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Although this has been a hot and humid summer, I thought I would try driving with my air conditioning off. However, as soon as I turned it off there seemed to be more humidity inside the car than outside. I'm not making this up. Is this possible? It actually got so bad that the windshield fogged up - and it was 34C outside.


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Hey Earl, I believe you - so don't sweat it.

Sorry, there was no way I could hang that pun out to dry.

Most people don't know that air conditioning does more than cool the air. As you discovered Earl, it not only dries but cleans the air as well. Here's what happens:

When the A/C is turned on, this system works in the same manner as the cooling system for your engine. That is, it removes heat from the passenger compartment and expels it to the atmosphere. The major difference being the medium that is used. Instead of coolant transferring the heat away, the A/C system uses refrigerant.

Simply, refrigerant is a liquid that has an extremely low boiling point. This matters because the lower the boiling point of any liquid, the easier it is for that liquid to remove heat. Think of water boiling on a stove. Its boiling point is 100C. This means that up to the 100 degree point, as the stove element is heating up, the water is taking that heat away. In essence, there is a balance of the temperature of the element to the water - until the element gets so hot that the water can't take the heat away any longer. At that point, the water starts to boil like crazy, trying to keep up with the heat removal.

A/C refrigerant works the same way, but the difference is that it doesn't have to remove heat at the 100 degree level.

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Modern refrigerants such as R134A have a boiling point of -26.3C. This means that if it is not controlled or contained, it will try to remove heat (cool) anything it touches to -26.3C. If it was poured onto a table at room temperature, it would almost instantly boil and turn into a vapour - all the while freezing anything it came in contact with. This is why propane tank refilling stations take such care when refuelling a 20-pound propane tank. Same thing, because propane boils at -42C.

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Inside the A/C system in your car, R134A is being pumped through a series of high-pressure hoses and, what appear to be forms of radiators. The first "radiator" that the refrigerant comes in contact with is under your dashboard and it's called the evaporator. As the name implies, it allows the liquid refrigerant to evaporate. Remember the part about pouring R134A on a table: if allowed to evaporate (or boil), it will remove heat at an incredible rate. This heat removal or evaporation or cooling or whatever you want to call it, is what cools the air entering the passenger compartment. The heat that is absorbed is given off to the atmosphere by the second "radiator" - the Condenser. This is located in front of your engine's radiator, behind the grill.

With this rapid cooling going on, the fins of the evaporator get extremely cold. Any air that passes by these fins on its way into the passenger compartment will also be cooled - so much so that any moisture (humidity) in that air will condense on the evaporator fins and stick to those fins. This removes the humidity or moisture from the incoming air. The air leaving the evaporator is now very dry. This is the difference you noticed, Earl. You may have also experienced drying eyes after long periods of A/C operation. This is because the air is now so dry in the cabin that it affects anything with moisture in or on it.

As the moisture collects on the evaporator, dust and other contaminates stick to the water droplets that have formed on the fins. This has the added benefit of cleaning those particles out of the air.

As an aside, this system is so effective that most car manufacturers employ the A/C system any time the "Defrost" mode is selected on the dash. In defrost; the A/C system will dry the windshield much faster than by using heat alone.

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There you go Earl, I know you're not making this up, this is why it's called air conditioning - not just air cooling.

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About the Author
Globe Drive columnist

As associate dean of Motive Power programs at the British Columbia Institute of Technology, Rob MacGregor has nearly three decades' experience in training the province's automotive technicians. He has written extensively on car mechanics, appears regularly on television and is a member of the Autotmobile Journalists Association of Canada. More

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