by someone who said that if the engine is already warm then running the aircon on hot with blowers pointing at the windows is the fastest way to demist them on a wet day as the heat accelerates the process by evaporating the moisture from the windows and then the AC removes the moisture from the air = clear windows very quickly.

[Zoobeef]

The question is, does your air con work? Or does it need recharging?

The quickest way to demist is:
Fans on full
Outside air
Temp on hot
Air con on
Blowers pointing at your screen

[Cactussed]

Ac has a dryer which dehumidifies the air which is why cars with ac don't fog up in wet weather, regardless of the temp you set it at.

However, the ac generally doesn't work below 5 degrees, so you have to rely on either warming the car up first or hitting it with outside air as stated in winter