AC Short Cycling? Why Your AC Keeps Turning On & Off
6 common causes of frequent AC cycling with Portland-specific solutions. Learn why short cycling damages your system and how to fix it.
Serving Portland, Beaverton, Lake Oswego, West Linn, and the greater metro area since 2008.
Owner & Lead HVAC Technician
🚀 Quick Diagnosis: Normal vs Short Cycling
✅ Normal AC Cycling:
- • 15-20 minutes on during peak cooling
- • Longer off periods when load is light
- • Consistent temperature maintenance
- • Proper dehumidification
- • Lower energy bills
⚠️ Short Cycling (Problem):
- • On/off every 5-10 minutes
- • Constant running with little cooling
- • Poor humidity control
- • Higher energy bills
- • Excessive compressor wear
Portland Tip: Short cycling is common in older homes with oversized AC units installed during energy crisis years (1970s-80s).
Understanding AC Short Cycling in Portland
AC short cycling—when your system turns on and off frequently—is one of the most common and damaging problems we encounter in Portland homes. While normal AC operation involves 15-20 minute cycles, short cycling means the system runs for just 5-10 minutes before shutting down, only to restart moments later.
This constant starting and stopping prevents your AC from properly dehumidifying the air, reduces energy efficiency, increases wear on expensive components like the compressor, and can lead to premature system failure. In Portland's variable climate with occasional heat waves, short cycling becomes even more problematic as systems struggle to maintain consistent cooling.
After diagnosing thousands of short cycling AC systems, I've found that most cases stem from just a few common causes. This guide covers the 6 most frequent reasons Portland homeowners experience short cycling, with clear steps for what you can fix yourself and when to call our professional technicians.
6 Causes of AC Short Cycling (In Order of Likelihood)
1. Oversized AC Unit (Most Common Cause)
Why It Causes Short Cycling
An oversized AC cools your home too quickly, reaching the thermostat temperature setpoint in just 5-10 minutes. The system then shuts off, but the temperature rises rapidly, causing it to restart immediately. This prevents proper dehumidification and increases compressor wear.
Symptoms
- • AC runs 5-10 minutes, off 5-10 minutes
- • Home feels clammy (poor dehumidification)
- • Higher energy bills despite short run times
- • AC struggles during Portland heat waves
Diagnosis
Check your system's age and installation history. Units installed in the 1970s-80s energy crisis era were often deliberately oversized to save energy, but this creates short cycling problems.
Professional Solution
Manual J load calculation and potential system replacement. Cost: $3,000-7,000 for properly sized system.
Portland Context
Many Portland homes built pre-1990 have oversized systems from energy efficiency push. Our mild climate made oversizing seem effective, but it causes short cycling during rare heat waves.
2. Dirty/Clogged Air Filter
Why It Causes Short Cycling
A dirty filter restricts airflow across the evaporator coil, causing it to get too cold and potentially freeze. The system senses the restriction and shuts down for protection, then restarts when it thaws. This creates the classic short cycling pattern.
Symptoms
- • AC runs short bursts with ice on coils
- • Reduced airflow from vents
- • System works but doesn't cool well
- • Filter hasn't been changed in months
DIY Solution
- Locate and remove air filter
- Check if dirty or clogged
- Replace with new filter (MERV 8-13)
- Monitor cycling pattern for improvement
Cost & Prevention
$5-20 for filter. Change monthly during cooling season to prevent short cycling.
Portland Context
Our high pollen season (May-October) with cottonwood and other trees rapidly clogs filters, causing short cycling if neglected.
3. Thermostat Problems
Why It Causes Short Cycling
Faulty thermostats can send false signals to the AC, causing it to turn on/off inappropriately. Heat from nearby appliances, poor location, or failing sensors create temperature readings that don't match room conditions.
Symptoms
- • AC turns on when room is already cool
- • Thermostat reading doesn't match room temperature
- • Inconsistent cooling throughout house
- • Very short cycles (under 5 minutes)
DIY Troubleshooting
- Check thermostat location (away from heat sources)
- Replace batteries if applicable
- Ensure thermostat is set to "COOL" mode
- Check for proper calibration
Professional Service
If thermostat is faulty, replacement needed. Cost: $50-300 depending on type.
Cost: Basic thermostat $50-150, smart thermostat $200-300.
4. Frozen Evaporator Coil
Why It Causes Short Cycling
When the evaporator coil freezes, ice prevents proper heat transfer. The system senses the problem and shuts down for protection, then restarts when ice melts, creating short cycling. The underlying cause (usually airflow restriction) must be fixed.
