You notice a damp patch under the indoor unit, water marks on the wall, or a drip coming through the ceiling below a loft or ducted system. At that point, the question is straightforward: why is my AC leaking, and how quickly does it need attention? In most cases, a leaking air conditioner is not a minor nuisance. It is a sign that condensate is not draining properly, airflow is restricted, or a component has failed.
A small amount of condensation inside an AC system is normal. Water outside the system, on your floor, ceiling, or walls, is not. Left alone, it can damage plaster, flooring, electrics, and furnishings, especially in London flats, offices, and converted properties where pipe runs and drainage routes are often tight or awkward.
Why is my AC leaking indoors?
The most common reason is a blocked condensate drain. Your air conditioning system removes moisture from the air as it cools the room. That moisture collects as condensation and should flow safely away through a drain line. If that line becomes blocked with sludge, dust, algae, or debris, the water backs up and starts dripping from the unit.
This is one of the faults we see regularly in both homes and commercial spaces. In a well-used office, restaurant, or retail unit, filters and coils can gather dirt quickly. In a flat or rental property, maintenance may simply have been missed. The result is the same: the system still produces condensation, but the drainage path cannot cope.
Another common cause is a dirty air filter. When airflow is restricted, the evaporator coil can become too cold and start to freeze. When that ice melts, it can produce more water than the drain tray was designed to handle, and the excess ends up leaking indoors. This often starts as a performance problem before it becomes a visible leak. Rooms take longer to cool, the unit runs for longer, and then the dripping appears.
A damaged or misaligned drain tray can also be the issue. Older systems, especially in tenanted properties or ageing commercial units, may have trays that are cracked, corroded, or no longer sitting correctly. If the tray cannot catch and direct condensate properly, water escapes before it reaches the drain line.
Common causes of AC leaks
There is no single answer to why is my AC leaking because the fault can come from several points in the system. Some are simple maintenance issues. Others need an engineer with the right tools and refrigerant handling certification.
Low refrigerant is one example. If refrigerant levels drop due to a leak, pressure inside the system changes and the evaporator coil can freeze. Once the ice thaws, water can spill over. This is not something to ignore or top up casually. Refrigerant problems need proper diagnosis, leak testing, repair, and recharging to the correct specification.
A failed condensate pump is another possibility. In some properties, especially where the indoor unit sits below the drain outlet level, the system relies on a pump to move water away. If that pump stops working, the water has nowhere to go. This is common in basement spaces, offices with adapted layouts, and some wall-mounted systems in awkward positions.
Installation faults can also cause persistent leaking. If the unit is not level, if the drain pipe has the wrong fall, or if insulation has been fitted poorly, condensation can collect in the wrong place or form on pipework outside the tray. In newer installations, this points to workmanship. In older buildings, it can be a result of movement, alterations, or previous patch repairs.
Then there is simple wear and tear. Seals degrade, connections loosen, and pipe insulation breaks down over time. In a Victorian terrace conversion or a busy commercial property, that ageing process can be sped up by limited access, dust, vibration, and long operating hours.
What you can check before calling an engineer
If water is actively dripping, switch the unit off first. That reduces the chance of further overflow and helps protect nearby electrics and finishes. If safe to do so, place a towel or container under the leak to limit damage.
Next, check the filter. If it looks clogged with dust, that may be restricting airflow and contributing to icing. Some filters can be removed and cleaned, depending on the unit type and manufacturer guidance. If you are unsure, do not force any panels open.
You can also check whether the drain outlet is visibly blocked, if it is accessible. On some systems, there may be obvious dirt or standing water in the tray area. However, avoid dismantling the unit or attempting to clear drains with makeshift tools. It is easy to push debris further into the line or damage internal components.
If the system has been leaking for more than a short period, check the surrounding area as well. Staining on walls, swollen skirting, damp ceiling plaster, or a musty smell can indicate ongoing water escape rather than a one-off overflow. That matters because the repair may need to address both the AC fault and the resulting property damage.
When a leaking AC becomes urgent
Some leaks can wait a few hours for a booked visit. Others should be treated as urgent.
If water is close to sockets, lighting circuits, ceiling void electrics, or server equipment, the risk is no longer just water damage. If the unit is in a tenanted property, shared commercial space, or managed block, delays can quickly become more expensive than the repair itself. One leaking cassette unit above a suspended ceiling can affect multiple rooms before anyone realises how far the water has travelled.
Urgency also increases if the AC is part of a business-critical environment. Shops, offices, clinics, and hospitality spaces often rely on cooling for comfort, equipment protection, or compliance. A leak combined with poor cooling may point to a broader system fault that needs prompt attention.
Landlords and property managers should also act quickly because recurring leaks often trigger complaints, decoration damage, and disputes about maintenance responsibility. In mixed-use London buildings, what starts as a leak in one unit can become a ceiling issue in another.
Why leaks are common in London properties
London buildings present their own challenges. Older homes converted into flats may have limited service voids and improvised pipe routes. Modern developments often pack HVAC systems into tight cupboards or ceiling spaces where routine servicing gets overlooked. Commercial fit-outs can add further complexity, especially when layouts change but drainage and condensate pumps are not upgraded to suit.
That is why the cause is not always obvious from the symptom. The visible drip may be at the front of the unit, but the real issue could be a blocked line several metres away, poor pipe insulation in a ceiling void, or a pump failure hidden behind access panels. A proper diagnosis matters more than a quick guess.
How engineers usually fix the problem
The repair depends on the fault. A blocked condensate line may need flushing and cleaning. A dirty filter and coil may require a full service. A cracked tray might need replacement. A failed pump will need testing and changing if it cannot be restored.
If refrigerant is involved, the job becomes more technical. The system must be pressure tested, the leak identified, repaired correctly, and the charge reset to manufacturer standards. Simply adding refrigerant without fixing the leak is not a proper repair and usually means the problem returns.
Where installation is at fault, the remedy may involve correcting the pipe fall, re-levelling the unit, replacing insulation, or reworking part of the condensate route. That is why transparent diagnosis is important. The right engineer should explain whether the issue is maintenance-related, component failure, or an original installation defect.
How to stop your AC leaking again
Regular servicing is the best way to prevent repeat leaks. That means cleaning filters and coils, checking drain lines, testing pumps, inspecting trays, and confirming the system is operating at the correct pressures and temperatures. For landlords and businesses, planned maintenance is usually far cheaper than emergency call-outs, water damage repairs, and tenant disruption.
It also helps to act early. If your AC is cooling poorly, making unusual noises, switching on and off too often, or producing a damp smell, do not wait for visible water. Leaks often follow earlier warning signs.
For homes and businesses across London, the practical answer is simple: treat a leaking AC as a fault, not as normal condensation. If the cause is minor, it can often be sorted quickly. If it is not, early intervention limits damage, keeps the system safe, and gets your cooling back where it should be. When water is coming from an AC unit, speed and proper diagnosis matter more than guesswork.