The annual service, itemised
Most of what is on a maintenance invoice is cleaning, looking and measuring. Here is the whole list, split by what needs instruments, what needs certification, and what needs a hose and an hour.
What you are actually paying for
A tune-up is not one thing. It is a list, and the items on it need very different people.
The green half needs a hose, a screwdriver and an hour. The amber slice needs an inexpensive instrument and tells you more than everything else combined. Only the blue end genuinely needs a technician — refrigerant work requires EPA Section 608 certification and venting is illegal, a run capacitor holds a charge that can kill after the power is off, and combustion is diagnosed with an analyser because carbon monoxide has no smell.
Knowing which is which is worth money in both directions: it is the difference between paying for work that was not done, and doing work you should not be doing.
Where the numbers come from
The same rule as the rest of the trades tools here: derive what can be derived, ask for what cannot.
The three constants every technician carries — 1.08 for sensible heat, 0.68 for latent, 4.5 for total — are not constants at all. They are sixty minutes an hour times the density of air times its specific heat, and air thins as you climb.
A job measured in Denver with the textbook 1.08 reads roughly 22% high — enough to condemn a healthy system or pass a sick one. Every calculator here derives the figure from the altitude you enter.
What cannot be derived is asked for. A charging chart belongs to your equipment, a temperature rise range is printed on your data plate, and a target superheat depends on the weather on the day. Nothing here invents any of them, and a reading offered without its target is reported as a reading rather than a verdict.