How the Air Conditioner Works

• An air conditioner contains three main sections. A pump compresses a gas contained inside the air conditioner's sealed system and forces the hot vapor into the unit's condenser coil. As waste heat radiates from the coil, the gas becomes a hot pressurized liquid and flows to the evaporator coil. As a controlled flow of hot liquid passes through the evaporator, it changes to gas and the coil chills. A fan blows warm air over the cold coil, and moisture condenses onto the coil as the cool air flows into the room or into the ducts of the central air system.
  
  

Waste Water

• Window air conditioners use the bottom of the unit's metal cabinet to channel condensation to the exterior of the building. A small drain hole in the rear of the unit lets water out. Drain pipes connected to the opening channel the water into ground level drains or onto the lawn. The unit must tilt down towards the outside or water backs up into the home. Debris in the cabinet's drain could also cause problems. Any water draining into the room from an upstairs unit saturates the floor and leaks through to the ceiling below.
  
  

Ducts

• Central air conditioning systems usually place the air conditioner outside the building, circulating interior air to and from the unit through a sealed system of ducts. Ducts carry cold, dry air to all parts of the home, delivering the air through vents in room ceilings or floors. Humid attic air contacting the cold metal of an air conditioning duct causes water to condense on the outside of the duct. Condensation pools below, damaging the ceiling. Correctly installed ducts include an outer blanket of insulation and a sealed moisture barrier covering. Hot humid air never contacts the cold duct.
  
  

Correcting Problems

• To prevent condensation on ducts, wrap the ducts with insulation rated from R6 to R8 and cover with an impermeable moisture barrier. Inspect the cabinet drain at the rear of any upstairs window unit to ensure water flows out. To clear a clogged drain, take the unit out of the window and disassemble the cabinet outside the house. Clear away all dirt and debris before reinstalling, and check the new base angle with a carpenter's level. Older air conditioning systems could have unusual problems. Air conditioners installed in attics could physically settle over the years, reversing the water flow in drains, or mice and insects could clog the drain pan and pipes.