Driving in winter brings an assortment of extra considerations, but one that you may often overlook until you're in the vehicle and already driving is the issue of fogged windows. Using your HVAC (Heating, Ventilation, Air Conditioning) system is the most common solution to this, but it does not always work. In such an event, you can be at risk due both to reduced visibility and the inevitable distraction of rubbing sleeves and gloves on the windows, trying to clear a view. It is therefore important to know why this perpetual fogging occurs, and how to solve it.
The Simple Cause
What may come as a surprise to many motorists is that the cause of this - whether it is fog or indeed frost on the inside of the windows - is something that can be easily controlled with a simple button or switch right there in the car. The reason for the fog is almost always due to the 'recirc' feature, denoted by a pictograph of a car, within which is a small arrow bending back on itself. This means the car's HVAC system is in recirculation mode.
Recirc is a handy feature most of the time. What it does is close off the entry of outside air into the HVAC system, and instead it recirculates air already within the car. This is especially useful when you are driving in hot or cold weather, as the recirculated air is already cooled or warmed towards a more comfortable level. It can also help a little if you are passing by a place with an unpleasant smell, such as a particularly pungent farm, or are driving behind something that is throwing up a lot of dust.
The problem comes in cold weather because recirculated air is air which, in part, has already been breathed by the driver and passengers. This has two consequences of importance; First, when you breath out, you expel moisture along with the air. This moisture then has the chance to gather on windows and, due to the cold temperature, it will quickly condense - leading to fogging and icing. Second, the recirculated air will have a slightly depleted oxygen content, meaning you could be at risk of lethargy. Combined with the decreased visibility, you can see how this situation goes from inconvenience to dangerous.
The Simple Solution
Fortunately, as you might already have guessed, the solution to this is a very simple one: Deactivate the recirc feature. The HVAC system is best for keeping clear the front and rear windows, but the sides are especially vulnerable to fogging and icing. Therefore, it is generally best to use the recirc feature to give the interior of the car an initial warmup and then, after a few minutes, turn it off. It will continue warming at a slower rate, but it will be using fresh air from the outside, which will be far less prone to causing visibility problems for you as a driver.
Modern HVAC systems are often more sophisticated, and less prone to causing these problems to begin with (for example by drawing in a small amount of outside air even in recirc mode), but if you find yourself with fogged windows, or know someone else with the same problem, keep in mind that the solution might be as simple as turning off the recirculation mode.
These tips and more can be found on our fleet maintenance blog.