When the A/C Dies in a Heat Wave

What should you do first when the AC breaks in a heat wave?

First, look at the people, then the equipment. Ready.gov defines extreme heat as temperatures above 90 degrees with high heat and humidity for at least two to three days, and says it causes the most deaths of any weather hazard each year. If anyone in the house is old, very young, ill or on heat-sensitive medicines, move them to a cooled place before the house warms up.

Then work through this order in the first hour:

1. Set the thermostat to OFF so a failing part is not restarted over and over.
2. Run the quick checks below.
3. If they do not fix it, call for repair and describe exactly what you saw and heard.
4. Close blinds on sunny sides and leave the oven off.
5. Line up a cooled place for the afternoon and night: a relative, a library or a cooling center.

The National Weather Service's heat warning advice is to stay in an air-conditioned space as much as possible, including overnight.

How do you keep people safe while you wait?

Get anyone at risk into air conditioning, and keep fans for when the house is still below 90°F. The CDC says to use fans only if indoor temperatures are less than 90°F, because above that a fan can increase body temperature. To find a place with A/C, dial 2-1-1, or ask your local health department.

Drink water before you feel thirsty and take cool showers. Check on older neighbors and relatives at least twice a day, which is the CDC's advice for caregivers in its older adults page. Do not stop or change medicines without talking to a doctor; insulin can lose strength in the heat.

Know the signs that mean 911. Ready.gov lists a body temperature above 103°F as a heat stroke sign, and CDC/NIOSH adds confusion, slurred speech, seizures and loss of consciousness. Call 911 now, stay with the person, and cool them with cold wet cloths or ice on the head, neck, armpits and groin, or soak their clothing with cool water. Don't give a confused or unconscious person anything to drink. For heat exhaustion (heavy sweating, dizziness, nausea, weakness), move them somewhere cool and call a doctor if it gets worse or lasts more than an hour. The heat illness page has the full lists.

Which quick checks are safe?

Three checks are safe for any owner, and they rule out the simple causes before you pay for a visit. Do them with the thermostat set to OFF.

1. Thermostat. Put in fresh batteries, confirm the mode is COOL and the setting is below room temperature, then give it a few minutes after switching back on, since many systems delay the compressor restart.
2. Breaker and disconnect. Look for a tripped breaker labeled for the A/C, furnace or air handler, and reset it once. If it trips again, leave it off and call.
3. Filter. Choked airflow can ice up the evaporator coil, per the Minnesota Commerce Department. If you see ice, leave cooling off and let it thaw; never chip it.

Also look, without touching, at the drain pan under the indoor unit. Water there may mean a float switch has shut the system down to prevent an overflow; never bypass or jumper a float switch to get cooling back. Do not open electrical panels on the equipment. If you're unsure, call a licensed technician.

Which parts fail most in extreme heat?

No primary source ranks heat-wave failures, but the usual suspects are the electrical parts that start the compressor and fan. NREL's measure guideline on A/C diagnostics lists faulty contactors, capacitors, condenser fan motors and compressors among the causes of low equipment reliability. Those parts work hardest on days when the system rarely shuts off.

Capacitors, small cylinders that help start the motors, hold a dangerous charge even with the power off, so diagnosing and replacing one is a licensed technician's job. Supply-house capacitor listings (PlumbingSupplyAndMore, owned by the people who run Swelter Almanac) show what a technician fits, so you can follow the repair estimate.

Refrigerant leaks are the other big one. Refrigerant is not used up; a low charge means a leak, and EPA's Section 608 rule limits refrigerant sales to certified technicians. Do not buy a "top-up" can or sealant for a home system.

Is a portable generator safe to use?

Only outdoors, far from the house, with working CO alarms inside. The CPSC says never to use a portable generator inside homes, garages, basements, crawlspaces, sheds or other enclosed areas, even if doors or windows are open. Run it at least 20 feet from the home with the exhaust pointed away from windows, doors and vents; Ready.gov adds attached garages to that list.

Install battery or battery-backup CO alarms on every level and outside sleeping areas. If one sounds, get outside right away, then call 911. CPSC suggests models with an automatic CO shut-off certified to PGMA G300-2023 or UL 2201-2023. Never use a gas stove or oven to heat the house, and never run a charcoal grill, camp stove or generator indoors.

If the whole power supply is out, Ready.gov says to discard refrigerated medicines after more than a day without power, unless the label says otherwise.

How do you find a repair fast?

Call a licensed HVAC contractor, give a clear symptom list, and ask for the earliest slot and a written estimate. Say whether the outdoor unit runs, whether there is ice and whether a breaker tripped, so the right parts come on the first trip.

Check the contractor's state license and ask whether the technician carries EPA Section 608 certification if refrigerant may be involved. For an example of what a local company's A/C service page lists, see Central Plumbing HVAC's air conditioning page (Central Plumbing HVAC, owned by the people who run Swelter Almanac); it works in Bucks and Montgomery County, Pennsylvania. Elsewhere, look for a licensed company near you.

Renters have extra rights. The Arizona courts' A/C legal FAQ says a landlord's normal 10 days to repair shrinks to five days for a health or safety issue, and a Phoenix rental A/C must cool to 82 degrees. Elsewhere, check your city's code. In Texas, PUCT rule §25.483 bars disconnection for nonpayment on National Weather Service heat-advisory days. For bill trouble, LIHEAP covers cooling and emergency services (usa.gov). Once it is fixed, the maintenance page covers the spring checks that catch some faults early.

Numbers for the first hours

• 103°FHeat stroke warning sign, body temperatureReady.gov
• 90°FIndoor temperature above which a fan can raise body temperatureCDC
• 20 feetMinimum distance from a running generator to the homeCPSC (2026)
• 2 visitsDaily check-ins for older adults in heatCDC
• 5 daysArizona landlord repair time for health or safety issuesArizona Judicial Branch

More questions

When the heat turns serious

• SectionHeat safety at home
• Heat safetyOlder adults in a heat wave
• A/C in extreme heatWhy the system falls behind
• SectionA/C in extreme heat, all pages