One of the most inconvenient and costly home repairs to ceiling/floor damage due to evaporator coil condensate overflow is preventable.
If you have upper floor- or attic-mounted evaporator coils (they're the ones inside your house; the condenser is the one on the outside), make sure you check the drain lines and drain pans. If you see moisture dripping from one of the little pipe stubs under the eaves (that's where they usually are), go check the drain pan under the evaporator (usually located in the attic). If it has water in it, the primary drain line is clogged and you should call a professional.
Each unit has a primary drain line to the house sewer/plumbing drainage (often visible under sinks) to handle the amazing amount of moisture an evaporator coil can produce (up to 50 gallons on a very hot, very humid day!). If the primary drain line becomes clogged, the drain pan under the evaporator coil and its attached secondary drain line are supposed to handle the water. Often, they do not, resulting in costly and unsightly ceiling and floor damage.
We highly recommend you have your HVAC system serviced by a professional in the spring and fall. To kill fungus, prevent organic buildup, and keep your primary drain running smoothly, pour one cup of a 50/50 solution of bleach and water into the opening at the condensate drain line where it leaves the evaporator coil. Doing this in the spring and fall will also help prevent condensate from backing up and flooding.
Make sure the drain pan opening to the secondary drain line (usually white PVC) is not clogged with organic material, insulation or other debris. If the line is visible, make sure it has a downhill slope to the outside (they are often jostled about or stepped on in the attic).