During a heat wave, set your air conditioner to a temperature that keeps the people in your home safely cool, and check the actual indoor temperature, not just the setting on the screen. There is no single thermostat number that is safe and comfortable for everyone.
About 78°F (26°C) is a commonly suggested summer energy-saving starting point when it is comfortable. It is not a medical safety threshold or a rule to follow when someone remains overheated. Lower the setting when needed, and move to another cooled location if your home cannot stay safely cool.
Start with the occupants, not an energy target
Carrier’s thermostat guidance treats summer setpoints as a balance between comfort and energy use. A heat emergency changes that balance: health takes priority over meeting a recommended savings number.
The CDC’s heat-health guidance recommends air conditioning or access to a cooled location and checking on people who may be at greater risk. Older adults, infants, pregnant people and those with certain medical conditions may need additional precautions. Follow an individual’s clinician-provided heat plan where applicable; do not change medication to manage heat without medical advice.
A thermostat in a cool hallway does not establish the conditions in an upstairs bedroom. Check the rooms people actually occupy with a suitable indoor thermometer, especially overnight. A room that feels unusually humid or stuffy also deserves attention even when the thermostat appears satisfied.
Set a reasonable target, then watch the room
- Select cooling mode. Make sure a schedule, vacation setting or utility program is not unintentionally raising the target while someone needs cooling.
- Choose a setting that provides comfortable cooling for your household. Do not force 78°F on someone who needs a cooler environment.
- Compare the setpoint with the actual temperature. If the room remains hotter and keeps warming, lowering the displayed target repeatedly is not a substitute for another cooling plan.
- Check people as well as equipment. Unusual weakness, dizziness, headache, nausea or other concerning symptoms need attention, not another hour of thermostat experimentation.
Confusion, collapse or suspected heat stroke is an emergency: call emergency services and move the person to a cooler place while following the dispatcher’s instructions. The CDC heat-illness chart distinguishes warning signs and first responses. For less severe symptoms or individual medical concerns, obtain appropriate medical advice promptly.
Two cooling myths that cause confusion
“Set it much lower to cool faster.” On a conventional fixed-capacity system, an extreme setting does not create extra cooling capacity. Trane’s thermostat explanation describes why a lower target generally means a longer run rather than faster cooling. Variable-capacity systems have more complex controls, but an unrealistically low target still cannot remove the equipment’s capacity limit.
“An AC can only cool a house 20 degrees below outside.” This confuses two different measurements. Carrier explains that the familiar temperature drop concerns air moving through the cooling equipment, not a universal outdoor-to-indoor limit. A technician also interprets that air-temperature difference alongside humidity and airflow; it is not a stand-alone homeowner pass/fail test.
Long operation during severe weather is not automatically evidence of damage. More useful warning signs include a rising indoor temperature, newly weak airflow, ice, leakage, electrical trips or abnormal sounds and odors.
Reduce heat gain without modifying the AC
Close sun-facing shades, keep exterior doors and windows closed while cooling, and avoid unnecessary indoor heat from cooking or other appliances during the hottest period. Keep normal air inlets and outlets unobstructed.
Check the specified filter using the unit’s safe maintenance procedure. Do not remove it to increase airflow; see why running without the required AC filter is not a repair. Do not put a sprinkler on the outdoor unit, cover it with an improvised shade, change fan wiring or open its electrical enclosure. Any clearance or cleaning work should follow the manufacturer’s instructions.
Have a backup plan before the house becomes unsafe
CDC advises using fans only when indoor temperatures are below 90°F (32°C); above that, a fan can increase body temperature. A fan is not equivalent to an air conditioner and does not make a dangerously hot room safe.
If cooling fails, move to an air-conditioned location. In the United States, 2-1-1, a local health department or emergency management agency may help identify cooling centers. Plan for children, pets and anyone relying on electrically powered medical equipment.
For a system that is running but not cooling adequately, use our safe cooling-troubleshooting guide while arranging service. Do not remain in unsafe heat to finish diagnosing the AC. This is general heat-health and equipment guidance, not a personalized medical temperature prescription.
