Fogged Up Windows


Fogged up windows
Recirculate switch can lead to problems

How to deal with fog on the inside of your windshield

Most motorists know to use the defrost position of your HVAC (Heating, Ventilation, Air Conditioning) system to clear the inside of the windshield. What many don’t know is that this may activate a system that can create a film on the inside of all windows, a dangerous film of moisture. We have all seen vehicles driving down the road with the side windows all steamed up and the occupants frantically rubbing away with their hands or gloves trying to clear a path so they can see out. What they do not realize is that the problem I caused by re-circulated air.

The issue is the ‘recirc” or recirculate feature of the HVAC system, generally depicted by a little pictograph of an arrow turning back on itself, within the outline of a car

Fogged up windows?
Recirculate switch separate from a/c

This “feature” closes off outside air, recirculating the air inside the vehicle to help warm things up more quickly in winter or cool the interior down more quickly in summer. It can also be used to shut off outside air when driving behind a vehicle emitting noxious fumes or creating a lot of dust.

But left in that position too long and you have problems, serious problems. Recirculate means just that – the air occupants have already used is being re-circulated and you are breathing it again. With each breath, more oxygen is extracted from the air as you breath in and more moisture is added to the air in the vehicle when you breath out.

Things may warm up or cool down more quickly, rut that moisture-laden air, when it comes into contact with the cool inside of the windows/glass surfaces quickly fogs up the glass. The use of the fan helps keep things in control as far as the windshield and rear windows are concerned, but the side windows soon become blocked.

At the same time, the amount of oxygen in the air inside the vehicle is decreased, making it pretty easy to become lethargic and not at your best.

HVAC systems have become more sophisticated in recent years and this is less of a problem with those that include automatic climate control to remove moisture. Other systems automatically draw in some fresh air to mix with recirculated air.

When you see a car with the side windows fogged up, do the driver a favour and tell them to check the recirc switch. And for goodness sake do not use your hands to clear glass, it results in oily deposits that only make visibility worse.

About Richard 166 Articles
At the age of five I was already obsessed with all things automotive being able to identify the make and model of car by just the sound of its engine going down the street in front of our house in the small town on the south shore of Nova Scotia. Although I have been covering and writing about the automotive scene for more than 40 years and the light still grows brightly.