Why Is My AC Freezing Up? Complete Portland Troubleshooting Guide
8 common causes of AC freezing with step-by-step solutions. Learn why your Portland AC ices up and how to prevent costly compressor damage.
Serving Portland, Beaverton, Lake Oswego, West Linn, and the greater metro area since 2008.
Owner & Lead HVAC Technician
🚨 Emergency: Your AC Is Freezing - Act Now!
TURN YOUR AC OFF IMMEDIATELY!
Running a frozen AC destroys the compressor ($1,500-$2,500 repair). Let it thaw completely before restarting.
Most Common Causes:
- 70%Dirty air filter
- 15%Low refrigerant (leak)
- 10%Blocked/closed vents
- 3%Thermostat set too low
- 2%Dirty evaporator coil
What to Do Right Now:
- 1. Turn AC thermostat to OFF
- 2. Switch to FAN ONLY mode
- 3. Let thaw 6-12 hours
- 4. Check/replace air filter
- 5. Ensure all vents are open
- 6. Call pro if refreezes
Portland Tip: Our dry climate means less humidity, but pollen season (May-Oct) clogs filters fast, causing freezing.
Understanding AC Freezing in Portland Homes
An AC unit freezing up is a serious problem that requires immediate attention. After 17 years serving Portland homeowners, we've diagnosed thousands of frozen AC systems and know that ignoring ice buildup leads to expensive compressor failure—the heart of your cooling system.
Portland's climate creates unique challenges for AC freezing. Our mild but variable weather means systems don't get regular use, leading to dust accumulation and corrosion. Meanwhile, our occasional heat waves (80-100°F) put sudden stress on systems that haven't been properly maintained.
This guide covers the 8 most common causes of AC freezing, with clear steps for what you can fix yourself and when to call our EPA-certified technicians. Each cause includes Portland-specific context and prevention strategies to keep your system running smoothly year-round.
8 Causes of AC Freezing (In Order of Likelihood)
1. Dirty/Clogged Air Filter (Most Common - 70%)
Why It Causes Freezing
A dirty filter restricts airflow across the evaporator coil. Without proper air movement, the coil gets too cold and frost forms. This is the #1 cause of AC freezing in Portland homes.
Symptoms
- • Ice on evaporator coil (indoor unit)
- • Reduced airflow from vents
- • AC runs constantly but cools poorly
- • Higher energy bills
DIY Solution
- Turn AC off at thermostat
- Locate and remove air filter
- Check if dirty/clogged (hold up to light)
- Replace with new filter (MERV 8-13)
- Turn AC back on and monitor
Cost & Portland Context
$5-20 for filter. Change monthly during pollen season (May-October) when cottonwood and other trees release allergens that clog filters rapidly.
2. Low Refrigerant (Leak)
Why It Freezes
Refrigerant absorbs heat from your home. When levels are low, the evaporator coil pressure drops too far, causing it to freeze. Leaks are common in older Portland systems (10+ years).
Symptoms
- • Ice on copper refrigerant lines
- • Hissing sound from outdoor unit
- • Gradual loss of cooling capacity
- • System runs longer to cool
⚠️ Professional Only
EPA certification required for refrigerant handling. Never attempt DIY.
Professional Diagnosis
- • Electronic leak detection
- • Pressure testing
- • Leak repair (braze copper lines)
- • System evacuation and recharge
Cost
$200-500 (leak detection + repair + recharge)
3. Blocked or Closed Air Vents
Why It Freezes
Closing vents in unused rooms reduces total airflow. The system tries to cool with less air, causing the evaporator coil to get too cold and freeze. Never close more than 20% of vents.
Symptoms
- • Ice on indoor unit only
- • Some rooms too cold, others too warm
- • AC struggles to maintain temperature
- • Increased run time
DIY Solution
- Check all air vents in home
- Open any closed vents
- Remove furniture/boxes blocking vents
- Ensure return air grilles are clear
- Restart AC and monitor
Rule: Never close more than 20% of your home's vents. This disrupts airflow balance and causes freezing.
