Visible mold on your basement walls, carpet, or drywall is a problem. However, the presence of mold in HVAC (heating, ventilation, and air conditioning) systems poses a serious risk to you, everyone residing in your house, and even your pets. When your furnace or air conditioner runs, the fan turns on, and the airflow will spread mold spores throughout your entire home if your HVAC system has mold. Every room and area will be contaminated with mold spores. Your mold problem could get worse the longer the system is used while polluted.