Heat Pump Auxiliary Heat Running Constantly | Defrost Cycle, Thermostat Issues Portland
If your aux heat light never shuts off, your electric bill is about to get ugly. Here’s how to figure out what’s going wrong — and what you can actually fix yourself before calling in a tech.
Read Time
15 min
Service Area
Portland Metro
License
CCB #187834
Owner & Lead HVAC Technician
Auxiliary heat that won’t shut off is one of the most expensive problems I see on heat pump service calls around Portland. That backup heat — usually electric resistance strips — is only supposed to kick in during genuinely cold snaps or while the outdoor unit defrosts. When it’s running nonstop, you’re burning through electricity at 2–3x the normal rate, and something deeper is wrong with the system.
Understanding why auxiliary heat runs constantly and how to diagnose the problem can prevent thousands of dollars in unnecessary energy costs. Portland’s cold, humid winters create unique challenges for heat pump operation, making proper diagnosis and maintenance essential for efficient heating.
Below, I’ll walk through the most common causes, step-by-step diagnosis, safety concerns, and the Portland-specific factors that make this issue so common in our area.
How Heat Pump Auxiliary Heat Works
Auxiliary heat activates when the heat pump can’t maintain the target temperature, typically below 25-35°F depending on efficiency. It provides backup heating using electric resistance elements, costing 2-3 times more than heat pump operation. Constant auxiliary heat indicates the heat pump isn’t working properly, not that you need more heat.
⚠️ Energy Cost Warning
Aux heat stuck on can add $200–400+ per month to your heating bill during a Portland winter. Electric resistance heat is dramatically more expensive than gas or efficient heat pump operation. If yours won’t shut off, that’s a red flag that needs attention now — not next month.
Auxiliary Heat Running Constantly - Symptoms & Urgency Levels
Auxiliary Heat Runs 24/7
Thermostat malfunction, low refrigerant, frozen outdoor coil
Shut off heat pump immediately, call emergency service
Portland Cost: $400-800 + emergency
Auxiliary Heat Runs Most of Time
Dirty air filter, low airflow, thermostat calibration
Check filters, recalibrate thermostat, schedule service
Portland Cost: $150-350
Auxiliary Heat Activates Above 35°F
Low refrigerant, dirty outdoor coil, airflow restrictions
Check refrigerant levels, clean coils, verify airflow
Portland Cost: $200-500
Heat Pump Defrosts Frequently
Cold weather, humidity, normal operation, or ice buildup issues
Monitor frequency, ensure proper drainage
Portland Cost: $100-250
Inconsistent Heating Performance
Thermostat issues, low refrigerant, electrical problems
Check thermostat calibration, verify refrigerant charge
Portland Cost: $150-400
Outdoor Unit Makes Unusual Noises
Defrost cycle operation, ice cracking, mechanical issues
Check for ice, ensure proper defrost drainage
Portland Cost: $100-300
Why Auxiliary Heat Runs Constantly
Thermostat Malfunction
Description:
Thermostat incorrectly calls for auxiliary heat or has wrong settings
Symptoms:
Auxiliary heat runs constantly regardless of temperature
Solution:
Recalibrate thermostat, replace if faulty ($100-300)
Prevention:
Annual thermostat inspection and calibration
Portland Impact:
Portland’s variable weather can cause thermostat confusion
Low Refrigerant Charge
Description:
Insufficient refrigerant prevents heat pump from reaching target temperature
Symptoms:
Auxiliary heat compensates for weak heat pump performance
Solution:
Find leak, recharge system ($300-800)
Prevention:
Annual refrigerant level check
Portland Impact:
Older Portland homes often have refrigerant leaks
Dirty Air Filter
Description:
Clogged filter restricts airflow, preventing heat pump efficiency
Symptoms:
Heat pump struggles, auxiliary heat activates to maintain temperature
Solution:
Replace air filter ($10-50)
Prevention:
Change filter monthly, more often during pollen season
Portland Impact:
Critical during Willamette Valley pollen season
Frozen Outdoor Coil
Description:
Ice buildup prevents heat absorption, forcing auxiliary heat use
Symptoms:
Heat pump ineffective, auxiliary heat runs continuously
Solution:
Defrost coil, address underlying cause ($200-600)
Prevention:
Ensure proper defrost drainage, maintain clearances
Portland Impact:
High humidity and cold weather increase freeze risk
Cold Weather Performance
Description:
Heat pump reaches efficiency limit in extreme cold
Symptoms:
Auxiliary heat activates in very cold weather (normal below 25°F)
Solution:
Normal operation - may need auxiliary heat upgrade
Prevention:
Ensure proper sizing for Portland climate
Portland Impact:
Arctic outbreaks can challenge even good heat pumps
Defrost Cycle Issues
Description:
Defrost system fails or doesn’t work properly
Symptoms:
Ice buildup, reduced heating, auxiliary heat overuse
Solution:
Repair defrost system ($400-900)
Prevention:
Annual defrost system inspection
Portland Impact:
High humidity affects defrost effectiveness
Step-by-Step Auxiliary Heat Diagnosis
Check Thermostat Settings
Action:
Verify thermostat is set to heat pump mode, check temperature differential settings
Tools:
Thermostat manual, multimeter for testing
Time: 5-10 minutes
Inspect Air Filter
Action:
Remove and check air filter for dirt/clogging, replace if dirty
Tools:
New air filter
Time: 5 minutes
Check Refrigerant Levels
Action:
Professional pressure testing to verify refrigerant charge
Tools:
Manifold gauges, pressure testing equipment
