An indoor central air conditioning unit may be leaking water due to a buildup of excess condensation on the internal cold evaporator coil, a blocked condensate drainage pipe, a rusted condensate drain pan, a frozen evaporator coil or improper installation. If water is noticed beneath an air conditioning unit, it is important to unplug the unit first before attempting to diagnose the problem.
Most air conditioning units have a dehumidifier built into the design. This dehumidifier works to pull water out of the air by passing it over a chilled evaporator coil in the body of the unit. On a particularly humid day, the dehumidifier pulls more moisture out of the air than normal. The water builds up on the evaporator coil in excessive amounts and could overflow the drainage pipe or leak through the unit's drain pan. If the drainage pipe becomes obstructed by dirt, leaves or insects, the water formed on the evaporation coil can overflow and leak out of the unit.
Ice can also build up on the evaporator coil if the air conditioning unit is not properly ventilated due to a dirty air filter. Once the air filter is changed, the ice melts from the coil and could leak out of the unit. Finally, if the air conditioning unit was installed on a slant, this could cause the water to run off of the drain pan and leak out of the unit.