Furnace Not Heating? Complete Portland Troubleshooting Guide
12 common reasons your furnace isn't working and step-by-step solutions. Portland natural gas furnace troubleshooting for homeowners.
Owner & Lead HVAC Technician
🚨 Emergency Furnace Diagnosis: Is It Heat or Cold Air?
❄️ Furnace Running But Blowing Cold Air:
- • Thermostat set to "Fan On" (not "Auto")
- • Pilot light or ignition failure
- • Dirty flame sensor
- • Gas supply problems
- • Limit switch tripped (overheat)
🔥 Furnace Not Running At All:
- • Tripped circuit breaker
- • Thermostat issues (dead batteries, wiring)
- • Gas valve closed
- • Control board failure
- • Cracked heat exchanger (dangerous!)
Portland Tip: Our mild winters mean furnaces sit unused for months, leading to dust buildup. First cold snap (October) often reveals issues.
Understanding Furnace Heating Problems in Portland
After 17 years diagnosing furnace problems throughout Portland, I've seen every imaginable heating failure—from simple thermostat issues to dangerous cracked heat exchangers. Portland's climate creates unique challenges: our mild but variable winters mean furnaces sit dormant for months, only to face sudden demands during rare cold snaps. Meanwhile, 95% of Portland homes use natural gas furnaces, making gas supply and venting critical concerns.
This comprehensive guide covers the 12 most common reasons Portland furnaces stop heating, with clear steps for what you can safely diagnose yourself and when to call our NATE-certified technicians. Each issue includes Portland-specific context, typical repair costs, and safety considerations. Remember: while many furnace problems are simple fixes, some indicate serious safety issues requiring immediate professional attention.
Whether your furnace is blowing cold air, not running at all, or cycling erratically, this guide will help you understand what's happening and what to do next. We'll start with the most common (and easiest to fix) problems and work toward more serious issues.
12 Common Causes of Furnace Not Heating (In Order of Likelihood)
1. Dirty/Clogged Air Filter (Most Common - 80%)
Symptoms
- • Furnace runs but house doesn't warm up
- • Longer than normal run times
- • Higher energy bills
- • Reduced airflow from vents
- • Furnace shuts off prematurely
Why It Happens
Your air filter traps dust, pollen, and debris. When clogged, it restricts airflow through the furnace, causing the heat exchanger to overheat and the limit switch to trip, shutting down the furnace for safety. This creates a cycle of short run times with no heat production.
DIY Solution
- Locate furnace filter (usually in return air duct)
- Remove and inspect - hold up to light
- Replace with new filter (same size, MERV 8-13)
- Ensure correct airflow direction arrows
- Test furnace operation
Cost & Portland Context
$5-20 for replacement filter. Change monthly during Portland's pollen season (May-October) when cottonwood and grass pollen heavily loads filters.
2. Thermostat Problems
Symptoms
- • Furnace doesn't respond to temperature changes
- • Display shows wrong temperature
- • Furnace runs constantly or not at all
- • Clicking sounds from thermostat
Common Issues
Dead batteries, poor location (near heat sources), incorrect settings, or faulty wiring. Portland homes often have thermostats in hallways or near windows where temperature readings are inaccurate.
DIY Troubleshooting
- Replace batteries (even if digital thermostat)
- Check if set to "HEAT" mode
- Verify temperature setting above room temperature
- Ensure thermostat isn't in direct sunlight or draft
- Test by turning up 5 degrees
Professional Service
If thermostat is faulty, replacement needed. Cost: $150-350 including programming.
Portland Tip: Older homes often have thermostats that don't work with modern furnaces. Check compatibility when replacing.
3. Pilot Light or Ignition Failure
Symptoms
- • Furnace clicks but burners don't ignite
- • Pilot light visible but won't stay lit
- • Smell of gas when furnace tries to start
- • Furnace runs on fan only
Types of Systems
Portland has both standing pilot systems (constant small flame) and electronic ignition systems (lights only when heating). Standing pilot systems are more common in older homes but require regular maintenance.
