Why Is My AC Running Constantly? | AC Repair

Common Reasons an AC Runs Constantly

Several conditions can cause an air conditioner to run longer than expected. Some are simple to check, while others require professional diagnosis and repair.

Thermostat Settings or Thermostat Problems

Start with the thermostat. Confirm it is set to cool and that the target temperature is lower than the current indoor temperature. Check that the fan setting is on auto, not on.

When the fan is set to “on,” the blower can continue running between cooling cycles. This may make it seem like the air conditioner is operating constantly even when the outdoor unit and compressor are not actively cooling.

A thermostat set too low can also lead to long runtime. For example, trying to maintain 68°F during an extremely hot Georgia afternoon may require the system to run continuously because the indoor temperature may not be able to reach that setting.

A malfunctioning thermostat, inaccurate temperature sensor, poor thermostat placement, wiring issue, or programming error can also cause the system to run longer than necessary. Thermostats located near direct sunlight, kitchens, supply vents, exterior doors, fireplaces, or other heat sources may not accurately represent the average temperature of the home.

Dirty or Restrictive Air Filter

A dirty filter is one of the most common causes of poor AC airflow. As it fills with dust, pet hair, pollen, and debris, it becomes harder for the blower to pull air through the system. Restricted airflow can reduce cooling performance, increase runtime, and contribute to a frozen evaporator coil.

ENERGY STAR recommends checking HVAC filters monthly during months of heavy heating or cooling use. If the filter appears dirty, replace it; at minimum, filters should be changed every three months.

Use the correct filter size and type for your HVAC system. A highly restrictive filter can also create airflow problems if your ductwork, blower, or filter cabinet is not designed to handle it. If you are considering a high-MERV or HEPA filtration upgrade, ask an HVAC professional to confirm that it is compatible with your equipment.

Blocked Vents or Return Grilles

Your air conditioner needs a clear path for air to move through supply vents, rooms, return grilles, ducts, filters, coils, and the blower. A blocked supply register can limit conditioned air entering a room, while a blocked return grille can restrict the air available to the system.

Check that furniture, rugs, curtains, storage, pet beds, and other objects are not blocking vents or returns. Keep most supply registers open. Closing several vents in an attempt to redirect cooling can increase static pressure, reduce overall airflow, and make the system work harder.

Dirty Coils

Your air conditioner uses two main coils: the indoor evaporator coil and the outdoor condenser coil. The evaporator coil absorbs heat from indoor air, while the condenser coil releases that heat outdoors.

If either coil becomes dirty, heat transfer can be reduced. The AC may need to run longer to cool the home. Dirty indoor coils can also restrict airflow and contribute to freezing. Dirty outdoor coils can make it harder for the system to release indoor heat.

Coil cleaning should be performed by a trained HVAC technician. Evaporator coils are delicate and connected to refrigerant, drainage, and electrical components. Attempting to clean coils without the right tools or training can damage the equipment or create a larger problem.

Refrigerant Leak or Low Refrigerant

Refrigerant is essential to the air-conditioning process. It absorbs heat indoors and carries it outside for release through the condenser coil. If the refrigerant level is low because of a leak, your AC may struggle to remove heat, run longer than normal, blow warmer air, or freeze at the indoor coil or refrigerant lines.

Refrigerant is not a consumable product that should need routine refilling. If the system is low, a qualified technician should locate the source of the leak, repair it when appropriate, and verify the system is charged according to manufacturer requirements.

Possible signs of a refrigerant-related issue include:

• AC running constantly without reaching the selected temperature.
• Warm or mildly cool supply air.
• Ice on refrigerant lines or indoor equipment.
• Hissing or bubbling sounds near refrigerant lines.
• Higher energy use and declining cooling performance.

Duct Leakage or Damaged Ductwork

Ductwork carries cooled air from the AC equipment to each room. If supply ducts leak, cool air can escape into an attic, crawl space, garage, wall cavity, or other unconditioned area. The air conditioner then has to run longer because less conditioned air reaches the living spaces.

Return-duct leaks can pull hot, humid, dusty, or unconditioned air into the system. This increases the AC’s workload and can affect comfort and indoor air quality.

ENERGY STAR estimates that, in a typical home, about 20% to 30% of the air moving through the duct system may be lost through leaks, holes, or poorly connected ductwork.

Duct leakage is especially important in homes with ducts located in hot attics or damp crawl spaces. A professional inspection can identify disconnected ducts, damaged flexible ducts, poor insulation, unsealed connections, undersized returns, and other airflow concerns.

Frozen Evaporator Coil

A frozen evaporator coil can prevent your AC from cooling effectively. Once ice builds up, the system may continue running while airflow becomes weaker and the home becomes warmer.

Common causes of a frozen coil include:

• Dirty or restrictive air filter.
• Blocked return or supply airflow.
• Dirty evaporator coil.
• Blower motor or blower-control issue.
• Low refrigerant from a leak.
• Ductwork or static-pressure problems.

If you see ice on refrigerant lines, the indoor coil area, or outdoor equipment, turn the thermostat from “cool” to “off.” Do not chip ice with sharp tools or continue forcing the system to cool. Contact a qualified HVAC technician to identify why the coil froze.

Outdoor Unit Problems

The outdoor condenser unit must release heat effectively. Leaves, grass clippings, mulch, branches, or stored items around the unit can restrict airflow. Keep the area around the condenser clear, but do not remove panels or use a pressure washer on the coil fins.

Electrical or mechanical problems can also affect outdoor-unit performance. A failed capacitor, contactor, fan motor, compressor, control board, or wiring connection may prevent the system from operating correctly. If the outdoor unit hums, makes unusual noises, repeatedly shuts off, trips a breaker, or runs without cooling, schedule professional AC service.

Incorrect AC Sizing

An air conditioner that is too small may struggle to keep up with the home’s cooling load during the hottest weather. It may run continuously without reaching the thermostat setting.

The system size should be based on a professional load calculation that considers more than square footage. Factors such as insulation, windows, building orientation, ceiling height, air leakage, ductwork, occupant load, and local weather conditions affect cooling requirements.

An oversized AC can also create comfort problems, although it is more likely to short-cycle—turning on and off in brief, frequent cycles—than to run continuously. The U.S. Department of Energy notes that oversized equipment often runs in short cycles and can operate less efficiently than properly sized equipment.