Water heater — serviced twice a year (per heater)
Added to an annual package (ready, custom or institution); not sold on its own.
Preventive heater service: descaling and a safety-valve check, twice a year.
Unit: per heater. Carried out during your scheduled annual subscription visits, with a photo report after each visit.
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.