Delta-T vs Superheat / Subcooling — Which Check to Run

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The Field Rule

Run a delta-T check when the question is “is the air actually picking up (or dropping) temperature?” Two dry-bulbs. That is the delta-T calculator and the measurement guide.

Run superheat or subcooling when the question is “is this system charged, restricted, or leaking?” Metering device picks the method — piston SH, TXV SC. That is the SH/SC calculator, now live, and the refrigerant-side sibling writeup Superheat/Subcooling vs Delta-T.

When the Delta-T Check Tells You More

• First pass on a no-cool / weak-cool before hoses go on. A 9°F drop with a roaring blower is a capacity story. A 27°F drop is an airflow story. You just saved a recovery cylinder or a wild goose chase in the attic.
• Icing and limit trips. High cooling split plus ice is low CFM until proven otherwise. High heating rise plus a tripping limit is the same physics on a heat exchanger.
• After a filter or blower change. Re-ΔT is the before/after the customer understands. SH on a piston also moves with load — worse as a demo.
• You left the manifold in the truck. Split is legal as a screen. It is not legal as a final charge.

Delta-T is a poor leak detector and a poor TXV test. A valve that will not hold superheat will not show up in a 19°F split until capacity is already gone.

When the Superheat / Subcooling Check Tells You More

• Any weigh-in or recover. There is no honest “add until delta-T is 20.” Outdoor ambient and indoor wet-bulb move split. OEM charts are SH or SC.
• Leak vs restriction vs overcharge. The SH+SC matrix on SH/SC troubleshooting needs gauges. Split cannot tell a plugged drier from a 12-ounce leak.
• TXV holding (or not holding) superheat while subcooling is on target — a valve problem, not a 2°F error in air split.

How to take those readings: SH/SC guide.

The Sequence That Does Not Waste a Trip

1. Delta-T vs the humidity-adjusted band. High, low, or on target.
2. High cooling split → ESP, then CFM. Gauges after airflow is honest.
3. Low cooling split with static on target → SH or SC for the metering device. See delta-T troubleshooting.
4. Only then weigh refrigerant. Charging a restricted duct to “fix” a 28°F split is a callback.

The Wrong Diagnostic First

Common miss: tech sees 18°F split, calls it charged, never measures piston superheat. The system is light and the split is being held up by low CFM. Other miss: tech recovers to “fix” a 26°F split; the media filter was the entire plant. Match the failure mode.

You cannot convert delta-T to superheat. A 20°F air split is not 20°F SH. Different fluids, different locations. Use both tools; do not translate between them.

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