HVAC Sulfur Smell Causes | Rotten Egg Smell from Furnace or AC
Why HVAC smells like rotten eggs (sulfur). Complete diagnosis guide for sulfur odors. Portland gas leak safety and solutions.
Read Time
15 min
Service Area
Portland Metro
License
CCB #187834
Owner & Lead HVAC Technician
🚨 EMERGENCY WARNING
If you smell rotten eggs or sulfur from your HVAC system, EVACUATE IMMEDIATELY. This could indicate a NATURAL GAS LEAK, which can cause explosions or fires.
DO NOT:
- • Use electrical switches or appliances
- • Make phone calls inside the house
- • Light matches or create sparks
- • Try to fix it yourself
DO:
- • Leave the house immediately
- • Call NW Natural: 800-882-3377
- • Call 911 if needed
- • Go to a neighbor's house
A sulfur or rotten egg smell coming from your HVAC system is one of the most serious warnings you can encounter as a homeowner. While the smell might seem like a minor annoyance, it often indicates dangerous conditions that require immediate action. Natural gas, which is odorless, has sulfur compounds added specifically to create this detectable rotten egg smell when leaks occur.
In Portland, where natural gas is the primary heating fuel for most homes, sulfur smells from HVAC systems are taken very seriously. While not every sulfur odor indicates a gas leak, all require professional investigation to rule out life-threatening dangers. This guide covers the most common causes of sulfur smells in HVAC systems and provides clear safety instructions for each situation.
Understanding when a sulfur smell is an emergency versus a less serious issue can help you respond appropriately and protect your family's safety.
Why Natural Gas Smells Like Rotten Eggs
Natural gas is odorless in its pure form. Utilities add mercaptan (a sulfur compound) to give it a distinctive rotten egg smell, making leaks detectable. This safety measure has saved countless lives by alerting people to dangerous gas leaks before they cause explosions or fires.
7 Causes of Sulfur Smells in HVAC Systems
Natural Gas Leak (EMERGENCY)
Critical⚠️ Gas leaking from furnace or connections - evacuate immediately
Symptoms:
Strong rotten egg smell, especially when furnace runs
Action:
Call NW Natural 800-882-3377, evacuate, call 911
Cost: Emergency response free, repair $200-$800
Dead Animal in Ductwork
MediumRodents, birds, or small animals die in ducts and decompose
Symptoms:
Intermittent sulfur smell, stronger when heat runs
Action:
Professional duct cleaning and sanitization
Cost: $300-$800
Sulfur Bacteria in Humidifier
LowBacteria grow in humidifier water, producing hydrogen sulfide
Symptoms:
Sulfur smell when humidifier runs, visible mold/slime
Action:
Deep clean humidifier, use antibacterial solution
Cost: $100-$200
Sewer Gas from Dry Trap
MediumHigh-efficiency furnace condensate trap dries out, allowing sewer gas in
Symptoms:
Sulfur smell from vents, worse after furnace cycles off
Action:
Refill condensate trap with water
Cost: $100-$150
Cracked Heat Exchanger
HighFurnace heat exchanger crack allows exhaust gases into home
Symptoms:
Persistent sulfur smell, soot around furnace, condensation on windows
Action:
Replace heat exchanger, safety inspection required
Cost: $1,500-$3,500
Gas Valve Leak
CriticalFurnace gas valve develops small leak, releasing gas with odorant
Symptoms:
Sulfur smell at furnace, especially when off
Action:
Shut off gas, evacuate, call NW Natural
Cost: $200-$500 valve replacement
Old Gas Line Corrosion
CriticalCorroded gas lines leak small amounts of gas
Symptoms:
Persistent low-level sulfur odor throughout home
Action:
Evacuate, call NW Natural for inspection
Cost: $500-$2,000 pipe replacement
Emergency Response for Sulfur Smells
Immediate Evacuation
- •Leave house immediately - do not use electrical switches
- •Go to neighbor's house or safe distance
- •Do not use cell phones inside or near house
- •Take pets and family members with you
Why: Natural gas can explode from any spark
Call Emergency Services
- •Call NW Natural at 800-882-3377 first
- •Then call 911 if evacuation needed
- •Provide exact address and description of smell
- •Stay away from house until cleared
Why: Professional gas leak detection required
If No Gas Leak Found
- •Re-enter house only when utilities clear it safe
- •Call HVAC technician for other causes
- •Do not operate furnace until inspected
- •Monitor for carbon monoxide
Why: Other causes still need professional attention
Portland Gas Safety Resources
Emergency Contacts
- •NW Natural Gas Emergency: 800-882-3377 (24/7)
- •Portland Fire Department: 911
- •Poison Control (CO): 800-222-1222
- •HVAC Emergency: 503-698-5588
- •PGE Power Emergency: 800-743-5000
Prevention Measures
- •Annual furnace safety inspection
- •Install carbon monoxide detectors
- •Keep furnace area clear and clean
- •Regular ductwork inspection
- •Humidifier maintenance and cleaning
Carbon Monoxide Warning
Some sulfur smell causes (like cracked heat exchangers) can also produce carbon monoxide, which is odorless and deadly. Always install CO detectors near sleeping areas and test them monthly. In Portland, CO poisoning is a risk during winter months when furnaces run continuously.
CO symptoms: headache, nausea, confusion, drowsiness. If suspected, evacuate and seek medical attention.
Diagnosing Sulfur Smells (After Gas Leak Ruled Out)
Safe Diagnostic Steps
- 1
Confirm No Gas Leak
Only proceed after NW Natural confirms no gas leak. Never investigate sulfur smells yourself until gas is ruled out. In Portland, gas leaks are taken extremely seriously by utilities.
- 2
Check Furnace Operation
Note when smell occurs - during heating cycles, continuously, or when system is off. This helps identify if it's furnace-related or from ductwork/dead animals.
- 3
Inspect Humidifier (If Applicable)
Check humidifier for mold, slime, or standing water. Clean thoroughly or replace if bacteria growth is suspected. Portland's damp climate can promote bacterial growth in humidifiers.
- 4
Check Condensate System
For high-efficiency furnaces, check condensate trap water level. Dry traps allow sewer gas (with sulfur compounds) into the home. Refill with water if empty.
- 5
Professional Duct Inspection
Have licensed technician inspect ductwork for dead animals or blockages. Professional camera inspection may be needed for hard-to-access areas.