AC Not Working in a Heat Wave: How to Stay Safe Until Repair

When the air conditioner fails during a heat wave, deal with the people before the machine: anyone at higher risk should go somewhere air-conditioned, because the Centers for Disease Control and Prevention (CDC) says fans may feel good but will not prevent heat-related illness when it is really hot. Then check the thermostat setting and the air filter, and call a contractor if the system still is not cooling. Renters should call the landlord or property manager, who arranges the repair. Keep in mind that on days hotter than the system was designed for, a working air conditioner can fall behind.

Who needs a cool place first

Some people are more vulnerable to heat than others. CDC lists people aged 65 and older, infants and young children, people with heart disease or asthma, and pregnant women among the groups at higher risk, along with anyone working or exercising outdoors.

Medicines are part of the picture. CDC notes that many medicines can make people dehydrated or overheated on hot days, and says not to stop or change a medicine until you talk to your doctor. A physician is the person to ask how heat affects a specific condition or prescription.

Where to go when the house is too hot

CDC says that even a few hours in air conditioning helps. It suggests contacting the local health department to find air-conditioned shelters, or spending time in a public library or shopping mall.

Ready.gov, the federal preparedness site run by FEMA, gives the same advice and adds a second one: check on family members, older adults and neighbors. CDC’s HeatRisk dashboard shows the local heat forecast by ZIP code, which helps decide how long the house can wait.

Fans have limits. Ready.gov says not to rely on a fan as the primary cooling device, because fans move air but do not lower body temperature or prevent heat-related illness.

Signs that heat is becoming dangerous

CDC says to seek medical care right away for anyone with symptoms of heat-related illness such as muscle cramps, headaches, nausea or vomiting.

Heat stroke is more serious still. CDC’s occupational health institute (NIOSH) lists its signs as confusion or slurred speech, loss of consciousness, hot dry skin or heavy sweating, seizures and a very high body temperature, and its first instruction is to call 911. While waiting, NIOSH says to stay with the person, move them to a cool, shaded area, remove outer clothing, and cool them with cold water, wet cloths or ice on the head, neck, armpits and groin.

Keeping the house cooler while you wait

A few steps slow the heat gain inside. Ready.gov recommends covering windows with drapes or shades, and CDC suggests using the stove and oven less and taking cool showers or baths.

Weather-stripping doors and windows, also on the Ready.gov list, is a longer-term job, but it keeps hot air out for every future heat wave as well.

Ready.gov also lists a window air conditioner, installed with insulation around it, among preparations for extreme heat. One cooled room can shelter a person at higher risk while the central system waits for a repair.

Why the first heat wave of the summer matters

In U.S. data, the first heat wave of a summer has been the more dangerous one. A study of 43 U.S. communities from 1987 through 2005, published in Environmental Health Perspectives in 2011, found that deaths rose 5.04% during the first heat wave of a summer, against 2.65% during later ones.

The same study found that mortality risk rose 2.49% for every 1°F increase in heat wave intensity, and that the effects were more pronounced in the Northeast and Midwest than in the South.

For the equipment, ENERGY STAR suggests checking the cooling system in the spring, before contractors get busy. Its maintenance checklist lists what that checkup covers: cleaning the indoor and outdoor coils, checking the refrigerant level, adjusting blower components for proper airflow, and tightening electrical connections.

Broken, or just overwhelmed?

A system can be working and still lose ground on the hottest days. Load calculations size equipment to hold a set indoor temperature at a set outdoor design temperature. ENERGY STAR’s design report for certified new homes uses 75°F indoors for cooling and limits the outdoor design temperature county by county: 93°F in Frederick County, Virginia, for example.

When the outdoor temperature climbs past the design point, the equipment is working beyond the conditions it was sized for. A slow rise indoors on those afternoons does not by itself mean a fault.

Two items from ENERGY STAR’s maintenance checklist are within a homeowner’s reach:

• The thermostat settings. The system should be set to cool, and to a temperature below the current room temperature.
• The air filter. ENERGY STAR advises inspecting, cleaning or changing it once a month.

When a professional is needed, and what to ask

Call a cooling contractor when the thermostat and filter are fine and the house keeps warming even on days that are not extreme. From there the diagnosis needs instruments at the equipment. A written note of indoor and outdoor temperatures at a few times of day gives the technician something to set against the design conditions the system was sized for.

Questions worth asking:

• Is the technician certified under EPA Section 608? The U.S. Environmental Protection Agency (EPA) limits the purchase of the refrigerants used in home air conditioners to certified technicians and their employers.
• Will the visit include refrigerant charge, airflow and electrical checks? ENERGY STAR lists all three in a checkup.
• If the refrigerant is low, where is the leak, and will it be repaired? EPA notes that system leaks harm the environment and raise operating and maintenance costs.
• If replacement comes up, what does the load calculation for the house show? The International Residential Code (Section M1401.3) requires new equipment to be sized from that calculation.