AC Running Constantly: Reasons and How to Fix

Why Your AC Runs Constantly and How to Fix It

An air conditioner that runs non-stop is concerning for both your electricity bill and the health of your system. While some continuous operation is normal during extreme heat, constant running under typical conditions signals a problem. This guide helps you identify why your AC won't stop and what to do about it.

When Constant Running Is Normal

Expected Continuous Operation

Before troubleshooting, know when constant running isn't a problem:

• Extreme heat days (40°C+): Even properly sized ACs may run continuously
• Inverter ACs: Designed to run continuously at low power—this is efficient
• Initial cool-down: First 1-2 hours after AC starts on a hot day
• Very low thermostat setting: Setting 20°C when it's 40°C outside requires constant operation

When It's a Problem

Investigate if:

• AC runs constantly but doesn't reach set temperature
• Non-inverter AC never cycles off, even on moderate days
• Bills significantly higher than previous years for same conditions
• Room feels uncomfortable despite continuous operation

Common Causes and Solutions

1. Dirty Air Filter

How it causes constant running:

• Restricted airflow reduces cooling efficiency
• Less cold air reaches the room
• AC runs longer trying to cool
• May never reach set temperature

Solution:

1. Turn off AC
2. Remove and inspect filter
3. Clean or replace if dirty
4. Run AC for 1-2 hours and observe cycling behavior

Time to fix: 15 minutes. Cost: $0-20

2. Undersized AC Unit

How it causes constant running:

• AC doesn't have enough capacity for the space
• Runs at maximum but can't remove heat fast enough
• Never reaches thermostat setpoint
• Problem existed since installation or room modification

Signs of undersizing:

• Issue present since AC was new
• Worse on hot days, somewhat better on mild days
• Room never quite reaches desired temperature

Solution:

• Calculate proper BTU requirement using our calculator
• Compare to your AC's rated capacity
• If undersized, consider supplemental unit or replacement
• Short-term: Reduce heat gain (close curtains, use fans)

Time to fix: Varies. Cost: $0 for mitigation, $500-2000 for new AC

3. Low Refrigerant

How it causes constant running:

• Refrigerant absorbs heat from air
• Low levels mean less heat absorption per cycle
• AC must run longer to achieve same cooling
• May never reach set temperature

Signs of low refrigerant:

• Ice forming on refrigerant lines or coils
• Hissing or bubbling sounds
• Gradual decline in performance over weeks/months
• Air from vents not as cold as it used to be

Solution:

• Professional diagnosis required
• Technician must find and fix leak, then recharge
• Simply recharging without fixing leak wastes money

Time to fix: 1-3 hours. Cost: $150-400

4. Thermostat Problems

How it causes constant running:

• Thermostat reads wrong temperature (too high)
• AC never "thinks" room is cool enough
• Keeps running trying to reach impossible target

Causes of thermostat errors:

• Located in hot spot (near window, lamp, or electronics)
• Direct sunlight on thermostat
• Dirty sensor affecting readings
• Calibration drift in older thermostats
• Dead batteries causing erratic behavior

Solution:

1. Compare thermostat reading to accurate room thermometer
2. If different, relocate thermostat or recalibrate
3. Replace batteries if applicable
4. Consider smart thermostat with remote sensors

Time to fix: 30 minutes - 2 hours. Cost: $0-200

5. Dirty Condenser Coils (Outdoor Unit)

How it causes constant running:

• Condenser releases heat from refrigerant
• Dirty coils can't release heat efficiently
• Refrigerant stays warmer, reducing cooling capacity
• AC works harder and runs longer

Signs of dirty condenser:

• Outdoor unit looks dirty or covered with debris
• Plants or obstructions around unit
• Higher electricity bills over time

Solution:

1. Turn off AC at breaker
2. Clear all debris from around unit (leaves, grass, trash)
3. Gently spray coils with garden hose (low pressure)
4. Ensure 2-foot clearance on all sides
5. For heavy buildup, professional cleaning recommended

Time to fix: 30-60 minutes. Cost: $0 DIY, $100-200 professional

6. Duct Leaks or Blockages

How it causes constant running:

• Cool air escapes into attic or walls before reaching rooms
• Blocked ducts prevent proper airflow
• AC cools spaces you can't feel
• Rooms never reach target temperature

Signs of duct problems:

• Some rooms significantly warmer than others
• Weak airflow from certain vents
• Hot attic despite AC running
• Higher bills without comfort improvement

Solution:

• Inspect accessible ductwork for gaps or damage
• Seal visible leaks with mastic or metal tape
• Professional duct testing for hidden leaks
• Consider duct cleaning if heavily contaminated

Time to fix: 1-4 hours. Cost: $50-500

7. Heat Gain Exceeding AC Capacity

How it causes constant running:

• Excessive heat entering faster than AC can remove it
• Poor insulation, single-pane windows, or direct sun
• AC was sized correctly but conditions changed

Solution:

• Add insulation to attic and walls
• Install reflective window film
• Use thermal curtains on sunny windows
• Add shade trees or awnings outside
• Seal air leaks around doors and windows

Time to fix: Varies. Cost: $50-2000+ depending on scope

Troubleshooting Flowchart

1. Check filter first: Clean/replace → Wait 2 hours → Still running?
2. Check thermostat: Compare to room thermometer → Accurate?
3. Check outdoor unit: Clean if dirty → Clear obstructions
4. Check for ice: If present, turn off and call professional
5. Check airflow: All vents open and unblocked?
6. Verify sizing: Use calculator to confirm adequate capacity
7. If all above okay: Call HVAC professional

Energy Cost of Constant Running

AC Size	Normal (8 hr/day)	Constant (16 hr/day)	Extra Monthly Cost
1 Ton	$30	$60	$30
1.5 Ton	$45	$90	$45
2 Ton	$60	$120	$60

Fixing the cause of constant running can save $30-60 per month or more.

When to Call a Professional

• Ice on refrigerant lines or coils (likely refrigerant issue)
• Hissing sounds (refrigerant leak)
• DIY troubleshooting didn't help
• System more than 10 years old
• Electrical issues or burning smell

Conclusion

AC running constantly usually indicates a fixable problem: dirty filter, thermostat issue, dirty outdoor unit, or inadequate sizing. Start with simple checks—filter, thermostat, and outdoor unit—before moving to more complex causes. For refrigerant issues, duct problems, or if basic troubleshooting fails, professional diagnosis ensures proper repair and restores efficient operation.