AC Not Cooling Enough? 15 Causes, Safe Checks, and When to Call a Pro

When the AC runs but rooms still feel warm, it is more than a comfort issue—it can signal airflow problems, heat-load changes, or mechanical faults that reduce capacity. In Southern California’s long cooling season, small issues pile up fast. Below we explain how to judge performance, what you can safely check, the most common reasons an air conditioner is not cooling, and why some fixes require a qualified, insured HVAC professional.

How to tell if your AC is underperforming

• Temperature split: A healthy system typically delivers a 18–22°F drop between return and supply air. If your return is 78°F and supply is 70°F, that 8°F split is too low.
• Run time: If the system runs constantly yet can’t reach setpoint, capacity or airflow may be insufficient.
• Uneven rooms: Hot spots point to duct leaks, zoning issues, or poor distribution rather than a bad compressor.
• Humidity and stickiness: Excess humidity often means low airflow, dirty coils, or refrigerant problems.

Quick, safe checks homeowners can do

• Filter: Inspect and replace if dirty. A clogged filter is the top cause of low cooling and frozen coils.
• Thermostat: Verify COOL mode, correct setpoint, and fan set to AUTO (not ON if humidity is high). Replace low batteries.
• Vents and returns: Open supply registers and keep returns unblocked by furniture or rugs.
• Outdoor unit airflow: Clear leaves, grass, or debris at least 18–24 inches around the condenser.
• Breaker and disconnect: If tripped, reset once. If it trips again, stop and seek qualified help.
• Drain line: If you see water around the air handler, a float switch may be shutting cooling off to prevent overflow.

15 common reasons your AC is not cooling enough

1. Dirty filter or coil: Dust restricts airflow and slashes capacity. Evaporator or condenser coil cleaning restores heat transfer.
2. Frozen evaporator coil: Often from low airflow or low refrigerant. Ice reduces cooling to nearly zero; powering off and addressing the root cause is essential.
3. Low refrigerant or a leak: Cooling depends on precise charge. Leaks require EPA-certified handling; insurers commonly require licensed service.
4. Duct leaks and poor insulation: Leaky or under-insulated ducts dump cool air into attics/garages, starving rooms and wasting energy.
5. Incorrect system size: Undersized units can’t meet peak loads; oversized units short-cycle, leaving rooms humid and uneven.
6. Blocked condenser or failed condenser fan: Without outdoor heat rejection, capacity plummets and compressor life is at risk.
7. Thermostat misplacement or miscalibration: Thermostats in sun or near supply vents read wrong and end cycles too early.
8. Blower issues and low static pressure: Wrong fan speed, dirty wheel, or failing ECM motor cuts airflow and cooling output.
9. Zoning or damper faults: Stuck dampers push air to the wrong rooms; one area freezes while another bakes.
10. Metering device problems (e.g., TXV stuck): The system can’t feed the coil properly, causing low capacity and temperature split.
11. Condensate safety lockout: A tripped float switch prevents cooling to avoid water damage; the root cause is often a clogged drain.
12. Electrical components: Weak capacitors, pitted contactors, or voltage issues cause hard starts, short cycling, and warm air.
13. Building pressure and infiltration: Leaky envelopes or strong exhaust (kitchen/bath fans) pull hot air in, overwhelming capacity.
14. Heat-load changes: New windows, occupants, electronics, or poor attic ventilation can increase cooling demand beyond design.
15. Aging equipment: Older systems lose efficiency and capacity; even if they run, they may not meet current loads or comfort targets.

What you can do vs. what needs a pro

Homeowners can replace filters, clear leaves, open vents, and confirm thermostat settings. However, tasks involving refrigerant, sealed electrical components, coil cleaning with chemicals, charge verification (superheat/subcool), duct leakage testing, and static pressure diagnostics should be left to qualified HVAC professionals. Handling refrigerant without certification is unsafe and illegal, may void warranties, and many insurance policies or HOAs require licensed, documented service for coverage.

Design matters: sizing, ducts, and airflow

When cooling is consistently marginal—even after maintenance—design may be the culprit. Proper load calculations (Manual J), equipment selection (Manual S), and duct design (Manual D) ensure the right capacity, correct airflow to each room, and balanced comfort. In hot, dry-to-moderate Southern California conditions, attention to attic insulation, duct sealing, and ventilation pays off in stable temperature, better humidity control, and lower bills.

Preventive maintenance checklist

• Replace filters on schedule (often every 1–3 months, more in wildfire or heavy-use periods).
• Clean coils and straighten fins as needed; maintain clear airflow around the condenser.
• Flush the condensate line and confirm float switch operation to prevent water damage.
• Verify proper refrigerant charge and temperature split under design conditions.
• Measure total external static pressure and adjust fan speed for target airflow.
• Inspect and seal ducts; insulate in unconditioned spaces.
• Review thermostat placement, schedules, and calibration.

Why many fixes aren’t DIY

Air conditioning systems combine high-voltage electricity, pressure vessels, and chemicals that demand training and specialized instruments. Missteps can destroy a compressor, create leaks, or nullify manufacturer warranties. In addition, insurers frequently require that refrigerant and electrical work be performed and documented by licensed professionals to maintain coverage on HVAC equipment and any resulting water or property damage claims.

About #1 AC Guys

#1 AC Guys is a fourth-generation family HVAC company based in Southern California with 80+ years of engineering experience. Our mission is healthy indoor air for a healthy life. We serve both residential and commercial properties, including large and complex projects. Our engineers have completed factory training at Fujitsu (Japan), Mitsubishi Elektrik (Thailand), Midea (China), Gree (China), and Hier (China).

Service area

We serve Los Angeles County and nearby areas including Orange County, Ventura County, and Western Riverside County. Typical communities include Los Angeles, Pasadena, Glendale, Santa Monica, Long Beach, Anaheim, Irvine, Santa Ana, Huntington Beach, Oxnard, Thousand Oaks, Ventura, Riverside (western areas), and Corona.

Key takeaway

If your air conditioner isn’t cooling enough, start with safe checks—filters, thermostat, vents, and outdoor clearance. When symptoms persist, the cause is often airflow, refrigerant, or design-related and requires proper diagnostics. Professional evaluation protects your equipment, safety, warranties, and insurance coverage while restoring dependable comfort.