Why does my AC struggle on the hottest days?

Most systems are sized to keep the house roughly 20°F or so cooler than outside. Above about 100°F, a dirty coil or low refrigerant makes that gap hard to hold.

Is it safe to set the thermostat very low to cool faster?

It won't cool faster, and it can freeze the coil. Set it to a comfortable temperature and let it run.

AC Repair · Modesto, CA

AC Repair in Modesto, CA

When the AC fails on a 105°F afternoon in the valley, the house can become unsafe quickly, especially for older people, babies and pets. Here's what to check and when to get help.

AC repairs, new systems and furnaces
Modesto and nearby Stanislaus County towns
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Quick checks

Confirm the thermostat is on Cool and set below room temperature, and that its batteries are good. Check the breakers for the furnace or air handler and the outdoor unit. Look at the filter, because a clogged one can stop cooling almost entirely and freeze the coil.

The outdoor disconnect

If the indoor fan runs but the outdoor unit is silent, check the disconnect box beside it. Gardeners and pool workers sometimes switch it off by accident.

Diagram: the outdoor unit has its own disconnect box on the wall nearby. If it is switched off or the block is pulled out, the outdoor unit will not run.

Heat-related failures

Extreme heat is hard on capacitors, contactors and fan motors, which are some of the most common summer failures in the valley. A unit that hums but won't start, or whose fan doesn't spin, often has a failed capacitor.

Dirty outdoor coils

Dust from farm fields, cottonwood fluff and lawn clippings clog the outdoor coil, so the system can't shed heat. On very hot days a dirty coil can make the compressor overheat and shut off.

Ice on the lines

Frost on the copper lines or the indoor coil means airflow or refrigerant problems. Turn the system off and let it thaw, and check the filter. Running it frozen can damage the compressor.

Keeping cool meanwhile

Close blinds on sunny windows, run fans, avoid using the oven, and spend the hottest hours in the coolest room or a cooling centre. Check on neighbours who live alone.

Older systems

If the system uses the old refrigerant R-22, repairs involving refrigerant get expensive, because that refrigerant is no longer made. It may be time to price a replacement; see AC installation.

Get it fixed

Tell the technician what the system is doing and what you've checked. Send a request and an HVAC technician will get in touch.

Frequently asked questions

Why does my AC struggle on the hottest days?: Most systems are sized to keep the house roughly 20°F or so cooler than outside. Above about 100°F, a dirty coil or low refrigerant makes that gap hard to hold.
Is it safe to set the thermostat very low to cool faster?: It won't cool faster, and it can freeze the coil. Set it to a comfortable temperature and let it run.

Other AC services

AC InstallationNew systems sized for valley heat, with permits and duct testing.Furnaces & HeatingGas furnaces, heat pumps and no-heat problems on foggy winter mornings.Swamp Cooler ReplacementSwitching from an evaporative cooler to central AC or a heat pump.AC Tune-UpsSpring checks that catch failures before the 100°F days.Duct Testing & SealingLeakage testing, sealing and replacing ducts in hot attics.Air Filtration & SmokeBetter filters and settings for smoky days and dusty valley air.Thermostats & ZoningSmart thermostats, schedules and zoning for hot upstairs rooms.

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