Air Conditioner Not Cooling
A measurement led approach to poor cooling, starting with airflow rather than the gauges.
Safety
Compressor: Low pressure vapor is compressed into hot, high pressure discharge gas.
Symptom
Symptom
The system runs continuously but the space will not reach setpoint.
Symptom
Supply air is not much cooler than return air, or the indoor coil is iced.
Possible causes
- Restricted airflow from a plugged filter, dirty coil, closed registers or undersized ductwork.
- Dirty condenser coil preventing heat rejection.
- Refrigerant undercharge from a leak.
- Overcharge, often from a previous top up without measurement.
- Metering device underfeeding or a liquid line restriction.
- Failing compressor unable to pump properly.
- Duct leakage in an unconditioned space.
- Load exceeding capacity on an extreme day, or equipment undersized for the building.
What to check, in order
- 1Inspect the filter, indoor coil and blower wheel before any gauges go on.
- 2Measure total external static pressure and confirm airflow against the manufacturer table.
- 3Measure the temperature split across the indoor coil, using wet bulb for a proper reading.
- 4Inspect and, if needed, clean the outdoor coil, then confirm condenser fan operation.
- 5Connect gauges and take superheat and subcooling together with line temperatures.
- 6Interpret the pair of readings rather than pressures alone.
- 7Verify supply voltage and compressor amp draw against the rating plate.
- 8Check accessible duct runs for disconnections and leakage.
High superheat with low subcooling
Undercharge. Find and repair the leak before adding refrigerant
High superheat with high subcooling
Restriction between the condenser and the evaporator, often a plugged drier or a failing metering device
Low superheat with high subcooling
Overcharge
Low superheat with low subcooling
Overfeeding metering device, or a compressor that is not pumping
Both readings normal but poor comfort
Airflow, duct losses, or a load and sizing problem rather than a refrigerant problem
Handle refrigerant properly
Next step
When a coil has iced, shut the compressor off and run the blower to thaw it completely before taking any refrigerant readings. Measurements on an iced system are meaningless. After the repair, verify airflow, superheat or subcooling and temperature split, and document the readings.
Symptoms
Step-by-step: Air conditioner runs but doesn't cool
Work through the checks in order rather than guessing.
Practice & Mastery
- Air Conditioner Not Cooling Troubleshooting Scenarios2
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