Water dripping from an air conditioning indoor unit is one of the most alarming things to discover — particularly when it is dripping onto a ceiling, a piece of furniture or a floor below. Here is a clear guide to why air conditioning units leak water, which causes are minor and which need immediate attention, and what to do right now if yours is leaking.
First — Is It Actually Leaking or Just Condensing?
Before anything else, confirm that what you are seeing is actually water from the unit and not condensation forming on cold pipework. In very humid conditions, the refrigerant pipes running from the indoor unit to the outdoor unit can develop surface condensation where their insulation has deteriorated. This looks similar to a leak but is a different problem with a different fix (re-insulating the pipework).
Cause 1 — Blocked Condensate Drain (Most Common)
This is the cause of the vast majority of water leaks from air conditioning indoor units. When the system cools the air, moisture condenses on the evaporator coil and drips into a drain pan below the coil. This water drains away through a condensate drain pipe to the outside (or to an internal drain point).
Over time, algae, mould and debris accumulate in this drain pipe, eventually causing a partial or complete blockage. Water backs up in the drain pan and overflows into the room. You will typically see water dripping from the front or bottom of the indoor unit.
Immediate action: Switch the system off to stop further water production. Place a towel or container under the unit. Call us to clear the drain. This is a straightforward fix — we clear the blockage, flush the drain, and treat it with a biocide to prevent recurrence. Drain clearing is included in our annual service.
Cause 2 — Frozen Evaporator Coil
If the air filter is heavily blocked, the evaporator coil can freeze — ice forms on the coil surface. When the system is eventually switched off, or when the ice becomes excessive, it melts rapidly and floods the drain pan beyond its capacity, causing a significant water discharge from the unit.
Signs of this cause: the unit has been running but not cooling well, you may notice reduced airflow from the unit, and when you open the front panel you may see ice on the internal components.
Immediate action: Switch the system off. Check and clean the air filter. The ice will defrost over 1-2 hours. Once defrosted, check the drain pan has cleared and run the system in fan-only mode for 30 minutes to ensure all moisture has drained before switching back to cooling mode. If the problem recurs after filter cleaning, low refrigerant may be the underlying cause — call us.
Cause 3 — Disconnected or Damaged Drain Pipe
The condensate drain pipe can become disconnected from the drain pan connection point — either through vibration over time or through a poor original installation. If the pipe falls away from the indoor unit, water collects in the drain pan and overflows. This typically causes a more sudden and significant leak than a blocked drain.
What to look for: Water coming from behind or above the unit rather than dripping from the front. Switch off and call an engineer — reconnecting and properly securing the drain pipe requires access to the rear of the indoor unit.
Cause 4 — Incorrect Installation (Drain Pan Not Level)
The indoor unit must be installed perfectly level for the drain pan to drain correctly. If a unit has been installed with a slight tilt towards the front or to one side, water collects at the low point and eventually overflows. This is an installation error that typically causes intermittent leaking from day one rather than developing over time.
Cause 5 — Refrigerant Leak Causing Over-Cooling
A refrigerant leak can cause the evaporator coil to run at an abnormally low temperature, condensing significantly more moisture from the air than usual. This can exceed the capacity of the condensate drain system, particularly in high-humidity conditions. If you have a leaking unit and the drain appears clear, low refrigerant may be a contributing cause.
What to Do Right Now
Switch off the system. Protect the area below from water damage. Clear the filter and check if the drain is blocked. If the problem persists or the water is significant, call us. A leaking unit left running causes progressive water damage to ceilings, walls and flooring that is typically much more expensive to repair than the air conditioning fault itself.
Call 07833 053749 for emergency drain clearing. We cover East Grinstead, Surrey, Sussex and South London with same-day attendance for urgent leaks.