How Extreme Heat Affects AC Performance

Why ACs struggle in extreme heat

When a heat wave pushes outdoor temperatures to extremes, air conditioners face physics that work against them. Your system must move heat from inside your home to hotter outdoor air. As the ambient temperature rises, the temperature difference across the condenser shrinks, head pressure climbs, the compressor works harder, and AC performance and efficiency drop. That is why even a healthy unit can run longer, deliver warmer supply air, and still fall short of the thermostat setpoint during peak hours.

Capacity can fall 10–30% in high ambient temperature conditions, and Energy Efficiency Ratio (EER) and SEER/SEER2 ratings no longer reflect real-world output. Central AC, heat pumps in cooling mode, and ductless mini-splits all experience this effect, particularly on west-facing homes and low-shade lots common across Southern California.

The physics in simple terms

• Reduced temperature differential: With less difference between indoor and outdoor conditions, less heat is rejected per minute.
• Elevated refrigerant pressure: Head pressure rises, increasing compression ratio and compressor amp draw.
• Air density and airflow: Hot, thin air offers less mass flow over the condenser coil, reducing heat transfer.
• Ductwork losses: In attics that hit 130–160°F, duct leakage and poor insulation further erode capacity.

Signs your AC is overwhelmed by a heat wave

• Longer runtimes and the thermostat hovering a few degrees above setpoint
• Hot upstairs rooms, uneven cooling, and high indoor humidity
• Outdoor fan running continuously and louder compressor operation
• High-pressure switch trips or breaker trips
• Short-cycling after sunset if the system overheated during the day

Components under the most stress

• Compressors and fan motors: Higher compression ratios mean higher temperatures and wear.
• Run capacitors and contactors: Heat accelerates failure, causing hard starts and erratic cycling.
• Condenser and evaporator coils: Any dirt or matted fins become a major bottleneck under extreme load.
• Refrigerant circuit: Marginal charge shows up as poor subcooling or superheat when the sun is at its peak.
• Ductwork: Leaks and crushed runs waste precious tonnage exactly when you need it most.

What you can safely do during a heat wave

• Set realistic targets: 76–78°F with continuous fan or auto; avoid frequent changes that cause extra cycling.
• Pre-cool early: Start cooling in the morning to bank coolth in walls and furnishings before peak hours.
• Seal the envelope: Close blinds on sun-exposed windows; use exterior shading where possible.
• Reduce internal loads: Delay oven and laundry; switch to LED lighting; use ceiling fans to boost comfort.
• Change the filter: A clean filter improves airflow and helps avoid evaporator freeze-ups under strain.
• Clear the condenser: Remove debris and keep 2–3 feet of clearance around the outdoor unit.

What requires a qualified HVAC professional

Some tasks are unsafe, code-sensitive, or require specialized instruments. In fact, many insurers and warranties require a licensed, qualified HVAC specialist to perform repairs and adjustments. DIY refrigerant handling, electrical work, or non-permitted modifications can void coverage and introduce hazards.

• Measure and set refrigerant charge using superheat and subcooling under high ambient conditions.
• Check high-pressure controls, fan speeds, and verify the need for a high ambient kit where appropriate.
• Test static pressure, correct airflow, and balance or seal ductwork exposed to attic heat.
• Deep-clean coils using the correct chemicals and rinse methods to restore heat transfer.
• Evaluate capacity, SEER2/EER trade-offs, and zoning; commission variable-capacity systems or mini-splits.
• Address electrical stress points: capacitors, contactors, and wire terminations.
• Verify code compliance, permitting, and documentation required by insurance and manufacturers.

Designing for extreme heat in Southern California

In Los Angeles County and neighboring areas such as Orange County, Ventura County, and Western Riverside County, design choices that look minor on paper can determine comfort during a heat wave.

• Right-size by load calculation: Proper Manual J/S approach prevents oversizing that harms humidity control and causes short-cycling.
• Duct design and insulation: Low-static, sealed ducts with adequate R-value reduce attic heat penalties.
• Ventilation and IAQ: Balanced ventilation, filtration, and dehumidification protect healthy air indoors—our core mission is healthy air in your home for a healthy life.
• High-efficiency equipment: SEER2 and strong part-load performance help, but condenser placement, shading, and airflow are equally critical.
• Mini-splits and multi-zone systems: Useful for additions, west-facing rooms, and mixed-use spaces common in both residential and commercial buildings.

Commercial and complex projects

Large homes, multi-story properties, and commercial spaces face added challenges: rooftop unit exposure, long refrigerant runs, and high internal loads from people and equipment. Strategies include staged or variable-capacity systems, demand-controlled ventilation, energy recovery, and thoughtful controls programming that pre-cools and modulates during peak hours.

Safeguards for reliability in a heat wave

• Annual maintenance before summer: Clean coils, verify charge, test capacitors and contactors.
• Smart controls: Gradual setpoint changes and lockouts that prevent aggressive setbacks.
• Condenser placement: Shaded, well-ventilated locations away from recirculating hot exhaust.
• Envelope upgrades: Attic insulation, radiant barriers, and window films reduce peak loads.

FAQ and common myths

Will oversizing fix heat-wave discomfort?

Not reliably. Oversized systems short-cycle, control humidity poorly, and can still struggle if ductwork and envelope are weak. Proper sizing plus airflow and duct upgrades perform better.

Does adding refrigerant help in extreme heat?

Only if charge is actually low. Overcharging elevates head pressure and can damage the compressor. Charge should be set by superheat/subcooling by a qualified technician.

Should I run exhaust fans?

Use kitchen and bath fans briefly; they pull conditioned air out and draw hot air in. Focus on source control and short, targeted use.

Who we are

#1 AC Guys is a fourth-generation family of engineers based in Los Angeles, serving Southern California. We bring 80+ years of engineering experience to residential and commercial HVAC, with a focus on big and complex projects. Our team has factory training at Fujitsu (Japan), Mitsubishi Elektrik (Thailand), Midea (China), Gree (China), and Hier (China). We serve Los Angeles County and nearby areas, including Orange County, Ventura County, and Western Riverside County—covering communities such as Los Angeles, Long Beach, Pasadena, Santa Monica, Anaheim, Irvine, Santa Ana, Thousand Oaks, Ventura, Camarillo, Corona, Temecula, and more.

When heat waves hit, plan for physics, not just for nameplate ratings. With the right design, maintenance, and qualified care, your air conditioner can stay safer, more reliable, and closer to setpoint—even on the hottest days.