The answer to 'when to service evaporator coil' is that the evaporator coil should be serviced and cleaned annually as part of a regular HVAC maintenance routine. The evaporator coil is responsible for absorbing heat from the indoor air, so it can become coated with dust, dirt, and other debris over time. This buildup can reduce the coil's efficiency and lead to higher energy bills, as well as potential damage to the HVAC system if left unchecked.
It's recommended to have a professional HVAC technician service the evaporator coil once a year, typically in the spring or fall when the system is not in heavy use. They will thoroughly clean the coil, check for any damage or leaks, and ensure the entire HVAC system is operating at peak efficiency.