Symptoms
- • Ice visible on indoor air handler
- • AC runs short bursts then shuts off
- • No cooling during short run periods
- • Water leaking from indoor unit
Immediate Action
- Turn AC off at thermostat
- Switch to fan-only mode to thaw
- Check and replace air filter
- Let thaw completely (6-12 hours)
- Fix underlying cause before restarting
Warning: Never chip ice off coil - damages aluminum fins permanently.
Cost: Diagnostic $100-150, repair $200-500 depending on cause.
5. Low Refrigerant (Leak)
Why It Causes Short Cycling
Low refrigerant pressure from leaks causes the evaporator coil to get too cold, leading to freezing. The system senses the problem and shuts down, then restarts when conditions change, creating short cycling. Leaks are common in older systems.
Symptoms
- • Gradual decrease in cooling capacity
- • Ice on refrigerant lines or coil
- • Hissing sounds from outdoor unit
- • AC runs but doesn't reach set temperature
⚠️ Professional Only
Refrigerant handling requires EPA certification. Never attempt DIY refrigerant work.
Professional Diagnosis
- • Electronic leak detection
- • Pressure testing
- • Leak repair and recharge
Cost
$200-500 for leak detection, repair, and recharge.
6. Electrical Problems
Why It Causes Short Cycling
Failing capacitors, contactors, or voltage issues cause the compressor to struggle, leading to safety shutdowns. The system tries to restart, but the electrical problem persists, creating short cycling. Common in systems 10+ years old.
Symptoms
- • AC hums but compressor doesn't start
- • Very short run times (under 5 minutes)
- • Frequent breaker trips
- • Burning smell from outdoor unit
⚠️ DANGER - Call Professional
ELECTRICAL HAZARD!
Do not attempt electrical repairs yourself. Can cause fire or shock.
Professional Electrical Work
Capacitor or contactor replacement. Cost: $150-400. Full electrical panel may need updating.
Cost: Capacitor $150-250, contactor $200-350, electrical service upgrade $500+.
Why Short Cycling Damages Your AC System
❌ Problems Caused by Short Cycling:
- Premature Compressor Failure
Constant starting/stopping wears out the most expensive component
- Poor Dehumidification
Short cycles don't remove enough moisture, creating damp indoor air
- Higher Energy Bills
Frequent starts use more electricity than efficient long cycles
- Reduced System Life
Overall lifespan cut by 30-50% due to excessive wear
✅ Benefits of Proper Cycling:
- Better Humidity Control
Longer run times effectively remove moisture
- Energy Efficiency
15-20 minute cycles use less total energy
- Extended Equipment Life
Reduced wear on compressor and components
- Consistent Comfort
Steady temperatures without temperature swings
Preventing AC Short Cycling in Portland
Monthly Maintenance
- • Change air filters monthly during cooling season
- • Monitor thermostat for accurate readings
- • Check for ice on coils during operation
- • Ensure proper airflow from all vents
- • Listen for changes in cycling patterns
Professional Service
- • Annual system inspection and tune-up
- • Load calculation to verify proper sizing
- • Refrigerant level check
- • Electrical system testing
- • Thermostat calibration
Portland-Specific Prevention
- • Schedule tune-ups before summer heat waves when short cycling becomes most problematic
- • Check for oversized systems common in homes built during 1970s energy crisis
- • Monitor during our rare 90°F+ days when systems work hardest
- • Clean cottonwood seeds from outdoor units during May pollen season
- • Consider programmable thermostats to prevent temperature swings
Frequently Asked Questions About AC Short Cycling
Short cycling usually means oversized AC unit (cools too fast), dirty air filter, thermostat issues, frozen coil, low refrigerant, or electrical problems. Check filter first, then call professional for diagnosis. Most cases are fixable without expensive repairs.
Yes! Frequent cycling prevents proper dehumidification, increases wear on compressor, raises energy bills, and can cause premature failure. Normal cycles should be 15-20 minutes; short cycling (under 10 minutes) damages your system and reduces lifespan by 30-50%.
Check/replace air filter, ensure proper thermostat location, verify correct sizing. If issues persist, call professional - may need electrical repairs or resizing. Most short cycling is caused by maintenance issues or improper installation.
Very short cycles (under 10 minutes) often indicate oversized unit, thermostat malfunction, or electrical issues. Normal cycles should be 15-20 minutes during peak cooling. Short cycling prevents dehumidification and increases energy costs.
Depends on cause: Filter $5-20, thermostat $50-300, capacitor $150-400, electrical $200-500, resizing full system $3,000-7,000. Professional diagnostic $100-150. Prevention through maintenance costs $100-150/year.
AC Short Cycling? Get Proper System Performance!
Professional AC diagnosis and optimization in Portland. Fix short cycling before it causes expensive damage.