Why is the air conditioner leaking water?
Water coming out of the outdoor drain pipe is usually normal. An indoor unit dripping inside, wetting the wall or overflowing the drain pan is not. This guide separates each symptom and explains the most likely causes and solutions.
Water is normal; where it appears determines whether there is a fault
In cooling or dehumidification mode, it is normal for the air conditioner to produce water and discharge it through the intended drain. What is not normal is finding water on the indoor unit, wall, ceiling or floor. In these cases, the condensate is not draining correctly or is forming somewhere it should not.
The most common causes are a blocked or incorrectly sloped drain, a drain pan that is not draining, a condensate pump, a frozen evaporator or external condensation on cold pipes and ducts.
Where does air conditioner water come from?
It is not water from the refrigerant or an internal water reservoir in the unit. It is mainly water vapour that was already present in the room air and condenses when it comes into contact with a sufficiently cold surface.
From indoor humidity to the drain pipe
The evaporator operates at a lower temperature than the return air. When its surface falls below the dew point, water vapour turns into liquid.
Quick diagnosis based on where the water appears
Select the symptom that most closely matches your case. The same unit can produce water normally while also leaking elsewhere, so the exact source needs to be identified.
Where do you see the water?
The location is the most useful clue for prioritising causes.
The indoor unit is dripping
The condensate is not reaching the drainage point or water is forming on an unintended surface.
Indoor unit dripping or water from the outdoor drain pipe: they are not the same
The most common mistake is to interpret any water as a leak. The system is supposed to produce and drain condensate, but it must do so through the designed route.
Water comes out of the outdoor drain pipe
In cooling or dehumidification mode, it is normal for the pipe to discharge condensate. If you are unsure about Cool, Dry or Heat, also see the air conditioner remote control modes and symbols. On humid days there may be more, while on dry days or under low load there may be little or none.
The indoor unit is dripping
This is not a normal outlet. The leak may come from the drainage system, the drain pan, melting ice, a pump, the installation or external condensation on a cold component.
In a heat pump, frost accumulated on the outdoor coil melts during defrost and water or vapour may come out through the base of the unit. This is different from indoor summer condensate and may be normal if the outdoor drainage is properly arranged.
Main causes when the drain pan fills up or the indoor unit drips
A full drain pan is not the root cause of the problem, but the result of water entering faster than it can leave. These are the most common causes, starting with drainage-related issues and moving towards more technical faults.
Blocked drain
Dust, biofilm, dirt, small insects or debris can restrict or block the passage. Water backs up until the pan overflows through the casing or air outlet.
Incorrectly sloped, kinked or submerged pipe
A gravity drain needs a continuous downward slope. A sag, rising section, tight bend or submerged end can slow the flow and cause water to back up.
Dirty, displaced or cracked drain pan
A pan containing sediment may overflow before the water reaches the outlet. If it is deformed, incorrectly fitted, corroded or cracked, water can leak even if the pipe is not completely blocked.
Very dirty filter or insufficient airflow
When air does not pass correctly through the evaporator, the coil can become too cold. Proper air conditioner filter cleaning is the first maintenance task to check. If the airflow restriction remains, it can encourage ice formation and, when the ice melts, create a sudden flow of water that may overwhelm the drain pan.
Faulty condensate pump or float switch
In installations without gravity drainage, a pump lifts the water. If it loses power, jams, the float switch fails or the discharge pipe becomes blocked, the auxiliary pan can fill up.
Frozen evaporator
Ice can form because of insufficient airflow, a faulty fan, a dirty coil or a refrigerant-circuit issue. When it melts, the water may overflow or continue dripping after the unit is switched off.
Indoor unit not level
The drain pan and outlet need to be in the position specified by the manufacturer. If the unit tilts to the wrong side or the mounting plate is not level, water can collect and overflow.
Very high humidity or continuous outdoor air infiltration
It does not usually cause a leak by itself in a correctly installed system, but it greatly increases condensate production. An excessively low setpoint can also worsen other problems; see the ideal air conditioning temperature. High humidity can expose a partially blocked drain or insufficient insulation.
Water running down the wall: four sources that are often confused
When the damp patch does not appear directly from the casing, the leak may be hidden behind the indoor unit, inside the trunking or at the wall penetration. Rainwater can also enter through a poorly sealed opening.
The path of the damp patch helps identify the source
A mark that starts directly below the unit usually points to the drainage system. A patch that appears after rain may come from the outdoor penetration. If the surface is cold and “sweating”, it may be condensation on a poorly insulated pipe.
Do not cover the stain or seal only from the inside without locating the source. If there is water inside the partition, the necessary route must be opened, dried and repaired before restoring the finish.
Frozen evaporator: why does it form ice and then leak water?
Ice is not normal condensation. When the coil becomes too cold, water freezes on the fins. When the unit stops, the ice melts and can produce more water than the drain pan can remove quickly enough.
The symptom may disappear as the ice melts, but the cause remains
A blocked filter or air inlet are causes you can check safely. There may also be a dirty coil, a faulty fan, an incorrect sensor reading or low pressure in the refrigerant circuit due to a refrigerant leak.
Condensation on ducts, grilles or suspended ceilings
In ducted systems, the water may not come from the drain pan. An outer surface that is too cold can condense moisture from the plenum or indoor air, especially in humid areas.
The solution is not to collect the water, but to prevent it from forming
A supply duct can be much colder than the air above a suspended ceiling. If the insulation is insufficient, compressed or the vapour barrier is left open, moisture reaches the cold surface and condenses.
The behaviour of a wall-mounted split should not be extrapolated to an air handling unit. In addition, the RITE requires thermal insulation when pipes contain fluids at a temperature below that of the environment through which they run.
