Fogging up

[gaza1994]

So this has been an issue for the past few days

All the windows in my car seems to instantly fog up when starting the car.

The fog disappears when I blast the ac over the window, but it's getting rather annoying when I need to go somewhere in a hurry.

Even after I drive the car and leave it for half an hour or so, hop in start the car, boom instant fog up

Any ideas what it could be?

[RUM4MO]

Sorry I can't say anything about this, many people on the Fabia/Polo/Ibiza have complained about this, I have only ever witnessed it happening once with my wife's 2015 Polo, obviously you need a high level of humidity in the car and very cold windows, with maybe a human being breathing inside it at when these conditions prevail, to just trigger this fogging up!

[SRGTD]

Do you keep your A/C on permanently? If not, switch it on and leave it on as it helps to dehumidify the air in the car.

[veteran]

Yup, caused by simple physics. It's bound to happen when conditions such as RUM4MO's explained prevail.

I seem to recall that VW recommend that when jumping into the car when the interior's cooled down quite a bit by icy weather, especially the inside surface of the windscreen, you DO NOT put the AC on but instead you just run the air blower to the windscreen only, at high speed. If the AC light activates turn it off. Open the driver's window a bit and basically just leave the engine to warm up, irrespective of leaving the car stationary and idling or whether moving off. On my Polo and on previous cars I've had, that sort of strategy has always seemed to work. Sure, for the first half-mile or so the inside glass will still be misted over a bit, but keep a dry rag by the gearchange and wipe the glass just before you move off, so that you can obviously see where you're going. It's little trouble, all things considered. Be patient and the blower air coming through will soon be sufficiently warm to clear the glass. After that, the interior temperature should only get warmer and the screen will remain cleared (unless you stop and leave the car to completely cool down again). Obviously, the situation will take longer to right itself if you get into the car with wet clothing, or a wet umberella, or if there's an undetected leak from the heater to the car's interior. Also, if the outside air temperature is excessively low it'll take longer for the glass to warm up. Really, the worst thing you can do is to keep the windows shut and put the AC on. That's more than likely why you're getting the problem.

[d-ash]

Yup, caused by simple physics. It's bound to happen when conditions such as RUM4MO's explained prevail.

I seem to recall that VW recommend that when jumping into the car when the interior's cooled down quite a bit by icy weather, especially the inside surface of the windscreen, you DO NOT put the AC on but instead you just run the air blower to the windscreen only, at high speed. If the AC light activates turn it off. Really, the worst thing you can do is to keep the windows shut and put the AC on. That's more than likely why you're getting the problem.

I couldn't disagree more. A main benefit of AC is that it facilitates rapid demisting of the windows when they are damp. So long as the outside temp is above the minimum operating temp for AC (around 5degC), then it works like magic and quickly clears the windscreen. Otherwise, why is AC set to activate automatically when you direct air to the windscreen? The AC dries the in-car atmosphere and, with the aid of the blower directed at the windscreen, dries the mist on the screen.

I imagine the reason why VW advise against attempting use of aircon in icy weather is that it is automatically disabled when outside temp is less than 5degC.

Often when getting into the car when wearing damp outer clothing I find the windows will mist up, but this is always quickly resolved with a blast of AC air to the windscreen for a few minutes.

[iichel]

For defogging the window, the ideal scenario would be, warm and dry air.
Warm air to raise the condensation point of the air (allowing air to take up more moisture) and dry air to aid in evaporation of the water on the window.

The AC will produce dry air, but at the same time it's very cold. I guess there is something that could apply to both.
When driving into tunnels in the winters, sometimes I notice the windscreen fogging up. Simply pressing the AC button often solves it.
I guess there is an argument for both.

Prevention is the best solution

[veteran]

d-ash,

There are two things going on here, as alluded by iichel. To avoid a fogged screen, you need not only dry air but also warm air. As far as I'm aware, AC systems are designed first and foremost for providing cold air into the cabin in hot summer months, they're not meant to be heat pumps. Unless AC systems in cars have changed in design, the heat you get at the dash vents comes from the engine's coolant circuit, not the AC's circuit. Whilst directing fanned AC air on to the fogged screen in a cold cabin will undoubtedly have a drying effect directly at the glass, you're also prolonging the time for which the cabin temperature will remain low - because the AC's generating cold air, not warm air. In already saturated cabin air, this will make the matter worse. So, I can understand why it's recommended to use the method I mentioned, and to just use the heater blower and an open window. (The recommendation didn't originate from me, incidentally!).

