Clogs can happen when the drainage line fills up with mold, slime, and sludge. This may happen if your HVAC system hasn’t been cleared or treated in a while.
When the pipes and the system have been clogged, the water flow from the drainage pan to the plumbing pipes slows down. The system cannot efficiently remove the water out of the house. As a result, the drainage pan fills up with water at a faster rate. This leads to an overflow of the first pan or even the secondary drainage pan.