The truth about air conditioner units is that they leak water. One that leaks will drive you crazy attempting to seek out the reason for the matter. However, the common reason why your air conditioning unit leaks water is unit icing.
Air conditioners have a condenser coil and an evaporator coil. The purpose of the evaporator coil is to extract heat from space. The purpose of the condenser coil is to expel that heat to the atmosphere. Between the two coils is a refrigerant that transports that heat.
Air conditioner systems are designed with exact specifications and are delicately balanced between evaporator and condenser; the coils have to be the right size, the air flow has to be in the right amounts and the refrigerant has to be charged to specific amounts. If any part of the system gets off balance, the unit operates inefficiently, or sometimes, too efficiently.
Most air conditioning units have digital programmable thermostats which are meant to save energy by turning units off during times when it is expected that a building would not be occupied. They also protect units from operating outside their ideal parameters.
But there are times when you want to override the program, say perhaps because you are staying late at work one night and you want one or more units to run while you are still there. This typically isn’t a problem, but it can become one if you override the thermostat to run the unit continuously 24/7. It’s not unusual for this to happen in commercial air conditioning applications such as big box retail stores. The manager or an employee overrides the program and then forgets that they have done so. The unit is left to run continuously.
As mentioned above, to run properly, an air conditioner needs to have all its loads in balance. When a unit runs at night, there are very few loads. The space inside is relatively cool because the lights are off, the people are gone, and the computers are off. During this time, the evaporator doesn’t have to work very hard to remove heat because there is less heat to remove.
Outside it is very cool, so the condenser is also finding it very easy to expel heat to the atmosphere so the unit is running very efficiently, in fact too efficiently. With so little load, the evaporator coil gets colder, so cold that it can often drop below freezing. Cold air cannot hold as much moisture as warm air so it is typical for evaporator coils to condense water out of the air and when an evaporator coil drops below freezing that evaporated water starts to freeze. Essentially the evaporator coil becomes an ice machine! Ice will continue to build as long as the thermostat is on. Eventually, the ice block becomes so large that air can no longer pass through the evaporator coil properly. With no air blowing across the evaporator coil the unit can only blow outside air into space. Morning comes, and it warms up outside, now the unit is blowing warm outside air, the ice starts the melt and presto! You have a water leak and a unit blowing warm air.