Delta T, the temperature split.

Delta T is the difference between the temperature of the air going into the return and the air coming out of a supply vent. It is written as a Greek delta and a T because it is a change in temperature, and it is the single most useful number you can take without opening anything.

What a healthy number looks like

In normal conditions a working system drops the air somewhere around 15 to 20 degrees across the coil. Measure at the return grille and at a supply vent reasonably close to the air handler, let it run fifteen minutes first, and compare.

A split that is much smaller than that points at a system not removing much heat: low charge, a fouled coil, a failing compressor. A split that is much larger usually points at airflow, because air moving too slowly across the coil gets very cold on its way through.

Why Phoenix complicates it

The published ranges assume ordinary indoor humidity and ordinary outdoor temperatures. On a 115 degree afternoon with the sun on the roof and the return air already at 85, a smaller split is not automatically a fault. Dry desert air also changes the number, because less of the system’s capacity goes into pulling moisture out.

So delta T on its own does not diagnose anything. It tells you whether to keep looking. Pressures, amp draws and airflow are what settle it, and those are what a check-up measures and writes down.

Measuring it yourself

A cheap probe thermometer is enough. Take both readings at the same time, write them down with the date, and keep the note. Next summer the number that moved is worth far more than the number on its own.