Why does a home split air conditioner leak?

The indoor part of a domestic split A/C essentially consists of cooling coils and a blower fan. Air from the room is pulled over cold coils and the air is cooled. Colder air can hold less humidity than warm air, so water vapour condenses as liquid on the coils. The cold, dry air is blown back into the room.

Meanwhile, the water on the coils drips on to a gutter or drip tray. This has an outlet which typically connects to a pipe on or inside the wall, and the water drains to the domestic drainage or else to outside. The A/C drips water when the drip tray is full, normally because of a blocked outlet.

The solution is to remove the A/C case and clean out the drip tray and outlet. Owners' manuals usually say to leave this to an appropriately qualified technician.

As for prevention, the air inlet of a split unit includes filters to trap dust. It is this dust that finds its way on to the coils and gets washed into the drip tray by the condensing water. Keeping the air filters clean should decrease the amount of dust that gets on to the coils and therefore increase the period between sessions of cleaning out the drip tray.