If your AC runs constantly but the house won’t cool down, the system is either not removing enough heat or the house is gaining heat faster than the AC can remove it. Check airflow first (filter, vents, blower), then the outdoor unit, then the house itself (sun, attic, leaks). If the air at the vents isn’t noticeably cold, a refrigerant or compressor problem is likely.
Quick answers
- Healthy systems usually deliver air about 15–20°F colder than the air going into the return.
- Replace the filter and open all vents first.
- Clean the outdoor condenser coil.
- On very hot days, most ACs can only keep the house about 20°F below outdoor temperature.
- Weak temperature drop at the vents usually means a refrigerant or compressor issue.
Test the temperature split
Use a simple kitchen or digital thermometer:
- Measure the air at a return grille (where air goes into the system).
- Measure the air at a supply vent close to the indoor unit after the AC runs 15 minutes.
- Subtract.
| Temperature difference | What it suggests |
|---|---|
| 15–20°F | System is cooling properly; the house may be gaining heat too fast |
| 10–14°F | Possible airflow, coil or charge problem |
| Under 10°F | Likely refrigerant, compressor or major airflow problem |
1. Dirty air filter
A clogged filter reduces airflow, cooling and comfort, and can freeze the coil. Replace it. How Often Should You Change Your Air Filter?
2. Dirty outdoor condenser
If the outdoor coil is covered in dirt, cottonwood or grass, the system can’t reject heat. Rinse it with a hose (power off). How to Clean an AC Condenser Coil
3. Closed or blocked vents and returns
Rooms with closed vents stay warm and the rest of the system loses airflow. Open supply vents and keep returns clear of furniture and rugs.
4. Leaky ducts in the attic
Ducts that leak in a hot attic lose cooled air and pull in hot air. Signs include rooms far from the unit staying warm and dusty attic insulation near duct joints. Sealing with mastic or foil tape (not cloth duct tape) helps. A contractor can measure duct leakage.
5. Low refrigerant
A leak lowers cooling capacity gradually. You may notice the system runs longer every week. Ice on the lines and hissing are other signs. This requires a licensed technician. AC Refrigerant Recharge Cost
6. Failing compressor or capacitor
A weak run capacitor can let the compressor run inefficiently. A failing compressor may run hot and trip off. A technician can test both.
7. Too much sun and heat gain
The house itself can be the problem:
- Close blinds and curtains on east windows in the morning and west windows in the afternoon.
- Avoid using the oven and dryer during the hottest hours.
- Check attic insulation. Many older homes have far less than the recommended level.
- Make sure the attic is ventilated.
8. The system is undersized
If the house was expanded, or the original system was too small, it may never keep up on hot days. Compare your home’s square footage with the sizing chart in What Size AC Do I Need?
9. Thermostat placement or calibration
A thermostat in direct sun, near a lamp or by a kitchen reads too warm and runs the AC endlessly. Compare its reading with a separate thermometer. Move or recalibrate it if needed.
How to keep the house cooler during a heat wave
- Pre-cool the house in the morning, then hold a steady setting.
- Use ceiling fans in occupied rooms (counterclockwise in summer).
- Keep doors to cooled rooms closed only if each room has a return or a gap under the door.
- Avoid setting the thermostat extremely low: it won’t cool faster.
When to call a technician
If the temperature split is under 15°F with a clean filter and coil, call a professional. Ask them to check refrigerant charge, coil condition, blower speed and duct static pressure.
Frequently asked questions
Why does my AC cool at night but not during the day?
Heat gain is highest in the afternoon. A system that is marginally sized or slightly low on refrigerant shows it first on hot afternoons.
Should my AC run all day in extreme heat?
On the hottest days, long run times are normal. If it runs constantly on mild days, something is wrong.
Can a bad thermostat cause the house not to cool?
Yes, if it reads the wrong temperature or doesn’t call for cooling properly.