The AC runs all afternoon and the house keeps getting warmer

Normal strain or a fault

On a 100°F afternoon even a healthy system may run without stopping. The San Marcos station averages about 21 such days a year (NOAA 1991–2020 normals). What it should not do is lose ground steadily, blow air that feels barely cool, or fail to recover once the sun is down.

Warning signs that point to a fault:

• The air at the vents feels only slightly cooler than the room
• The indoor reading gains more than a few degrees after noon and does not come back by bedtime
• The copper line at the outdoor unit is warm instead of cool and sweating
• The outdoor fan runs but the unit is quiet, or the unit hums and the fan is still

What a technician checks

The temperature drop across the coil first. A healthy system typically cools the air by roughly 15 to 20°F; a much smaller drop with good airflow points at refrigerant or the compressor, while a large drop with weak airflow points at the filter, the blower or a crushed duct. Then the outdoor unit: capacitor, fan, a dirty condenser coil, refrigerant pressures.

What to have ready when you call

• The setting, and the indoor reading at morning and at late afternoon
• Whether the outdoor fan and compressor both run
• The install year and refrigerant from the outdoor data plate
• Your address and ZIP code, and attic access

Common questions

Should I turn the thermostat down further? No. A lower setting does not cool faster; it only keeps the system running longer.

Can washing the outdoor coil help? A coil packed with cottonwood fluff or grass clippings does lose capacity. Switch power off at the disconnect before touching it, and if a gentle rinse changes nothing, call.