DIY for Standing Pilot
- Find pilot light access door on furnace
- Turn thermostat to lowest setting
- Wait 5 minutes for gas to clear
- Hold pilot button while lighting with match
- Hold button 30 seconds after lighting
⚠️ Safety Warning
Never attempt DIY on electronic ignition systems. Call professional for hot surface igniters ($150-400) or flame sensors ($100-250).
Cost: Pilot assembly $100-200, igniter $150-400, flame sensor $100-250.
Additional Common Causes (4-12)
The remaining 9 causes with key details (full content in complete guide):
4. Tripped Circuit Breaker
Reset breaker at panel. If trips again, electrical issue. Cost: $150-400.
5. Closed/Blocked Vents
Open all vents, clear obstructions. Never close more than 20%.
6. Gas Supply Issues
Check gas valve on/off. NW Natural emergency: call immediately.
7. Flame Sensor Problems
Dirty sensor prevents ignition. Clean or replace: $100-250.
8. Limit Switch Failure
Overheat safety switch. Replace: $150-300.
9. Blower Motor Not Running
Fan circulates heat. Motor replacement: $400-800.
10. Cracked Heat Exchanger
Carbon monoxide danger! Replace furnace: $1,500-2,500.
11. Low Gas Pressure
NW Natural issue. Call utility company.
12. Control Board Failure
Furnace brain failure. Replace: $300-800.
When Furnace Problems Require Professional Service
🚨 Emergency - Call Immediately:
- Smell gas or rotten eggs
- Yellow or flickering flames
- Loud banging or explosions
- Carbon monoxide symptoms (headache, nausea)
- Furnace is 15+ years old and failing
🔧 Schedule Service Soon:
- Furnace runs but doesn't heat well
- Higher than normal gas bills
- Strange noises during operation
- Uneven heating throughout house
- Annual maintenance is overdue
Preventing Furnace Heating Problems in Portland
Seasonal Maintenance (Fall)
- • Schedule professional tune-up before heating season
- • Change air filter (October start of heating)
- • Test carbon monoxide detector
- • Check thermostat batteries
- • Inspect visible ductwork for damage
Monthly During Heating Season
- • Replace air filters monthly
- • Keep furnace area clear (2-foot clearance)
- • Listen for unusual noises
- • Monitor gas bills for spikes
- • Check pilot light (standing pilot systems)
Portland-Specific Furnace Care
- • Our mild winters mean furnaces sit unused for months - first cold snap reveals dust buildup
- • October "cold snap surprise" is common - schedule tune-up before heating season starts
- • Older Portland homes (pre-1980) often have undersized return air systems
- • Crawlspace furnaces prone to moisture issues - annual inspection essential
- • Natural gas systems (95% of Portland) require proper venting to prevent carbon monoxide
- • Cottonwood pollen season (May-June) affects filters even in winter if not changed
Annual professional maintenance costs $100-150 but prevents $500+ emergency repairs and ensures safe, efficient heating.
Frequently Asked Questions About Furnace Heating Problems
Most common causes are dirty air filter (80% of cases), thermostat issues, pilot light/ignition problems, or gas supply issues. Check filter first, then thermostat settings. If furnace is running but not heating, it could be a cracked heat exchanger - call professional immediately for safety.
Simple repairs like filter changes take 15-30 minutes. Thermostat replacement: 1-2 hours. Gas valve or ignition issues: 1-3 hours. Complex repairs like heat exchanger replacement: 4-8 hours. Emergency repairs available same day in Portland metro area.
Cold air from furnace usually means thermostat set to 'Fan On' instead of 'Auto', pilot light out, dirty flame sensor, overheated furnace tripping limit switch, or gas supply problems. Check thermostat first - it's the most common cause and easiest to fix.
Call immediately if you smell gas, see yellow flames, hear loud banging, have carbon monoxide symptoms (headache, nausea), or if furnace won't heat after checking basics. Also call if system is 10+ years old and failing frequently. Don't wait for Portland winter emergencies.
Costs vary by issue: Air filter $5-20, thermostat $150-350, flame sensor $100-250, igniter $150-400, blower motor $400-800, gas valve $300-600, heat exchanger $1,500-2,500 (often replace whole furnace). Diagnostic fee $100-150 waived with repair.
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