Ductwork is the part of a cooling system nobody sees and almost nobody services. In Qatar, fine dust enters through returns, gaps and construction openings, and settles along the duct run. Every time the system starts, some of it is carried back into the room.
Most properties in Qatar benefit from duct cleaning every two to three years, sooner after building work. It is not an annual requirement, and any contractor recommending it every year should be asked why.
Quoted after inspection, based on the number of zones and the length and accessibility of the duct run.
The usual signs are dust settling around the supply grilles soon after cleaning, a smell that returns whenever the system starts even after the units have been serviced, or recent construction or fit-out work in the property.
For most properties in Qatar, every two to three years, and sooner after building work. It is not an annual job. Filters and unit servicing are what need doing frequently.
Not when it is done properly. The debris is extracted under negative pressure, so it is captured rather than pushed back into the rooms, and grilles and diffusers are cleaned as part of the same visit.