AC Struggling During Extreme Heat: What to Check

Why air conditioners struggle in extreme heat

During a severe heatwave, even a healthy AC can seem weak. Outdoor temperatures, attic heat, high UV index, and indoor heat load all push the system near its limits. Most residential systems are sized to maintain comfort around typical design days, not a record-breaking afternoon. A normal temperature split is often about 15–22°F between return and supply air. When it is 100–110°F outside, seeing 75–80°F indoors can still be normal if airflow and refrigerant circuits are correct.

Before you assume the system is failing, review what to check safely, how to reduce heat load, and the signs that point to deeper HVAC troubleshooting by a qualified technician.

What to check first (safe, homeowner-friendly)

Thermostat settings

• Mode should be Cool. Set Fan to Auto to avoid blowing warm air between cycles.
• Lower the setpoint gradually in 2°F steps. Rapid drops can cause short cycling.
• Disable schedule holds that might be raising the setpoint midday.

Airflow matters

• Filter: A dirty filter is the top cause of AC not cooling. Replace or clean if clogged. Reduced airflow can cause a frozen evaporator coil and high static pressure.
• Vents and returns: Open supply registers and keep returns clear of furniture. Closed doors in tight homes starve airflow.
• Ice check: If you see frost on the indoor line or coil area, turn cooling Off and set Fan to On to defrost. Do not open sealed panels.

Outdoor condenser

• Clear 2–3 feet around the unit. Vegetation, trash bags, or walls trap hot air.
• Shade without blocking airflow helps. Direct sun increases head pressure; do not cover the top.
• Coil cleaning: With power off, a gentle garden hose rinse from top to bottom can wash away dust. Avoid high pressure and do not bend fins.

Electrical basics

• Verify the AC circuit breaker is on. A breaker that trips more than once suggests a failing capacitor, contactor, motor, or compressor. Stop and schedule service.
• Listen: A humming outdoor unit with a fan that will not start often points to a weak capacitor. This is not a DIY repair.

Performance clues that signal a deeper issue

Prolonged runtimes and rising indoor temps

If your system runs nonstop and cannot hold within 5–7°F of the setpoint during a heatwave, check airflow first. If airflow is fine, low refrigerant, duct leaks, or a failing compressor could be involved. Refrigerant circuits are sealed; handling refrigerant requires certification and is not safe or lawful as DIY.

Uneven rooms or weak vents

Leaky ducts, crushed flex, or poor balancing cause hot rooms. In attics common to Los Angeles County and nearby regions, duct insulation and proper sealing are critical to reduce heat load.

Short cycling

Frequent starts stop efficient cooling and point to an oversized unit, a clogged filter, a faulty thermostat, dirty condenser coil, or electrical problems like a failing capacitor or contactor.

Ice on lines or coil

Frozen evaporator coil indicates low airflow, dirty filter, closed vents, or low refrigerant. Thaw the system as noted above and address airflow. If ice returns, stop and call a qualified technician to prevent compressor damage.

Unusual noises or smells

Grinding, buzzing, or hissing can indicate motor issues or a refrigerant leak. A sweet or chemical odor is a red flag. Do not attempt to braze, open, or charge lines yourself.

Heat load reduction you can do today

• Close blinds on sun-facing windows; reflective film helps on high UV index days.
• Seal obvious air leaks around doors and attic hatches; improve insulation to R-38 or better where feasible.
• Avoid heat-generating activities at peak hours: ovens, dryers, halogen lighting.
• Use ceiling fans to improve comfort; they do not lower temperature but help sweat evaporation.
• Keep interior doors open for better return airflow, especially in homes without dedicated return ducts.
• Ensure the outdoor condenser has full airflow and is not recirculating its own hot exhaust.

Mini-splits and heat pumps in extreme heat

Modern inverter heat pump and mini-split systems can perform well in heatwaves if properly sized and commissioned. If a mini-split is not cooling, check the same basics: filter screens, clearances, mode and setpoint, and coil cleanliness. For multi-zone systems, one clogged head can affect overall performance.

When to stop DIY

Refrigerant, high voltage, and sealed components require licensed expertise. Many equipment warranties and insurance companies require work by qualified professionals for coverage—especially on refrigerant leaks, compressor replacements, and electrical parts. If breakers trip repeatedly, you see ice after restoring airflow, you suspect low refrigerant, or you hear electrical buzzing, stop and schedule service. It protects your system and your policy.

Our region and typical conditions

In Southern California, attic temps can exceed 140°F during a heatwave, and coastal humidity swings stress systems differently than inland valleys. #1 AC Guys supports Los Angeles County and nearby areas including Orange County, Ventura County, and Western Riverside County. Common cities we serve include Los Angeles, Pasadena, Long Beach, Santa Clarita, Anaheim, Irvine, Santa Ana, Huntington Beach, Oxnard, Ventura, Thousand Oaks, Riverside, and Corona.

About #1 AC Guys

We are a family business in Southern California with 80+ years in engineering across four generations of engineers. Our manufacturer-trained engineers have received factory education at Fujitsu in Japan, Mitsubishi Elektrik in Thailand, Midea in China, Gree in China, and Hier in China. We specialize in residential HVAC and commercial HVAC, especially large and complex projects where precise load calculation, duct design, and commissioning matter most. Our mission is simple and constant: healthy air at home for a healthy life.

Quick answers

1. Is 78°F inside on a 105°F day normal? Often yes if the temperature split is 15–22°F and airflow is good.
2. Should I run Fan On? Use Auto to maintain dehumidification; Fan On can help thaw a frozen evaporator coil temporarily.
3. Can shading the condenser help? Yes, if airflow is not obstructed. Do not wrap or cover the unit.
4. What if my AC not cooling after a storm? Check the circuit breaker and thermostat first; if the breaker trips again, stop and call a pro.

Key takeaways

• Start with thermostat, filter, airflow, condenser clearance, and basic electrical checks.
• Reduce heat load indoors to support your system during an extreme heatwave.
• If you suspect low refrigerant, electrical faults, or repeated icing, stop DIY. Many insurance policies require licensed service for claims.