On the inside of your house, you will have an evaporator unit that extracts heat from the air flowing into your home. It accomplishes this feat through the science involved in phase conversion, which occurs when a substance changes from a solid to a liquid or a liquid to a gas. In this case, liquid coolant converts to a vapor, and as it does so, it absorbs heat from its surroundings. While phase conversion is a good thing when your unit is running properly, it is a bad thing when your system starts to malfunction. If you see ice on your evaporator coils, you know something is not running like it should.
Signs of a Problem
When ice forms on your evaporator coils, it will block the flow of air to your ducts. Thus, if you don't have cool air billowing from your ducts on a hot day, you have your first clue that something is not right. The second thing you should do is listen to your system. If you hear the sound of your fan running, then you know that at least your fan motor is working properly. You should then go to your mechanical room and inspect your coils. If you find that they are coated with ice, then you know where the problem is.
Causes of Icing
Your AC unit should be designed so that the air flowing over you coils warms them enough to prevent icing while still having enough cold to lower the temperature in your home. If you don't have enough air flowing over your coils, they can get so cold that they begin to freeze the water vapor present in the air in your home.
What to Do about Icing
The first thing to do when you find that your coils are iced over is to turn off your system, place a bucket and some towels under your evaporator to catch the water that runs off of the coils, and let the ice melt. Next, you should find the cause of your problem. First, check your register—the air intake vent for your AC unit—to make sure nothing is blocking it. A pile of clothes or a piece of furniture can interfere with airflow enough to cause problems. Next, check your filter to make sure that it is clean. And finally, check your coils. If they are coated with grime, they can constrict airflow enough to cause problems.
Dealing with iced over coils should be something that you can take care of on your own. At the very least, you can handle the steps described above on your own. If you still have problems with your coils, you can call a professional (such as one from Allied Air Conditioning & Heating Corp) to pick up where you left off.