What can you safely check before calling a technician?
With an indoor leak, the priority is to prevent electrical damage and damp. There is no need to dismantle the casing, handle refrigerant or insert tools into the drain: a systematic visual check can already provide strong diagnostic clues.
Guided five-step check
Tick each step as you check it. This list does not replace professional intervention.
How to prevent the air conditioner from leaking water again
Prevention combines user maintenance, drainage checks and correct installation. No amount of filter cleaning can compensate for a pipe without the correct slope, a cracked drain pan or a duct without a vapour barrier.
Filters and airflow
Clean or replace the filters according to the manual and keep air inlets and outlets clear. Good airflow reduces the risk of ice formation.
Drain check before summer
Check that the outdoor drain pipe is clear, without kinks or submerged sections. On units with a pump, check operation and alarms.
Slope, level and sealing
After building work or moving the unit, check the unit level, pipe slope and sealing around the wall penetration.
Continuous insulation
Cold pipes and ducts need continuous insulation and a properly sealed vapour barrier, especially above humid suspended ceilings.
Quick diagnosis table based on where the water appears
This summary helps identify the likely cause; it does not confirm a fault. Inspecting the complete condensate route is what allows a blockage, installation defect or refrigerant-system problem to be distinguished.
| Symptom | Most likely interpretation | Common causes | First safe action | When to call a technician |
|---|---|---|---|---|
| Water comes from the outdoor drain pipe in cooling mode | Normal if it comes out at the intended drainage point and there is no indoor leak. | Normal condensation on the evaporator. | Check that it drains without causing a nuisance or wetting façades. | If there is no drainage for hours in very humid conditions while water is also appearing indoors. |
| The indoor unit is dripping | Not normal: the drain pan is not draining or receives more water than it can carry away. | Blocked drain, incorrectly sloped pipe, drain pan or pump issue, ice. | Switch off cooling mode, protect the area and check the outdoor end. | If it persists, or if there is ice, odour, a pump issue or electrical risk. |
| Full drain pan or sudden overflow | Condensate has accumulated inside the unit. | Blockage, reverse slope, submerged pipe, stopped pump or disconnection. | Do not handle the drain pan while the unit is energised; stop the unit. | As a priority if the drain pan is inaccessible or above a suspended ceiling. |
| Water running down the wall | It may come from the pipe penetration, a hidden connection or rainwater. | Loose drain, poor sealing, discontinuous insulation or outdoor water ingress. | Relate it to cooling/rain and document where it starts. | If the patch grows, smells, causes blistered paint or has a hidden source. |
| Drips after being switched off | It may be accumulated ice melting or water retained in the drainage system. | Dirty filter, low airflow, dirty coil or refrigerant-system issue. | Leave the unit switched off and check only the accessible filter. | If it recurs or there is a loss of performance. |
| Ice on the evaporator or pipes | This is not normal condensation. | Insufficient airflow, dirt or abnormal refrigerant-system conditions. | Stop cooling mode; do not restart repeatedly. | Whenever the ice returns after correcting a dirty filter. |
| Droplets on grilles or ducts | Surface condensation below the dew point. | Insulation/vapour barrier, air leaks, high humidity. | Avoid blocking grilles and determine whether the issue is localised or widespread. | When it affects the suspended ceiling, insulation or several grilles. |
| Water at the outdoor unit in heating mode | It may be normal during defrost. | Melting frost from the outdoor coil. | Keep the base clear and well drained. | If the water enters the building, pools or forms hazardous ice. |
Frequently asked questions about air conditioner water
Is it normal for a lot of water to come out of the outdoor drain pipe?
It can be normal on hot, humid days, after many hours of operation or in dehumidification mode. The important point is that the water exits through the intended drain, not from the indoor unit, wall or suspended ceiling.
Is it normal for the indoor unit to drip?
No. Indoor dripping indicates that the water is not draining correctly, the drain pan is receiving an abnormal flow or condensation is forming outside the intended route.
If no water comes out outside, is the drain blocked?
Not necessarily. With low humidity or low demand, very little condensate may be produced. It is more suspicious when the unit has been cooling for some time in a humid environment while it is also dripping indoors or the drain pan is full.
Can I keep using the unit if it is leaking water?
If the leak is indoors, it is sensible to stop cooling mode until the source is found. Continuing to use it can damage walls, ceilings, furniture or electrical components and encourage odours and mould.
Why does it only drip at night or on very humid days?
Higher relative humidity raises the dew point and increases the amount of condensate. It can also expose poor insulation on pipes, ducts or grilles that does not drip on dry days.
Why does it leak water after being switched off?
One possibility is that the evaporator froze during operation and the ice melts after shutdown. Water may also remain trapped because of incorrect slope and overflow when the airflow changes.
Can a dirty filter cause the leak?
Yes. It reduces airflow, can encourage the evaporator to freeze and worsens performance. But if the leak continues after cleaning it, the drainage, drain pan, pump and refrigerant circuit need to be checked.
How do I know if there is a condensate pump?
It is common when the drain cannot fall by gravity, for example in some cassette, ducted or upward-routing installations. It may be integrated or installed next to the unit and usually has a float switch, a small tube and sometimes an alarm.
Can I pour bleach or drain cleaner into the drain?
It is not advisable without specific instructions from the manufacturer. Some products can attack materials, dislodge dirt into inaccessible areas, affect a pump or generate fumes. Professional cleaning controls the access point, direction and recovery of the residue.
Is condensate water distilled water?
Not in the sense of pure, safe water. It may contain few minerals, but it has been in contact with air, the coil, drain pan and pipe, where it can pick up dust, metals and microorganisms. It should be treated as non-potable water.
