Furnace coils are not only typically inside the furnace assembly, but may be in their own casing, as well. Most of the problems associated with dirty coils come from the lack of access homeowners have to clean them.
The buildup of dust and dirt can cause problems. Dirty furnace coils are not as good at transferring heat energy, and you’ll start to see your system’s energy-efficiency dip. In severe cases, extremely dirty coils can trap heat inside your furnace, causing the system to overheat and trigging temperature failsafes that shut off the system entirely.
When the air conditioner or heat pump is in use during the summer, the cold refrigerant stored in the heat exchanger, reacting with the warm air in your house, can cause the exterior to generate liquid condensate. This can lead to ideal conditions (warmth and water) for mold and mildew to develop. For obvious reasons, you’ll want to deal with this before it negatively impacts your indoor air quality.