The five numbers that are the service
A visit where nobody measured anything was a visual inspection with a hose. These are the readings that tell you whether a system is working, and most of them need a thermometer.
Where you are
Altitude first, because it changes every other answer on this page.
Temperature split
Return air minus supply air, after fifteen minutes of running. Two thermometers and the single most useful thing a homeowner can measure.
Furnace temperature rise
Supply minus return on a furnace. The data plate gives a range, and outside it is a fault rather than a preference.
Airflow, and whether there is enough of it
With no flow hood, airflow is worked backwards from a known capacity and a measured temperature drop. It is only ever as good as the capacity figure it starts from.
Total external static pressure
What the blower is fighting. The most diagnostic number on a residential system and almost nobody takes it — a manometer costs less than one service call.
Is the filter big enough?
Past about 300 feet per minute a one-inch pleated filter both stops catching things and starts costing pressure. The fix is area, not a more expensive filter.
How to read these together
A big split and a high static are the same fault
These readings are not independent, and the most common pattern in residential work is a large temperature split alongside a static pressure well over the rating. Both are airflow.
Air that moves too slowly across the coil spends longer on it, so it loses more temperature — the split looks impressive while the system moves less heat than it should. The order to look in is filter, then blower wheel, then duct: a filter is minutes, a wheel is an hour, and duct is a project.
A small split is not automatically low refrigerant
It is the diagnosis most often reached and most often wrong. A small split also comes from too much airflow, from a compressor that is not pumping, and from a coil so cold it has iced and is no longer passing air at all.
Check the static pressure and the cfm per ton before anyone connects a gauge. A system that is genuinely low on refrigerant is leaking, and topping it up without finding the leak is not a repair — it is an appointment to do the same thing again next summer.
Why altitude is the first field on this page
The sensible heat constant is sixty minutes an hour times the density of air times its specific heat. Air is only 0.075 lb/ft³ at sea level; in Denver it is nearer 0.062.
So the memorised 1.08 is a sea-level figure. Used at 5,280 feet it overstates capacity by about 22%, which is enough to condemn a healthy system or pass a sick one. Every capacity and airflow figure on this page is derived from the altitude in that box.
Related
The annual service checklist · Superheat & subcooling · All HVAC tools