How Long Should Your AC Run on a 110-Degree Day?

On the hottest afternoons in Indio, a correctly sized and healthy central air conditioner may run almost continuously and still keep the house comfortable. Long run time alone is not a problem. The warning signs are a house that gets warmer and warmer even though the system never stops, air from the vents that is not much cooler than the room, and run times that are clearly longer than in past summers under similar weather.

Central air conditioners are sized for a design temperature, a very hot day that occurs only a small share of the year in that location. On days hotter than that, the system is expected to run continuously, and the indoor temperature may drift a few degrees above the setting by late afternoon. In the low desert, those days come often. Indio's average July high is close to 106 degrees and its record is 125.

A system that is too large is not the answer. Oversized units cool the air quickly, shut off, and start again, which is called short cycling. That wears parts and removes less moisture. A properly sized unit runs longer cycles, which is more efficient and more comfortable.

A common rule of thumb is that air coming out of the supply vents should be roughly 15 to 20 degrees cooler than the air going into the return grille. If you have a simple thermometer, you can check this yourself after the system has run for 15 minutes or so. The house should also cool down steadily overnight and in the morning, when outdoor temperatures drop.

If the system runs all day and the indoor temperature never comes back down in the evening, or the temperature difference at the vents is small, something is limiting its capacity. Common culprits are a dirty outdoor coil, a clogged filter, a low refrigerant charge and failing electrical parts.

Call for a check if the house keeps getting warmer through the afternoon and does not recover overnight, if the outdoor fan or compressor is not running while the indoor blower is, if ice forms on the copper lines or indoor coil, or if your electric use is far above the same month last year with similar weather. The page on AC not cooling explains how a technician narrows down the cause.

Turning the system off all day and then setting it very low at 5 p.m. asks the AC to remove a whole day's heat during the hottest, most expensive hours. In the desert, a moderate setting held steady, with a few degrees higher when you are away, usually works better and keeps the house safer.

Close blinds on sunny windows, avoid using the oven in the afternoon, run ceiling fans in occupied rooms and keep doors to unused rooms closed only if your system's returns allow it. Keep the outdoor unit clear and its coil clean so it can release heat efficiently.

When you call about long run times, a technician usually measures the temperature difference across the indoor coil, checks the refrigerant pressures and temperatures against the manufacturer's data, and measures the current draw of the compressor and fan. Those numbers show whether the system is delivering its rated capacity. If it is, the answer may be shade, insulation or ductwork rather than a repair.

Long run times on extreme days are part of life in Indio. What matters is whether the system still cools the air properly and whether the house recovers overnight. If it does not, a technician can find out why before a small problem becomes a breakdown. For every service, see AC repair Indio.