Most common causes
- Extreme outdoor heat. Most systems are designed to hold about 20 degrees below outdoor temperature. In Santa Clarita, where summer afternoons often go over 100F, a system may run nonstop and still sit a few degrees above the setpoint until evening.
- Dirty filter or coils. A clogged filter, dirty indoor coil, or dirty outdoor coil all cut cooling capacity, so the system runs longer for less result.
- Leaky ductwork. Ducts in a hot attic that are torn, crushed, or disconnected lose cold air before it reaches the rooms.
- Refrigerant leak. A low charge lowers cooling capacity. The system keeps running but cannot remove enough heat. The leak needs to be found and repaired.
- Undersized or aging system. A system that was undersized, or one that has lost efficiency with age, may not keep up after an addition, new windows, or attic changes.
- Heat gain in the house. Poor attic insulation, west facing windows without shade, and an unvented attic all add heat faster than the AC can remove it.
- Thermostat in a bad spot. A thermostat in direct sun or near a heat source reads high and keeps the system running.
What you can safely check
- Replace the air filter.
- Make sure all supply vents are open and return grilles are not blocked by furniture or rugs.
- Check the outdoor unit: clear debris and rinse the fins gently with a hose from the outside in, with the power off at the disconnect.
- Close blinds on sunny windows during the afternoon.
- Feel the air at a supply vent close to the indoor unit. If it is only slightly cool, that points to a system problem, not just heat load.
- If you can see any ducts in the garage or attic access without climbing into the attic, look for disconnected or crushed sections.
Call a technician if
- The house never reaches the setpoint even on mild days
- Vent air feels only slightly cool
- You see ice on the lines or coil
- Some rooms are much hotter than others and nothing you adjust helps
- Your energy bills jumped without a change in use
What affects the repair cost
Cleaning coils and replacing filters is maintenance level work. Duct sealing or repair depends on how much duct is damaged and how easy the attic is to work in. Refrigerant leak repair depends on where the leak is and the refrigerant type. If the system is undersized or near the end of its life, a load calculation helps decide between repair and replacement.