I suspect that with many cars now having fully integrated AC controls on the dash, drivers may be mistaking the de-fogging as resulting from application of the AC, whereas it's probably because it's coming from fanned engine heat instead.

This fogging up business seems to have been plagueing drivers for years and years, and it's happened across all manufacturers and models of cars, and I think drivers are probably having the problem because the first thing they go to, to remedy it, is the AC, rather than turning off the cold blast of AC air and just allowing the heater/blower instead to do its thing.

[iichel]

In addition to the source of hot air, mostly the engine coolant, a number of cars have an additional electrical heating element in the air distribution housing. This heater element is active when the engine has not reached sufficient temperature to open the thermostat.

[RUM4MO]

In addition to the source of hot air, mostly the engine coolant, a number of cars have an additional electrical heating element in the air distribution housing. This heater element is active when the engine has not reached sufficient temperature to open the thermostat.

That is normally the case with some premium cars that use smelly oil like fuel as they are too thermally efficient to provide cabin heat from the coolant system early on, or if at all in winter conditions - also factory fitted or an option in cold climate sales territories. My S4 hands out warm cabin air very quickly, but I don't think that it has auxiliary heating in it.

I still think that a lot of this is due to newish, but hopefully not brand new next series - Fabia/Polo/Ibiza having very poor cabin ventilation systems, okay even petrol engines have become more efficient, but even after me switching the AC system off at journey start and leaving it off, if possible, until the temperature gauge indicates 90C to speed up coolant and engine getting up to operating temperature, when switching the cabin heating system back on, its performance is nothing short of abismal certainly not what you should see as acceptable in a car being sold in 2015>, the 2002> version of Polo with petrol engines had a far better cabin heating system, so why couldn't VW Group provide similar or better systems in later cars? I seriously think that there are design errors in these systems, not fit for purpose.

Edit:- my 2000 VW Passat 4Motion with the 2.8V6 30V petrol engine had a normal system that worked well, again auto temperature control with AC - that is until its recirc etc flap motor failed, when it became as useless at heating the cabin as my wife's 2015 Polo is, warm air can be directed at the windscreen, but if you try to "flood" the lower area with warm air, it just does not happen, this gets reported on all VW Group forums for this platform of car.

[veteran]

I'm now quite confused over this issue. I reckon that d-ash might well be correct because, since posting here, I've had a look at what my Polo driver's manual says about defrosting the windscreen and it seems to support d-ash's contention. In fact, it recommends almost the opposite of the recommendation I originally saw in a VW bulletin on the Web some time ago.

There can be all sorts of mitigating circumstances, though. And the controls you put to use will depend on whether it's manual AC you've got, or integrated Climatronic.

Echoing what d-ash has already stated, the driver's manual says that the blower for the AC won't come on if the air temperature inside the cabin is below 3 degrees C, which clearly won't help dry the cabin air if the car's been left, say, outside overnight in sub-zero temperatures and the cabin temp's still below that temperature. Under those circumstances, it may be better to use the engine-heat blower for a short while (so-called air recirculation mode), just to get some warm air to the glass and to generally raise the cabin temperature. But the manual seems to suggest definitely not using the blower (air rec. mode).

As far as I can see, the Polo driver's manual (I refer to the Nov 2016 version) recommends to direct dashboard venting to the windscreen specifically, to close all windows and, if possible, to turn the AC on until it's cleared.

Perhaps drivers are just too impatient when beset with the problem?

[alexperkins]

On the topic of an extra heating element, both my Q7 and A3 have this (they call it auxiliary heating). I believe this is only present on the diesel variants though due to the slower heat up time compared to petrol engines

[gaza1994]

so just as an update - this doesnt seem to be an issue anymore (with the warmer weather etc - i didnt expect it to be)

but i got my car uber cleaned last week (when it wasnt that hot) and after that its fine (i think my windscreen was just a tad scruffy haha)

[RUM4MO]

Hum, but if you are still owning this car at the start of next autumn/winter then you will have this problem again?

[gaza1994]

Hum, but if you are still owning this car at the start of next autumn/winter then you will have this problem again?

probably :/