The Annual Tune-Up Is a Photograph. Failure Is a Film.

A spring tune-up is a good idea. A technician cleans the coil, checks the charge, measures the temperature split, looks at the capacitor. Real problems get found. None of that is in question here.

The question is what happens during the other 364 days.

The sampling problem

A cooling season is roughly 3,000 hours in North Texas. A thorough maintenance visit is perhaps two of them. That is a sample of well under one percent of the operating period, taken on a date chosen by scheduling convenience rather than by anything the equipment is doing.

For faults that are already present in March, this works. For faults that arrive in June, it structurally cannot — the next observation is eleven months away, and the intervening summer is the part that matters.

Whether the visit happens at all

The DOE's 2018 review cites survey work on homeowner behaviour: more than half of surveyed households reported having their heating or cooling system serviced in the last five years, and 25 % had a maintenance service contract.

Read that carefully. "Serviced in the last five years" is a much weaker statement than "serviced annually," and three-quarters of households had no maintenance agreement at all. The annual tune-up is not the baseline condition of American HVAC. It is a minority practice.

Which helps explain why field testing keeps finding faults in the overwhelming majority of installed systems. Most of that equipment is not being inspected on anything like an annual cadence.

The measurement is a single point

Even a perfect visit produces one reading. A technician measuring superheat on 14 April learns what the system is doing on 14 April, at that outdoor temperature, at that indoor load.

What that reading cannot tell you is the trajectory. A pressure that is slightly low and stable is a different animal from a pressure that is slightly low and has been falling for six weeks. Same number on the gauge; completely different prognosis. Distinguishing them requires history, and a once-yearly visit has none.

The incremental-savings finding

There is a sobering number in the DOE review here. Among the benefits it catalogues from repairing documented faults, it notes: "incremental savings from tuning up existing AC/HP systems are approximately 5 %."

Compare that against duct sealing and insulation at "33 % increased cooling capacity, 16–41 % increased seasonal system efficiency," or combined duct, airflow and charge correction at "12–47 % energy savings."

The logical reading is not that tune-ups are worthless. It is that a routine tune-up and a genuine fault correction are different activities producing different magnitudes of result — and a visit only becomes the second kind if it happens to catch a fault that is present and detectable that day.

The two are not competitors

Continuous monitoring does not clean a coil, tighten a contactor, or replace a capacitor. It cannot do any of the physical work, and a monitor is not a substitute for a licensed technician.

What it does is change what the technician is walking into. Instead of an annual look for whatever happens to be wrong, the visit can be triggered by something being wrong — with weeks of pressure and temperature history describing when the drift started. That is a better use of a skilled person's two hours than a calendar reminder is.