AC Dripping Water? 3 Common Causes Explained

Why an AC Unit Drips Water

The surface of the evaporator coil is cold during operation, which causes moisture in the air to condense into water droplets. Under normal conditions these droplets collect in the drain pan and exit through the drain pipe to the outside or a drainage system. If the drainage path is blocked, condensation builds up inside the unit and overflows — causing the indoor unit to drip.

Hong Kong's humid climate means the AC produces a large volume of condensation every day it runs. Any obstruction or slope issue in the drainage system quickly becomes apparent.

The Three Most Common Causes

First: a blocked drain pipe. Dirt, algae, or sludge inside the drain pipe is the most frequent culprit. The technician will clean the drain pan and clear the pipe, which may require partially dismantling the pipework.

Second: incorrect installation angle. The unit must be tilted at the right angle for condensation to flow toward the drain outlet. A loose bracket, a shifted unit, or an inadequate original installation angle can cause water to pool or flow the wrong way. Third: a frozen evaporator coil. Severe filter blockage, insufficient refrigerant, or a faulty fan can drop the evaporator temperature low enough to form ice. When the ice melts, the volume of water exceeds the drain pan capacity and causes heavy dripping.

• Blocked drain pipe: most common — usually requires a technician to clear
• Incorrect unit angle: bracket or installation position needs checking
• Frozen evaporator coil: switch off, let the ice thaw naturally, then investigate the cause
• Cracked or deteriorated drain pan: less common — replacement required

Immediate Steps When You Notice Dripping

Switch the unit off and cut the power as soon as you notice dripping, to prevent further water accumulation and any contact with nearby electronics. Place a towel or basin under the drip point and move away any electronic devices or important documents that could be damaged.

If there is a large volume of water or the wall already shows damp stains, a towel is only a stopgap. Arrange for a technician to come and identify the cause — repeatedly soaking up water with towels only delays fixing the actual problem.

When You Should Not Switch the Unit Back On

If the dripping is accompanied by an unusual smell, a large volume of water, damp walls, or there are power sockets nearby, do not restart the unit until a technician has confirmed the cause and completed a repair. Continuing to run it risks water penetrating further into the wall or creating an electrical safety hazard.

AC Dripping Emergency Checklist

• Switch off the unit and turn off its circuit breaker or power switch immediately
• Place a towel or basin under the drip point
• Move nearby electronics and important items out of the way
• Note the drip location and approximate water volume (useful information for the technician)
• Photograph any damp stains on the wall
• If an unusual smell or heavy dripping is present, do not restart the unit
• Contact a technician to arrange an on-site inspection

Frequently asked questions

I cleaned the filter — why is the unit still dripping?

Filter cleaning only addresses the airflow inlet. Blockages deeper inside the drain pan and drain pipe remain. If dripping continues after a filter clean, a technician needs to flush out the drain pan and clear the drain pipe.

My newly installed AC is already dripping — is it an installation problem?

A new unit dripping immediately after installation may have been set at the wrong tilt angle or have a poorly connected drain pipe. Contact the installation team to inspect and adjust. Do not attempt to dismantle the unit yourself.