Annually for most systems. Anything in a kitchen, salon, gym or server room benefits from twice a year because of the dust and duty cycle.
For systems above a certain refrigerant charge, yes, with records kept. We handle the checks and the paperwork.
Yes. We maintain plenty of systems we did not install.
Yes. We hold contracts for multi-site operators with visits scheduled around trading hours.
Usually noticeably. Clean filters and coils, correct charge and working controls are the difference between a system sipping and one straining.

