There have been many posts on the bulletin board regarding the climate control system, especially problems with window fogging. I don't yet have my A4, so I can't give any first hand experience particular to this car (but this FAQ should be updated with this info VERY shortly!). What I can do now is give some basics on how air conditioning works, and why NO CAR can dehumidify the air when it is below freezing outside. Air conditioning is one of those technologies that stays somewhat hidden... the typical Joe (or Jolene) doesn't get too excited about the inner workings of air conditioners, adding performance, or keeping their home condensing units washed & waxed. When they're hot, they turn it on. For this reason, this is going to be detailed, lengthy, and for most, painfully boring. For those of you who know most of this, I'm sorry.

How does air conditioning work?
The majority of air conditioning systems used for comfort cooling utilize a closed loop refrigeration cycle. A refrigerant (in a gaseous state) is pumped through the compressor, and is heated up by compression. This hot, high pressure gas is forced through the condenser coil, which is the heat exchanger in front of the radiator. As the refrigerant passes through the tubes in the condenser, it is cooled down to a temperature close to the outdoor temperature, due to the airflow passing over the tubes. There are generally metal fins attached to the tubes to increase surface area, which increases heat transfer efficiency. The refrigerant is under such high pressure from the compressor, that it will condense into a liquid at a much higher temperature than it normally would at normal atmospheric pressures. As the refrigerant condenses into a liquid, it gives off a lot of heat, due to the principle that a substance is going to a lower state of energy. As the refrigerant leaves the condenser, it is a liquid, somewhat warmer than the outdoor air, and still under high pressure. From here, the refrigerant passes through a refrigerant metering device. This is most easily imagined as a faucet that is only cracked open slightly. The compressor is trying to pump a relatively large quantity of gas, but once the now liquid refrigerant gets to the metering device, the hole is too small to allow a large quantity to pass through it. This restriction in the line is what is actually backing up the pressure in the condenser, instead of simply allow the refrigerant to be pumped freely in a circle. As this pressure builds up, the liquid is forced through the restriction faster, until a balance point is reached where the pressure is high enough to force a sufficient quantity to pass through the metering device. Once the refrigerant passes through this restriction, it is no longer under pressure. The drastic drop in pressure causes the refrigerant to now be above the boiling point, so some refrigerant immediately boils, which absorbs energy (heat), and lowers the temperature of the remaining liquid refrigerant to a point that is below the boiling point (it is this boiling of refrigerant while passing through the metering device that causes the hissing sound that people have complained about). This now cold liquid (with some gas) refrigerant now passes through the evaporator coil. This is similar to the condenser coil, but smaller in face area and thicker, so as to fit in the dash. As the cold liquid passes through these tubes, it absorbs heat from the air that is passed over them, and boils. The refrigerant exits the evaporator as a cool gas, is sucked back to the compressor to start the cycle all over again. Now that this is over, we can talk about the cooled air that this is all about:

Air conditioning and de-humidification
As air is passed over the evaporator (either from outside, or in the cabin, depending on whether or not recirc is enabled), it is cooled as it comes into contact with the cold surfaces of the tubing and fins. The amount of vaporous water air can hold is dependent on its temperature: as air is cooled, it looses capacity for holding water molecules (in the form of humidity). If air is holding all the moisture it can hold at a given temperature, it is said to be at its saturation point. This correlates to 100% relative humidity. If air, with a specific amount of moisture in it, is cooled below its saturation point, some water condenses out if it in the form of condensation. The air leaves the evaporator coil cooled, but at close to 100% humidity. This is because moisture stops condensing out of the air once enough has left for the air to be at or below the saturation point. If this air were heated up to its original temperature, it would be at a lower relative humidity than it was originally, due to its lower absolute moisture content.

