A basic clean deals with the parts of the system that collect dust fastest and are reachable without stripping the unit down. On a split system, that means cleaning the indoor filter and the air vents, plus a clean of the outdoor unit. It's the maintenance job - it keeps airflow strong and stops the gradual drop in cooling that comes from a clogged filter.

Routine cleaning is quick, and on a system that's used heavily it's the work that needs doing most often. What it doesn't do is reach inside the unit.

A deep sanitisation strips the indoor unit down so the components inside can be cleaned properly - the cooling coil and the blower barrel are cleaned and sanitised, the drain line is flushed, and the outdoor unit gets a full clean.

The distinction is simply access. A filter can be pulled out and washed. A coil and a blower cannot, and that's why they need a different type of visit.

Air passes over the cooling coil to be cooled, and the blower barrel is what pushes that air into the room. Both sit in a damp, constantly moving airstream, so fine dust settles on them and stays there.

A coated coil transfers heat less effectively, and a fouled blower moves less air. Neither is reachable from the filter grille, which is why cleaning them means opening the unit up.

Your AC pulls moisture out of the air, and that condensate has to drain away. The drain line is a common place for build-up, and a restricted line is one of the reasons an indoor unit can start dripping water. A drain-line flush is included in our deep sanitisation rather than the basic clean.

If your unit is currently leaking, read why an AC leaks water before booking anything - the cause matters.

• The system is cooling normally and you're keeping to a maintenance schedule.
• It's been cleaned properly fairly recently and you're topping up between deeper services.
• Airflow has dropped slightly and the filter is visibly dusty.

• Cooling has gradually weakened even though the filter is clean.
• There's a persistent musty smell when the system runs.
• The unit hasn't been opened up and cleaned internally for a long time, or you don't know when it last was.
• Airflow from the vents feels weak across the whole system rather than in one room.

These are our current AC cleaning options, taken straight from our live service catalogue.

It's worth being clear about this, because it's where people waste money. Cleaning addresses dirt and restricted airflow. It does not address a refrigerant problem, a failed fan motor, a control or thermostat fault, or an electrical issue.

If a system has been cleaned and the same problem returns quickly, cleaning it again is unlikely to be the answer. At that point the sensible step is a diagnostic visit, where a technician establishes what's actually wrong and quotes the repair in writing before any work starts. Our guide to an AC that isn't cooling covers the common possibilities.