AC Not Cooling
AC is running but supply air is lukewarm. Most common mistake on this call is connecting gauges before checking anything else — airflow problems look like charge problems on a gauge set, and on A2L refrigerants every connection is handling risk. Non-invasive checks first.
- Indoor airflow restriction — dirty filter, coil, blower wheel, or duct issue30-40%
- Outdoor airflow or condenser problem — dirty coil, weak fan15-25%
- Low refrigerant charge from an actual leak15-25%
- Electrical (weak cap, pitted contactor, struggling compressor) and metering-device issues
- 1
Measure return-to-supply temperature split (expect 16-22°F, broader 14-23°F by humidity)
Free information. Low split = capacity loss; high split = low airflow. Frames everything that follows.
- 2
Visual airflow triage — filter, evap face, blower wheel, flex, dampers, outdoor coil, condenser fan
If the evap is iced, stop and thaw before any refrigerant diagnosis — frozen coils invalidate every reading.
- 3
Pull total external static with a manometer before pulling gauges
Field targets ~0.4-0.6" normal, 0.8"+ trouble, return ≥0.4" alone flags restriction. High TESP keeps you on airflow.
- 4
Verify blower CFM and speed setup against the table (target 350-400 CFM/ton, ACCA QI within 15%)
Low airflow explains most 'low on charge' misreads — low suction, freeze risk, high split.
- 5
Inspect and clean indoor and outdoor coils, then recheck
Dirty condenser drives high head and high-subcool patterns that look like overcharge. Dirty evap distorts SH and SC.
- 6
Only now decide if gauges are warranted
If airflow, static, coil cleanliness, and blower setup are all proven and split is still abnormal, connect. A2L systems especially reward this discipline.
- 7
Charge by metering device — fixed orifice = target SH from chart; TXV = target SC from nameplate (~10°F ±3°F typical)
Read the charging chart on the condenser. Pattern-read SH/SC together to separate undercharge, overcharge, low airflow, TXV restriction, non-condensables, and poor compression.
- 8
Electrical checks — cap within ±6% of rated µF, contactor for pitting and voltage drop, compressor windings (C-R lowest, R-S = sum)
Hard starting and bad compressor are different problems. Don't condemn a compressor before ruling out airflow, restrictions, and electrical support.
Compressor failure call is justified only after airflow, charge pattern, and electrical support parts are all proven. Before that, it's a guess. End-of-life conversation if compressor or coil failure on an older system — repair cost vs. age, reliability, and refrigerant type.
On A2L systems (R-454B, R-32) every unnecessary connection is handling risk. Use A2L-rated recovery, vacuum, leak detector, and manifold — older A1 tools aren't acceptable. Lennox R-454B coils ship with factory leak-detection that can't mix with non-OEM sensors. Left-hand threaded hoses and a fire extinguisher on the truck.