Homeowners are aware that like any mechanical system, their HVAC needs routine maintenance and requires occasional inspections to find potential problems before a full failure occurs. The question many people don't understand is when an inspection is needed. Part of the confusion stems from the fact that although there is a rule of thumb concerning such an issue, the specifics of an individual situation may demand different answers.
Once or Twice per Year
Generally speaking, a unit should be inspected annually for routine maintenance and potentially needed repairs. A heat pump generally only needs an inspection once per year as it a single unit which provides both heating and cooling. A house which has separate heating and air conditioning units needs two annual inspections, although they can be conducted at the same time by the same contractor for convenience. Ideally, a maintenance schedule is kept and inspections are conducted at the same time each year.
Additional Inspections as Needed
If a unit begins making a strange noise or running louder than it has in the past, it's a good idea to have an additional inspection to find and fix the problem before more expensive repairs are required. If the air doesn't seem to be as cool as it was in the past or if power bills inexplicably rise one month it's likely because the unit is running as efficiently as it should and needs maintenance adjustments which will be determined with an inspection.
The bottom line is that an annual inspection finds potential problems before they arise. Additional inspections find and repair problems while they're still manageable. As a unit becomes older, it can still have years of perfectly good function if it is inspected and maintained accordingly in order to assure a potential problem doesn't go unnoticed and cause more damage which requires a full replacement.