Additional Causes of AC Freezing
The remaining 5 causes with key details:
4. Thermostat Too Low
Setting below 68°F causes coil to freeze. During Portland heat waves, don't go below 72°F.
5. Dirty Evaporator Coil
Dust buildup reduces heat transfer. Professional cleaning required. $150-300.
6. Blower Fan Failure
No air over coil = instant freezing. Motor replacement $300-600.
7. Collapsed Ductwork
Common in Portland crawlspaces. Restricts airflow like dirty filter.
8. Outdoor Temperature
Running AC below 60°F outdoor temp can cause freezing. Rare in Portland.
Step-by-Step: What to Do When Your AC Freezes
STOP! Turn AC Off Immediately
Running a frozen AC destroys the compressor. Turn thermostat to OFF position right now.
Switch to Fan-Only Mode
This helps circulate air and speeds up thawing without compressor damage.
Let Thaw Completely (6-12 Hours)
Don't try to chip ice off - can damage aluminum fins permanently. Let nature do the work.
Check Air Filter
Replace if dirty. This fixes 70% of freezing problems.
Ensure All Vents Are Open
Check every room. Never close more than 20% of vents.
Restart and Monitor
Turn AC back on and watch for 30 minutes. If it refreezes within 24 hours, call professional.
⚠️ Warning Signs to Call Professional:
- • Freezing repeats after filter change
- • Ice on outdoor refrigerant lines
- • Hissing sounds (refrigerant leak)
- • Burning electrical smell
- • System is 10+ years old
Preventing AC Freezing in Portland
Monthly Maintenance
- • Check and replace air filters monthly
- • Clean outdoor unit condenser fins
- • Ensure proper clearance around outdoor unit
- • Check that all vents are open and clear
- • Monitor system performance during heat waves
Professional Service
- • Annual tune-up before cooling season
- • Refrigerant level check
- • Electrical and capacitor testing
- • Ductwork inspection and sealing
- • Thermostat calibration
Portland-Specific Tips
- • Change filters monthly during pollen season (May-Oct)
- • Cottonwood trees release seeds that clog filters fast
- • Older homes may have undersized or leaky ductwork
- • Dry climate means less condensate but same freezing risk
- • Heat waves stress systems not maintained regularly
Cost of Prevention vs Repair
Annual maintenance: $100-150
Filter replacement: $5-20/month
vs.
Frozen AC repair: $200-500
Compressor replacement: $1,500-2,500
Frequently Asked Questions About Frozen AC
6-12 hours for complete thawing. Turn the AC off, switch to fan-only mode to help circulate air, and let the ice melt naturally. Never try to chip ice off the coil - this can damage the delicate aluminum fins permanently and require expensive replacement.
Absolutely not! Running a frozen AC unit will destroy the compressor, which costs $1,500-$2,500 to replace. The ice prevents proper lubrication and cooling of the compressor motor. Always turn off the AC immediately when you see ice forming.
It may partially thaw, but it often refreezes because the underlying problem (dirty filter, low refrigerant, etc.) hasn't been fixed. A frozen AC is a symptom of a larger issue that needs to be addressed, or the freezing will keep happening.
Costs depend on the cause: Air filter replacement $5-20, diagnostic visit $100-150, low refrigerant repair $200-500, evaporator coil cleaning $150-300, or compressor replacement $1,500-2,500 if severely damaged. Most freezing issues are caught early with proper maintenance.
Change air filters monthly (especially during Portland's pollen season), keep all vents open, maintain proper refrigerant levels, ensure thermostat is set above 68°F, and schedule annual professional maintenance. Prevention costs $100-200/year vs $500+ for repairs.
Frozen AC? Don't Risk Compressor Damage!
Same-day AC diagnostic and repair in Portland. Prevent $1,500+ compressor replacement.