Signs of Water
Mold requires moisture to grow. So if your heating system has been wet for a while, chances are that mold has begun to develop. Look for staining and water damage around ductwork, and check the insides of the ductwork for puddles and condensation. If you see water, fix the source of the leak immediately.
If someone in your family is experiencing symptoms of mold exposure, but you can't see mold anywhere in your home, it may be in your heating system. The New York State Department of Health warns people to look out for the following symptoms: throat irritation, headache, cough, breathing problems (wheezing and difficulty breathing), nasal and sinus congestion, skin rash and eye irritation. Your doctor can help you determine if mold is a possibility by administering a mold-allergy test.
If there is visible mold growth on your heating system or its metal ductwork, the U.S. Environmental Protection Agency (EPA) recommends that you have a professional take a look at it. For example, if you see a thick growth of what appears to be mold in the metal ducts or on the air registers, there is a good chance your entire system is infected with mold. The EPA says the only way to know for sure if the substance is mold is to have a laboratory examine it under a microscope. The EPA says some microbiology labs offer the test for around $50, as of 2010.
If you smell a musty odor, you may have a mold problem. Try to determine the source of the smell. If you notice that it gets worse the closer you are to the air ducts and vents, or if the musty smell is more powerful when the heating system is circulating air, have a professional check out your system.