Do the annual service yourself

A maintenance visit is usually sold at a fixed price for a fixed hour. If the technician spends forty minutes of it washing a coil you could have washed, the measurements at the end get the remaining twenty.

Do the items marked do it yourself the weekend before. Then the visit is spent on the things only they can do — charge, combustion, electrical — and you get more for the same money.

If you want one thing to judge a contractor by, it is whether they measure. A visit where nobody took a static pressure reading, a temperature split or a combustion sample was a visual inspection with a hose.

Ask for the numbers afterwards. A good technician will have written down superheat or subcooling with the conditions they were taken in, the static pressure, the temperature rise against the plate range, and the capacitor's measured microfarads against its rating. Those five numbers are the service. Everything else is housekeeping.

Any smell of gas — leave the building and call the utility from outside. Any soot, any yellow lazy flame, any carbon monoxide alarm. Ice on the refrigerant lines in cooling weather, which means stop running it before the compressor takes liquid. Oil stains around the outdoor unit, which is refrigerant leaking with the oil that carries it. A breaker that trips more than once. Water where it has not been before.

None of these are things to work around. They are things to stop for.