Draining Central Air Conditioners
Turn off the air conditioner, then switch off the breaker powering the unit to ensure no electrical current is flowing to the appliance.
Remove the drain or condensate pan from under the main unit by unfastening the retaining screws with a screwdriver. Pull the drain pan out and pour the water out onto the ground.
Replace the drain plan under the main unit and secure in place with a screwdriver.
Go to the outside of the house. Locate the air conditioner drain pipe -- typically a PVC pipe running from the attic space down along an exterior wall near the garage.
Hold a garden hose to the end of the AC drain pipe and turn the water on its fullest open volume, shooting water up into the pipe for at least 30 seconds to backwash it, then pull the hose away. Let debris drain from the pipe, then repeat until no debris drains out.
Go up to the main air conditioning unit and find the connection point of the drain pipe. Remove the cap from the pipe by hand, twisting it up and off. Pour bleach into the pipe to clean it out.
Switch the breaker back on, then turn the air conditioning unit back on.
Draining Wall Units
Turn the wall unit off and unplug it from the electrical outlet.
Go to the outside of the house and remove the unit's housing cover with a screwdriver to expose the components and drip pan.
Pull the drip pan out and turn it upside down, emptying it of water. Because these units do not generally have drain lines, you'll need to periodically drain the condensate pan.