1. Air leakage around air conditioner.
If warm air is able to enter around the air conditioner it will encounter cooler, dryer air. When they meet condensation will occur. If water leaking from front of air conditioner inspect to see if dripping from body of machine, or water droplets clinging to front area. To test, operate machine for 30 minutes and then use flashlight to check under front edge of base. Small water droplets here indicate an air leakage problem. Add foam insulation to stop warm air from infiltrating.
2. Drain hole blocked.
Rear of air conditioner base has a drain hole or groove to allow water to escape. If becomes blocked water can back up. To test, operate machine for 30 minutes and then inspect if draining properly. If appears blocked use a small piece of wood to open drain hole at rear of metal base. CAUTION: Never be tempted to drill holes into the air conditioner body to relieve water pooling. Severe damage can result.
3. Internal drains blocked.
There are small passageways that allow water to drain from front of air conditioner to the rear. If they become blocked water will pool at front of machine and overflow onto floor. If this happens the air conditioner will require removal from window and servicing.
4. Outside temperature too cold.
This occurs at end of cooling season. If outside temperatures drops below 60 degrees Fahrenheit at night the cooling coil may ice up. If no leakage at bedtime but water in front of machine in morning, suspect this problem. If this problem suspected turn off machine before bedtime and restart as day warms. Alternately, operate machine at night with selector switch in ‘fan only’ position. This will circulate room air during night but not allow cooling.
from article site.