So why can't the air conditioning system be used to de-fog below 32°?
First off, it has nothing to due with "freezing" the compressor. Even at the pressures and temperatures within the refrigeration system, the refrigerant is always well above its freezing point. What can freeze is moisture on (not refrigerant in) evaporator coil. The humidity in the air is removed by condensation forming on the evaporator coil. If this moisture is condensed on the coil at a temperature below 32°, it will collect on the coil in the form of ice. The ice layer on the surfaces of the coil will become thicker and thicker, until it is completely covered with ice. Once this happens, air can no longer pass through the coil, and the system would no longer be able to function. This is where compressor damage could happen: once the evaporator is covered with ice, and the airflow cut off, heat transfer ceases to happen. The refrigerant never boils into a gas, and can return back to the compressor in the form of a liquid. The compressor will attempt to compress it, but liquids are not compressible. Compressor damage is often the result. For this reason, automotive air conditioning systems are designed to prevent freezing on the coil. Audi appears to sense the outdoor temperature, and disable the system at some point above freezing. More conventional (manual climate control) systems simply measure the temperature of the coil, usually by sensing the pressure within the evaporator (which can be correlated to a temperature, given that the refrigerant is at the boiling point). These systems are designed to disable the system at some temperature above 32°, as the surfaces of the coil are somewhat lower than the air passing through it, to provide good heat transfer.

There are refrigeration systems that are designed to deal with evaporators below freezing: refrigerators and heat pumps are two common examples (in the heat mode, the outdoor section of a heat pump acts like an air conditioner trying to cool the outdoors). They go into a defrost mode, initiated either by a timer, which simply periodically defrosts the system at certain intervals (most refrigerators and older heat pumps), or by demand, by detecting operating conditions that suggest ice build-up (most newer & high efficiency heat pumps). They defrost the coils either with electric heaters (refrigerators), or reversing into the a/c mode (heat pumps). The advantage they have over automotive air conditioners is they can shut down the airflow over the coil during the defrost mode. This allows the coil to heat up, the ice to melt, and the moisture to drain away. An automotive system does not have this advantage. If the airflow through the climate control system were shut down in cold weather long enough for the coil to defrost (probably at least two minutes), the windows would fog in the mean time, simply from moisture given off from your body. In addition to this, when the defrost cycle was complete, the coil would still be wet (it can't possibly drain completely dry in two minutes). As soon as the fan were turned back on, the moisture on the coil would immediately be evaporated back into the air, suddenly increasing the moisture in the cabin, resulting in instant window fogging. This can be witnessed in a heat pump: immediately after the defrost cycle, a burst of water vapor can be seen being blown out of the outdoor section when the fan kicks back on. Of course, you don't want this to happen in your freezer either, but they have much longer defrost cycles than heat pumps do, which allows more of the moisture to drain away.

So why not use recirculated air (which is warmer) to prevent evaporator icing?
There are several reasons for this: First off, auto manufacturers are reluctant to do anything that involves preventing a good supply of fresh air into the cabin for extended periods of time, for obvious reasons. Secondly, the air conditioning system would not operate if the evaporator coil was significantly warmer than the condenser coil. You may recall from the introduction that refrigerant is pumped into the condenser, and the high pressure in this part of the system forces the refrigerant to pass through the metering device. If the condenser was significantly cooler than the evaporator, the pressure in the condenser would be lower than the evaporator, and the refrigerant would not flow. Commercial air conditioning systems that have a need to cool indoor spaces while it is very cold outside overcome this obstacle by reducing the airflow over the condenser. When the system senses cold outdoor ambients, usually by sensing pressure in the condenser, it reduces the airflow over the condenser, which reduces heat dissipation, keeping the condenser warm enough for proper system operation. This is not easily done in a car, given the condensers location in a spot that gets natural airflow when the car is moving (whether the fan is operating or not). This could be overcome with dampers, but the car would probably soon overheat, since the radiator would be affected by this as well.

But the single most important reason recirculation is not used to prevent icing is simply because it would not help matters at all. No matter how warm the air coming into the evaporator is, you can't cool that air below 32° without fear of icing the evaporator. If you can only cool the air to a temperature close to 32°, then you won't be removing any moisture. Since the air coming off of a cooling coil is close to being saturated (100% RH), it will always have a higher moisture content than air that is 100% RH at lower temperatures. You are actually better off using outdoor air at 30° (even when it's snowing & 100% RH) than you are using recirculated air that has been cooled to the limit (32°), because it will actually have a lower moisture content. This is also why pre-heating outdoor air with the heater core would not help. If outdoor air at 30° and 100% RH were heated, then cooled to 32°, no moisture would be condensed out of it, since it would still be 2° above the dew point.

