Heat wave cooling tips for North County homes

To keep cool in a North County heat wave, pre-cool your home in the morning, hold a steady thermostat setting instead of cranking it down, close blinds on sunny windows, and use fans to feel cooler. Keep the filter clean and the outdoor unit clear. If the AC fails and the house gets dangerously hot, go somewhere air-conditioned.

Inland North County heats up fast. Fallbrook's August highs average around 84°F, but heat waves push inland valleys from Fallbrook and Bonsall to Escondido, San Marcos and Temecula far above that, while the coast stays milder. Those are the days air conditioners are most likely to fall behind or quit, and the days Fallbrook Heating & Air hears about it. Most failures trace back to a few causes you can understand, and some you can prevent.

Why do air conditioners fail during heat waves?

Heat waves push every part of the system to its limit. The usual failures are a weak capacitor that finally gives out, a dirty outdoor coil that can't shed heat, low refrigerant, a clogged filter that freezes the indoor coil, and systems that were undersized from the start. Each one is worse when the AC runs all day.

Capacitors

The capacitor gives the compressor and outdoor fan motor the jolt they need to start and run. Heat wears capacitors out, and a weak one often fails on the hottest afternoon. Common signs: the outdoor unit hums but the fan doesn't spin, or the indoor blower runs but the air isn't cold. Capacitors hold a charge even with the power off, so leave this repair to a technician.

Dirty condenser coils

Your outdoor unit's job is to dump the heat it removed from your house. When the coil is coated in dust, pollen or grass clippings, it can't shed that heat, so pressures and temperatures climb. The system cools less, uses more electricity and can trip on safety limits. A gentle rinse with a garden hose, with the power off at the disconnect, helps. A full coil cleaning is part of a tune-up.

Low refrigerant and frozen coils

A refrigerant leak or a packed filter can freeze the indoor coil. You'll see ice on the copper lines or weak airflow from the vents. Turn the cooling off and run the fan to thaw it. Our guide to an AC frozen with ice on the lines covers what to check.

Undersizing

An AC sized too small for the house may be fine most of the summer, then run nonstop and still fall behind in a heat wave. Additions, new windows or a converted garage can leave an older system undersized. A proper Manual J load calculation sizes a system for your actual home; see what size AC do I need.

What thermostat setting is best in a heat wave?

Pick a steady, reasonable setting, such as 76–78°F, and leave it. Setting the thermostat to 65°F doesn't cool the house faster; it just makes the AC run longer and can freeze the coil. Pre-cool a few degrees in the morning while it's cooler outside, then hold steady through the afternoon.

• Pre-cool in the morning, when the AC works most efficiently and the house hasn't soaked up heat.
• Don't turn the AC off when you leave on a heat wave day. A house that hits 90°F inside takes hours to recover.
• Expect the indoor temperature to run a few degrees above setpoint in late afternoon. That's normal for many systems in extreme heat.
• On SDG&E time-of-use rates, the 4–9 p.m. on-peak window lines up with the hottest part of the evening. See our time-of-use thermostat guide.

Should I shade my outdoor AC unit?

Shade can help a little, but airflow matters far more. A tree, awning or shade sail overhead that leaves the top and sides open is fine. Never cover, wrap or box in the unit, and keep plants, fences and stored items well back from it. The fan has to pull in and push out a lot of air.

• Keep shrubs, weeds and stored items cleared from around all sides.
• Leave the top completely open; the fan blows hot air straight up.
• Rinse the coil gently with the power off if it looks dusty.
• Don't hose the unit while it's running or spray water at the electrical panel.

How do blinds and fans help?

Closing blinds and curtains on sun-facing windows keeps a lot of heat from entering, so the AC has less work to do. Fans don't lower the air temperature, but moving air makes you feel cooler, so you can be comfortable at a slightly higher thermostat setting. Turn fans off in empty rooms.

In practice:

• Close west-facing blinds before noon and south-facing ones through midday.
• Run ceiling fans in the rooms you're using.
• Cook outdoors or use the microwave instead of the oven.
• Run the dryer and dishwasher in the early morning or late at night.
• Open windows at night only if it's truly cooler outside and air quality is good.

A fan alone may not keep you safe in extreme heat. The CDC advises using fans only when the indoor temperature is below 90°F; above that, a fan can raise your body temperature. If the house gets dangerously hot, go somewhere air-conditioned.

How do I stay safe in extreme heat?

Drink plenty of water, stay in air-conditioned spaces as much as you can, and check on neighbors who are at higher risk. The County of San Diego lists infants, young children, older adults and people with chronic medical conditions among those most at risk of heat illness. If someone shows signs of heat stroke, such as confusion, slurred speech, fainting or a very high body temperature, call 911. The National Weather Service lists heat stroke as a medical emergency; see its heat illness guide.

The National Weather Service issues heat advisories and extreme heat warnings for San Diego County separately for the coast, valleys, mountains and desert, and any of them activates the County's heat plan. During heat events, the County runs Cool Zones, free air-conditioned places to stay cool, with a searchable list of sites by city and ZIP code. Never leave children or pets in a parked car.

When is a broken AC an emergency?

If your AC has stopped cooling during a heat wave, especially with an older adult, infant or someone with a medical condition at home, treat it as urgent. Call us at (760) 728-8716. While you wait, keep blinds closed, use fans, and move anyone at risk to a cooler place.

Before you call, a few quick checks:

1. Thermostat. Make sure it's set to COOL and below room temperature, and that the display has power.
2. Breakers. Check the breakers for the indoor and outdoor units. Reset a tripped one once; if it trips again, leave it off.
3. Filter. A packed filter can stop airflow. Replace it.
4. Ice. If you see ice on the lines, turn cooling off and run the fan only.

For more on repairs, see our AC repair page. If your system is old and failing often, our repair or replace guide can help you decide. A yearly tune-up catches weak capacitors and dirty coils before summer.