New Install No Cooling at Commissioning: Decision Tree

Why this matters

No cooling on a brand-new system at commissioning is almost always an installation error, not a defect. The failure set is different from a service call on an aging unit: unopened service valves, swapped or miswired thermostat leads, an unset or wrong-orifice metering device, a never-completed evacuation and charge, or a breaker/disconnect left off. Each one stops cooling cold and each has a fast confirmation. Working through this in order keeps you from condemning a new compressor or calling the factory when the real problem is a valve you forgot to back-seat. This tree walks the new-install no-cool from the simplest installer omissions to the charge and metering checks that close out the commission.

Symptom presentation

Newly installed split or packaged system. Thermostat calls for cooling, but the space does not cool: no cold air at the registers, or air moving but not cold, or nothing running at all. Discovered during the commissioning run.

Variants:

• Nothing energizes on a cool call: power, control wiring, or thermostat configuration.
• Indoor blower runs, outdoor unit does not start: low-voltage, contactor, or disconnect.
• Both units run but supply air is not cold: charge, service valves, or metering device.
• Cools briefly then trips/stops: high pressure, airflow, or a protection lockout (covered below at a high level).

Quick checks before gauges

Verify power and disconnects. Confirm the outdoor disconnect is in, the breaker is on, and line voltage is present at the contactor line side. A new install with the disconnect pulled for safety and never reinserted is common.

Confirm the service valves are open. Liquid and suction service valves left front-seated (closed) after the line-set connection will block refrigerant flow entirely. Back-seat or open both fully. This is the single most frequent new-install no-cool.

Check the thermostat and control wiring. Confirm the stat is configured for the correct equipment type and staging, the Y/G/C/O/B leads land correctly, and there is 24V on the call. A heat-pump stat set to the wrong reversing-valve logic (O vs B) or swapped Y/O will not cool.

Isolation tree

Branch 1: nothing runs on a cool call.

• No 24V control voltage: check the transformer, the C-wire, blown low-voltage fuse, and any tripped float switch or safety in series. Restore 24V.
• 24V present but no Y at the contactor: trace the thermostat call through the board and any pressure switches. Correct miswire or a stuck-open safety.
• Y present at contactor but contactor does not pull in: check the contactor coil and high/low pressure switches. Replace a defective contactor.

Branch 2: indoor runs, outdoor does not start.

• Disconnect/breaker off, or line voltage missing at the condenser: restore power.
• Contactor pulls in but compressor/fan do not run: confirm capacitor and motor windings (new-unit shipping damage is possible but rare). Verify the correct supply voltage and phase (on three-phase, a rotation/phase issue).
• Hard-start or protection lockout present: read the board diagnostics; address the specific fault.

Branch 3: both units run, air is not cold.

• Service valves closed: open fully. Re-verify with gauges.
• Never properly evacuated or charged: confirm the system was evacuated to target microns and charged by the correct method (weigh-in for the line length, then verify by subcool on TXV or superheat on fixed-orifice). A unit running on shipping/holding charge only will not cool to spec.
• Wrong or unseated metering device: confirm the correct fixed orifice/piston for the model and line length, or a properly installed TXV with the bulb mounted and insulated. A missing piston or a mismatched orifice starves or floods the coil.

Branch 4: airflow.

• New duct system with closed dampers, construction debris, a left-in filter cap, or an undersized return: low airflow makes the coil cold but delivers little capacity and can freeze. Confirm CFM and external static pressure against the design (ACCA Manual D) before condemning charge.
• Blower set to the wrong tap/speed for the tonnage: set per the installation instructions.

Branch 5: heat-pump-specific.

• Reversing valve energized in the wrong mode (O/B stat logic backward) so the unit runs heat on a cool call: correct the thermostat O/B configuration and verify discharge-line behavior.

Confirming the diagnosis

Once corrected, complete the commissioning measurements: line voltage and amp draw within nameplate, subcool to the manufacturer target on a TXV system or superheat to target on a fixed-orifice system, suction and liquid pressures consistent with ambient, supply-air temperature drop of roughly 15 to 22 F across the coil at design RH, and external static pressure within the blower table. Verify the thermostat satisfies and the unit cycles correctly.

Document charge method and weighed amount, evacuation micron level, static pressure, temperature split, and electrical readings on the commissioning sheet.

Remediation summary

References

• ACCA Standard 5 (HVAC Quality Installation Specification) - commissioning verification of airflow, refrigerant charge, and electrical.
• ACCA Manual D - duct design and airflow targets used to verify CFM and static pressure at commissioning.
• ACCA Manual S - equipment selection and charge/airflow verification context.
• EPA Section 608 of the Clean Air Act, 40 CFR Part 82 - recovery and handling when opening the system.
• Manufacturer installation instructions - required evacuation micron target, charge method, metering-device selection, and blower-tap settings for the specific model.