Why Does My AC Keep Running?

When Continuous Running Is Normal

Air conditioners are sized to handle a design load, not the most extreme afternoon of the year. On a very hot day, a correctly sized system may run nearly continuously and still hold the house comfortably. That is expected behavior, not a malfunction.

There is even an advantage to it: longer run cycles remove more humidity and produce steadier room temperatures than short bursts. An oversized system that satisfies the thermostat in eight minutes usually delivers worse comfort than a right-sized one running for forty.

When It Signals a Problem

The meaningful question is not whether the system runs long, but whether it reaches and holds the setpoint. A system running continuously while indoor temperature climbs through the afternoon is losing the race, and that indicates a capacity or load problem worth investigating.

Another useful comparison is history. If this summer's run times are dramatically longer than last summer's under similar weather, something has changed.

Equipment-Side Causes

• A dirty condenser coil reducing heat rejection
• Low refrigerant charge from a leak
• A dirty evaporator coil or restricted filter limiting airflow
• A blower running at reduced speed or with a fouled wheel
• A compressor that has lost pumping efficiency
• A thermostat that never registers the setpoint due to placement or calibration

House-Side Causes

House-side causes are frequently overlooked in favor of blaming the equipment. In homes with attic ductwork and modest insulation, distribution and envelope improvements sometimes accomplish more than new equipment.

• Duct leakage delivering cooled air into a hot attic
• Insufficient attic insulation increasing heat gain
• Large west-facing glass without shading
• Added square footage the original system was not sized for
• Weatherstripping and sealing gaps around doors and windows
• Heat-producing appliances and lighting during peak hours

Practical Steps for Long Run Times

Start with the filter, then walk outside and look at the condenser coil. Those two account for a large share of gradual capacity loss. Close blinds on sun-facing windows during the afternoon, and avoid running the oven and dryer during peak heat.

Then track the actual performance: note the indoor temperature at 9 a.m., 3 p.m. and 8 p.m. for a few days along with the thermostat setpoint. That record tells a technician far more than a general impression.

About Setback Schedules

Deep setbacks during the day mean the system must recover a large temperature difference during the hottest hours, which can produce very long afternoon run times and sometimes worse comfort overall. In this climate, moderate setbacks generally work better than aggressive ones.