Time: 30-60 minutes
Inspect Outdoor Unit
Action:
Check for ice buildup, proper clearances, defrost drain operation
Tools:
Flashlight, tape measure
Time: 10-15 minutes
Test Thermostat Operation
Action:
Verify thermostat accurately reads temperature and controls auxiliary heat
Tools:
Digital thermometer, thermostat testing tools
Time: 15-20 minutes
Monitor Defrost Cycles
Action:
Observe defrost cycle timing and effectiveness during cold weather
Tools:
Timer, thermometer
Time: 30-60 minutes observation
Critical Safety Considerations
⚠️ Electrical Overload
Description:
Constant auxiliary heat draws high current, risking electrical fires
Warning Signs:
Hot electrical panels, tripped breakers, burning smells
Immediate Action:
Shut off system immediately, call electrician
Prevention:
Proper electrical service sizing, regular inspections
⚠️ Carbon Monoxide Risk
Description:
Improper heat pump operation can affect combustion safety
Warning Signs:
Headache, dizziness, confusion, excessive moisture
Immediate Action:
Ventilate home, seek fresh air, call gas company if gas heat involved
Prevention:
Annual system inspection, CO detector installation
⚠️ Freeze Damage
Description:
Ice buildup can crack heat exchangers or refrigerant lines
Warning Signs:
Water leaks, hissing sounds, reduced performance
Immediate Action:
Shut off system, call service immediately
Prevention:
Proper defrost operation, winterization procedures
⚠️ High Energy Costs
Description:
Auxiliary heat costs 2-3 times more than heat pump operation
Warning Signs:
Surprisingly high utility bills, constant equipment operation
Immediate Action:
Monitor usage, schedule efficiency assessment
Prevention:
Regular maintenance, proper system sizing
Auxiliary Heat Cannot Be Manually Disabled
Auxiliary heat is a safety-controlled system that cannot be manually turned off. It activates automatically when the heat pump can'
Never attempt to bypass or disable auxiliary heat controls - this creates serious safety risks.
Portland-Specific Auxiliary Heat Challenges
Cold Winter Temperatures
Local Impact:
Auxiliary heat activation becomes more frequent and prolonged
Solution:
Ensure proper heat pump sizing, consider dual-fuel systems
Annual Cost:
$500-1,500 system upgrade
Energy Savings:
Reduces auxiliary heat runtime by 40-60%
High Humidity Effects
Local Impact:
Longer defrost times, more frequent auxiliary heat activation
Solution:
Professional defrost system tuning, humidity control
Annual Cost:
$200-500 annual optimization
Energy Savings:
Improves efficiency in damp conditions
Variable Weather Patterns
Local Impact:
Unnecessary auxiliary heat activation during mild periods
Solution:
Smart thermostat installation, temperature lockouts
Annual Cost:
$300-600 automation upgrade
Energy Savings:
Reduces auxiliary heat usage by 30-50%
Pollen and Air Quality
Local Impact:
Heat pump struggles, auxiliary heat compensates constantly
Solution:
High-quality filtration, frequent maintenance
Annual Cost:
$150-400 seasonal upgrades
Energy Savings:
Maintains efficiency during pollen season
Older Home Integration
Local Impact:
Thermostat compatibility problems, wiring issues
Solution:
Professional system integration, thermostat upgrade
Annual Cost:
$400-800 compatibility upgrades
Energy Savings:
Eliminates chronic auxiliary heat issues
Utility Rate Structures
Local Impact:
High operating costs when auxiliary heat runs constantly
Solution:
Efficiency optimization, auxiliary heat minimization
Annual Cost:
$200-600 efficiency improvements
Energy Savings:
Reduces winter heating costs by $300-600 annually
Preventive Maintenance Reduces Auxiliary Heat Usage
In my experience, Portland homeowners can cut aux heat usage by 60–80% with consistent maintenance and some system tuning. Annual tune-ups, filter swaps during pollen season, and refrigerant checks keep the heat pump running efficiently through our wet, cold winters. You’ll spend $250–500 a year on upkeep — but you’ll avoid $400–800 in wasted aux heat costs when January rolls around.
A well-maintained heat pump saves Portland homeowners $500+ on winter heating bills.
When to Call Professional Heat Pump Service
🚨 Emergency Service Required
- •Auxiliary heat runs 24/7 regardless of temperature
- •Heat pump produces no heat, only auxiliary heat works
- •Freezing outdoor temperatures with ice completely covering coil
- •Burning smells or unusual electrical issues
- •Temperatures below 20°F with inadequate heating
- •Carbon monoxide detector alarms
🔧 Schedule Service Soon
- •Auxiliary heat runs most of the time above 35°F
- •Heating costs higher than expected this winter
- •Heat pump is 5+ years old without recent service
- •Thermostat seems inaccurate or erratic
- •Annual maintenance is 12+ months overdue
- •Planning to replace thermostat or add smart controls
Why Professional Heat Pump Service Matters
Aux heat problems sit at the intersection of refrigerant systems, electrical controls, and environmental conditions — it’s rarely just one thing. A licensed tech can trace the issue through the thermostat wiring, check refrigerant charge with gauges, and verify the defrost board is working correctly. In Portland’s cold, humid climate, getting this diagnosis right the first time makes a real difference in your winter bills.
Our auxiliary heat troubleshooting service pinpoints exactly why your system is leaning on expensive backup heat — and gets the heat pump doing its job again.
A $150 diagnostic visit can save you $500+ in unnecessary aux heat costs over a Portland winter.