Customer Says No Cold Air But System Running Translation Decision Tree

Why this matters

"It runs but there is no cold air" is the single most common cooling complaint, and it maps to at least six distinct mechanical faults that share zero corrective actions. A tech who treats the phrase as a diagnosis instead of a starting point either guesses charge (and freezes the coil) or replaces a thermostat (and returns next week). The phrase tells you two things only: the blower is energized and the customer feels warm supply air. Everything else is unverified. This tree converts the complaint into a measured fault family in the first 20 minutes by separating "compressor not making refrigeration effect" from "refrigeration effect present but heat not reaching the conditioned space."

Symptom presentation

What the customer reports versus what the words actually guarantee:

• "System is running" usually means they hear the indoor blower. It does NOT confirm the condenser, compressor, or condenser fan are running.
• "No cold air" means the supply register feels neutral or warm to the hand. It does NOT distinguish a 2 F split from a 0 F split.
• "It was fine yesterday" anchors a sudden hard failure (contactor, capacitor, compressor) over a gradual one (slow leak, fouling).

Walk to the outdoor unit before opening a single panel. Note whether the compressor and condenser fan are running, and put a hand near the suction line. A cold, sweating suction line means refrigeration effect exists and the problem is on the air side or distribution. A warm suction line with the compressor running means the refrigeration side is failing.

Quick checks

Capture these in order; each one prunes whole branches:

1. Temperature split at the nearest supply and return registers. A split of 16 to 22 F means the equipment is making cold air and the complaint is distribution. A split under 8 F means the equipment is the problem.
2. Outdoor unit state: compressor running (clamp the common, expect near rated load amps), condenser fan turning, suction line cold and sweating.
3. Thermostat call: confirm Y is energized at the air handler board (24 VAC, Y to C). This separates a control miss from a mechanical miss.
4. Filter and return: pull the filter, check return grilles are open. A collapsed filter alone produces this exact complaint.

Isolation tree

Branch A: Split is 16 F or greater (equipment is cooling)

The machine works; the complaint is delivery. Check:

• Duct leakage in unconditioned space (attic supply trunk dumping cold air into the attic).
• A zone damper stuck closed to the affected area.
• Return air short-circuiting (a return in the same room as a leaky supply boot).
• Customer expectation: 95 F outdoor with a 78 F space and a 22 F split delivers 56 F supply air, which can still feel inadequate during pulldown. Verify the space is actually not reaching setpoint over runtime, not just that one register feels less cold than expected.

Branch B: Split under 8 F, suction line warm, compressor running

Refrigeration effect is collapsing. Measure suction and liquid pressures with saturation temperatures, then compute superheat and subcooling:

• High superheat plus low subcooling: undercharge or restriction. Leak search before adding refrigerant.
• Low superheat plus low subcooling: low airflow flooding the coil or a failed metering device.
• Both pressures near equalized with the compressor humming: failed compressor (open winding, dead valves, or seized).

Branch C: Split under 8 F, outdoor unit NOT running

The "running" the customer heard was only the indoor blower. Move to the no-start path: verify 24 VAC at the contactor coil, line voltage across the contactor, run capacitor microfarads against nameplate, and compressor/fan windings to ground.

Branch D: Indoor blower running, evaporator iced

A frozen coil produces strong airflow at first that fades to nothing. Set the system to FAN only and melt the ice fully before any pressure reading. Then chase the root cause (low airflow or low charge), not the ice itself.

Confirming diagnosis

The diagnosis is confirmed only when the measured numbers agree with the mechanism:

• Distribution fault: equipment split 16 to 22 F at the coil, but room-level delivered air weak; duct leakage or damper position located physically.
• Undercharge: subcooling below 5 F on a TXV system, superheat elevated, leak located with electronic detector or bubble solution.
• Low airflow: total external static high (above 0.8 inWC on equipment rated 0.5), low coil-outlet superheat, restriction located.
• Electrical no-run: a specific failed component proven with a meter (open capacitor, no 24 VAC to contactor, grounded winding).

Never close the ticket on a symptom that "went away" after a power cycle without a measured root cause.

Remediation

Match the action to the proven branch:

• Distribution: seal accessible duct leaks, correct damper or zone control, balance returns.
• Undercharge: repair the leak, evacuate to 500 microns, weigh in the nameplate charge, verify by subcooling.
• Low airflow: replace the filter, clean the coil air side, clear the blower wheel, correct duct restriction, re-measure static.
• Electrical: replace the proven-failed contactor, capacitor, or compressor; confirm corrected amp draw against nameplate.

References

• AHRI Standard 210/240-2023 Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment
• 40 CFR Part 82 Subpart F (EPA Section 608 Stationary Refrigeration and Air Conditioning Rules)
• ACCA Manual S Residential Equipment Selection, 2nd Edition
• ASHRAE Handbook of Fundamentals, 2021 Edition, Chapter 1 Psychrometrics