Central AC vent (homes) — cleaning per service (per vent)
Added to an annual package (ready, custom or institution); not sold on its own.
Cleaning the air vent and its filter.
Unit: per vent. The price is per service; quantity = number of vents × times a year.
Carried out during your annual subscription visits; the engineer confirms the numbers at the first visit: any excess is charged and any shortfall refunded. The price is fixed for the subscription year.