So how do you keep the windows defogged in cold temperatures?
If air is at 30° and 100% RH, then is heated to 70°, the RH at this temperature is somewhere around 30%. This is relatively dry air. Your body is warm and moist (some more than others!), and it gives off a lot of moisture through respiration & perspiration. Also wet clothing and carpeting give off moisture, especially when warmed. In a closed environment, all the moisture being added to the air raises the humidity level. The temperature of the glass in a moving car tends to be fairly close to the temperature outside, so its temperature is now below the dew point of the recently humidified air. The trick is to keep fresh air from outside (before being humidified) flowing over the glass, preventing the humidified air from coming into contact with the cold glass. If this fresh air is warm, this will also warm the glass to a point where it is above the dew point of the air it comes in contact with. Also, the more air you bring into the cabin, the more the humidified air is diluted and expelled. In other words, turn the fan on high, and keep warm air blowing through the defrost vents. This is an age-old method that has been used since the time before automotive climate control systems, and should still work today.

But why does my A4 seem to have more window fogging problems than my other cars?
Well, this is a much harder question to answer, especially since I don't actually own one yet (yet!). Here is my educated guess:

While in the auto mode, the A4 appears to always use the air conditioning compressor, as long as it is above its low temperature cut-out (reportedly somewhere around 38°). Even though the air may not be cold coming out of the vents, it is actually being cooled, then mixed with hot air from the heater core to give you tempered air. This is actually the way most automotive automatic climate control systems work... they run the air conditioning, and play with the fan speed to meet the cooling needs of the cabin, once they get down to low speed, and cooling is still not required, they start tempering the air with the heater core. This assures you nice dry air, at the expense of running the compressor more than you probably would in a manual system. In a manual system, you can duplicate the way an automatic system works by keeping the air conditioning compressor on at all times, but turning the temperature lever up when you get too cold. There may be some systems that use the compressor less than Audi at cold temperatures, but this appears to be causing (or contributing) to the problem. Because moisture is constantly being condensed on the evaporator coil, the coil (and much of the surfaces surrounding it), stay wet. Once the outdoor temperature reaches a point where the compressor can no longer be used, the air is still pulled through the same path as it was before, but the coil is no longer cold. The evaporator coil begins to act as a humidifier. This is where the troubles begin, and fogging can continue until the a/c system completely dries out. To make matters worse, if the air outdoors is very humid, it becomes difficult to remove the fog from the windows, even after the humidification has ceased. The air is simply not dry enough to promote fast evaporation off of the window surface.

So what is the answer to the A4's woes?
My interpretation of the problem is simply that many people are relying too much on the automatic system. If the above situation is the problem, then the answer is simple: take control of the climate control system. If you manually disable the compressor (by hitting the snowflake button) at a time when it is not needed (it's not hot out, and it is relatively dry), this will allow the coil and ductwork to dry out. Once the moisture is eliminated from the system, it should be a fairly simple matter to keep the windows defogged by the conventional means of using lots of warm air blowing across the glass. Unless you actually bring a lot of moisture into the cabin, this method should be sufficient. At times when you are wet, or you have a lot of hot & sweaty guests in your car, you can re-enable the compressor to introduce dry air to compensate (assuming it's above the low temperature cut-out point). Just remember that by doing so, you are re-introducing moisture in the system, which will have to be dried out at some point again. Of course, it is still important to periodically run the compressor, to ensure the shaft seal stays lubricated, but I suggest that you do this during hot or dry weather, so it can dry out again after you do this.

I have suggested this on the bulletin board in the past, and have gotten several responses that this helped or solved the problem. Once my car gets here (in February!), I will see what else I can learn. I will update this FAQ once I get done running the 6 months worth of errands I have been saving up! I will be happy to try to answer any further questions on this topic on the bulletin board, or by e-mail.