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Rob's Garage

What's that puddle under my car? Add to ...

Dear Rob:

I hope you can settle an argument I'm having with my neighbour.

After I have parked my Acura in my driveway for a few minutes, a puddle appears on the blacktop just under the right hand side in front of the passenger seat. My neighbour says it's just condensation from the air conditioning but I'm worried that it could be something else.

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He told me that he experiences this on his car, but I can't honestly say that I have noticed this happening on my car before.

Can you help me out Rob?

Thanks, Bob

Bob, the short answer is: your neighbour is right.

To be accurate, what you're experiencing is melting ice that forms naturally on the fins of a device located in front of the dashboard called an evaporator. The evaporator, like the entire A/C system, is full of refrigerant - a key device in the air conditioning system that cools the cabin of the vehicle.

Very 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 100 degrees Celsius. This means that up to the 100 degree point, the water takes that heat away - until the element gets so hot that 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. The refrigerant used in most vehicles these days is called R134a. Because its boiling point is approximately -26° C, it would freeze everything it comes in contact with if left uncontrolled.

The A/C system's job is to control this freezing point by controlling such things as the pressure of its working environment. Keeping refrigerant under pressure will increase the temperature at which it boils and in the right circumstances, this can be controlled to just above 0 degrees C, which of course is the freezing point of water.

Hang in there Bob; this is coming around to your question real quick.

This temperature at the evaporator works for a few great reasons:

1. Any air drawn across the fins will be cooled

2. When cold air crosses the fins, any moisture in that air will be forced to condense. This dries the air blowing into the cabin, increasing the effect of the A/C system, and

3. The condensate that forms on the fins becomes an air filter, trapping dust and pollen in the water, preventing it from entering the cabin.

When you turn off your car, this condensate and ice begins to melt and run off. This water drops off the evaporator fins, lands in the bottom of the evaporator holder (Plenum), and finds its way out the bottom through what's lovingly referred to as a "burp valve." This is a rubber one-way valve that permits water to run out, but will not allow dirt, water, road contaminants, to work their way back into the plenum.

While I'm on the subject, this burp valve can be a source of grief. If the condensate coming off the evaporator is laden with guck (technical term), the burp valve can plug. This causes the plenum to fill with water and guck. Left unchecked, it will start leaking into the passenger compartment. Even worse, mould can form inside the plenum and/or on the evaporator fins, contaminating the air in the cabin. On older vehicles this valve should be checked every spring as a precaution.

So Bob, the liquid you see forming under your car is indeed water and if you look straight up from the driveway, you should see the black rubber burp valve doing its job.

Who buys whom the next driveway beer Bob?

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