What it is

Two air-side temperature measurements tell you a lot about whether a coil is doing its job before you ever hook up gauges. On the indoor coil it's the evaporator split (also called delta T) — the difference between return-air temperature and supply-air temperature. On the outdoor coil it's the condenser split and approach — how the condensing temperature relates to the outdoor air going across it.

These are cheap, fast, non-invasive checks. They won't replace gauges, but they point you in the right direction and often catch an airflow problem the gauges alone would let you misread as a charge problem.

How it works

Evaporator split. Air gives up heat to the cold coil, so supply air comes out colder than return air. How big that drop is depends on two things: how much heat the coil is pulling (charge, coil condition) and how fast the air moves through it (airflow). Slow the air down and each pound of air sits on the coil longer and cools more — split goes up. Speed the air up and split goes down. So split is a product of both load and airflow, which is why you read it alongside airflow, not in a vacuum.

Condenser approach and split. The outdoor coil rejects heat to outdoor air, so the refrigerant condensing temperature sits above the outdoor air temperature. Approach is condensing temperature minus outdoor ambient — a tight approach means the coil is transferring heat well; a wide approach means it's struggling (dirty coil, low airflow, or a coil that's just overwhelmed). Condenser split is the air temperature rise across the outdoor coil (air leaving minus air entering) and reflects how much heat is being dumped and how the fan/coil are moving it.

In the field

Evaporator split:

  1. Measure dry-bulb return air in the return close to the coil and supply air in the supply plenum, out of line-of-sight of the coil so you're not reading radiant cold.
  2. Subtract. That's your split.
  3. Interpret it with humidity in mind — high indoor humidity (high wet-bulb) pulls split down because the coil spends energy condensing moisture (latent) instead of dropping air temperature (sensible).

Condenser approach:

  1. Read suction/liquid pressures, convert liquid pressure to condensing saturation temperature.
  2. Measure outdoor ambient in the shade at the coil inlet.
  3. Approach = condensing temp − ambient.

Normal values & targets

  • Evaporator split (delta T): roughly 16–22°F on a typical residential system at moderate indoor humidity. Drier indoor air pushes it toward the high end; humid indoor air toward the low end.
  • Airflow context: that split assumes airflow in the normal band (~350–400 CFM/ton). Low airflow inflates split; high airflow flattens it.
  • Condenser approach: commonly around 10–20°F depending on equipment efficiency — higher-efficiency condensers with more coil surface run a tighter approach. A widening approach over time on the same unit signals a fouling or airflow problem.
  • Condenser split (air temp rise across outdoor coil): often in the ballpark of 15–30°F depending on the unit; track trend more than the absolute.

Common faults & what they mean

  • High evaporator split (e.g., 25°F+): low airflow — dirty filter, dirty blower wheel, closed dampers, undersized return, blower set too low. Could also be low charge starving the coil. Check airflow first.
  • Low evaporator split (e.g., under 14°F): too much airflow, low charge that's flooding... no — low charge usually raises split; low split is more often high airflow, a low-load condition, very humid air (latent load), or a coil that isn't getting cold enough. Cross-check with refrigerant readings.
  • Wide condenser approach: dirty condenser coil, failing condenser fan, recirculating hot air, or overcharge stacking liquid in the coil. Clean and check airflow before touching charge.
  • Tight/normal approach but poor cooling: the condenser is fine; look indoors (airflow, coil, charge to the evaporator).
  • Split looks fine but capacity is low: could be balanced wrong — adequate split with low airflow still moves little total heat. Always pair split with an airflow figure.

Tech tips & gotchas

  • Split alone is not a charge verdict. A perfect-looking split with terrible airflow still means poor capacity. Always interpret split next to airflow (CFM/ton), not by itself.
  • Humidity moves the split. On a muggy day a healthy system can show a lower split because it's doing latent work. Don't add charge chasing a "low split" that's really high humidity.
  • Approach is a great trending tool. Log it on maintenance visits. The same unit's approach creeping wider season over season is an early warning of a dirtying coil or weakening fan.
  • Measure supply air away from the coil's line of sight. Pointed straight at a cold coil, an IR or probe reads colder than the actual air temperature and fools your split.
  • Let it run first. Take these readings after the system has run long enough to stabilize (10–15 minutes), not right off a cold start.

Safety / code notes

These are air-side, non-invasive checks — no refrigerant access required, which is exactly why they're a good first look before you connect gauges and risk losing charge. If you do proceed to gauges, recover/handle refrigerant per EPA 608 (40 CFR Part 82, Subpart F).