Start by turning the unit off at the thermostat or its controls. That stops more condensate being produced while you sort it out, and often slows or stops the drip.

• Put a container or towels underneath to catch the water and protect flooring, and move anything below the unit out of the way.
• Take a photo of where the water is coming from - it's genuinely useful for whoever attends.
• Note whether it's a slow drip or a steady flow, and whether it started suddenly.
• Book a visit rather than waiting to see if it stops. Water finds its way into ceilings and walls.

If water is running near light fittings, sockets, the indoor unit's electrics or any other electrical equipment, treat it as an electrical hazard. Don't touch the affected equipment or attempt to isolate anything yourself if doing so means going near the water, and get appropriate professional help straight away. If anyone is at risk, contact the emergency services.

A leak has several plausible explanations, and they need looking at to tell apart.

• A blocked or restricted drain line. Condensate that can't drain away has to go somewhere, and it overflows the unit instead. One of the more common causes.
• A dirty filter or restricted airflow. Reduced airflow across the coil can lead to conditions where water doesn't drain as designed.
• Dirty cooling components. Build-up on the coil can cause water to run off in the wrong place rather than into the drain pan.
• A drain pan problem - damage, corrosion or a pan that isn't draining properly.
• Condensation on pipework or on the unit itself, particularly where insulation has failed.
• A frozen coil. If ice forms on the coil and then melts, that meltwater can appear as a sudden flow of water well beyond what the drain was designed to handle.
• Installation or drainage design issues - a drain line without the fall it needs, or a unit that isn't level.
• Other component faults that need inspection to identify.

A leak doesn't automatically mean any one of these. Two systems with identical symptoms can have entirely different causes.

It depends entirely on the cause.

If the leak is down to a restricted drain line or heavy build-up on the filter and coil, then it's a maintenance issue, and a service that includes a drain-line flush and internal cleaning may well resolve it. That's part of what our deep clean covers - see what a deep clean includes and our AC cleaning options.

If the cause is a damaged drain pan, a drainage or installation fault, or a problem causing the coil to freeze, cleaning won't fix it. Those need diagnosis and repair. If the coil is freezing, it's also worth reading why an AC may stop cooling, as the two often appear together.

If you don't know which situation you're in - and from the outside it's often impossible to tell - book a diagnostic. A technician establishes the cause and quotes the work in writing before anything is done.