Most HVAC emergencies were scheduled maintenance
There is a pattern behind almost every unit that dies in a heatwave. The coil was dirty, so the unit ran hotter and longer. The charge was slightly low from a small leak nobody looked for, so it ran hotter still. The compressor did that for a season and then stopped.
Each individual step was a cheap fix. The end of the sequence is the most expensive component in the machine, replaced during the week you can least afford to be without it, at emergency rates.
Preventive maintenance is not a way to avoid all failures. It is a way to make sure the failures you do get are the random ones rather than the entirely predictable ones.
What a visit actually covers
Filters, belts, coil cleaning, condensate drains, electrical connections, and pressure and temperature readings taken and written down.
The readings are the part that matters most and gets skipped most. A single set of pressures tells you almost nothing. The same readings across three visits tell you whether the charge is drifting down, which is how a slow leak gets found while it is still a repair rather than a compressor.
Condensate drains deserve a specific mention because they cause damage out of all proportion to their cost. A blocked drain on a rooftop unit or an above-ceiling air handler puts water into the building, and it usually arrives in a ceiling rather than in the plant room.
Kitchens and refrigeration are a different risk
Comfort cooling failing is a bad day. Refrigeration failing is stock on the floor and a conversation with an inspector.
Walk-ins fail in a small number of repeatable ways: door seals that no longer close properly so the unit runs continuously, defrost cycles that have stopped working so the evaporator ices up, and drains that block and refreeze. None of these is expensive to check and all of them are visible on a scheduled visit before they become a loss.
Kitchen extract and make-up air belongs in the same conversation. It affects whether the space is workable, whether the grease load reaches the roof, and in many buildings whether the kitchen is compliant at all.
When to stop repairing
At some point on an old unit the honest answer is that the next repair is not worth doing.
We would rather have that conversation with the readings in front of us: what the unit is drawing, how far the charge has drifted, what has already been replaced, and what the failure would cost you at the wrong time of year. A restaurant, a clinic and a warehouse can own the same model of unit and get three different answers, because the cost of it failing is different in each.
Replacing on your schedule is nearly always cheaper than replacing on the equipment’s.