If you have an a/c you have on top of the furnace a condensation drain for that a/c.
It’s a plastic or copper pipe half to an inch in diameter that takes away the condensation produced by the evaporator coil of the a/c during the air cooling process. It’s the moisture from the air in your house that turns into water when it comes in contact with the cold evaporator coil on top of the furnace.
The condensation drain can be draining into the floor drain, into a condensation pump, into a laundry tub or in some instances directly outside through the wall.
Sometimes over the years this drain may become slow due to sludge buildup or –as it’s often the case – It can be completely plugged usually at the point of connection to the coil. As a result the condensation (water) produced during the cooling process fills the evaporator pan and flows freely into the furnace causing rusting, and in many cases destroying sensitive electronic and electric components such as computers, ignition modules, gas valves, relays and even motors.
How much water can that be? On a hot humid day the a/c in a average home can produce as much as 20-30 gallons of condensation !!
So a small simple dirt build up can cost big bucks and take years away from the life of a furnace.