The Gas Smell From Furnace You Shouldn't Ignore!

🛑 If you smell gas or suspect a leak, leave the building immediately. From a safe location away from the building, call 911 and your gas utility’s emergency line.

🛑 Do not wait for a stronger smell or try to diagnose the furnace. Leave immediately and make emergency calls from a safe location.

Key takeaways

• Leave first: A suspected gas leak needs immediate evacuation, even if the odor is faint.
• 🛑 Avoid sparks: Leave switches, thermostats, appliances and gas valves alone. Do not use a phone in the affected area.
• 🛑 Call from safety: Contact 911 and your gas utility’s emergency line once outside and away.
• 🛑 Stay out: Return only when emergency responders or the utility say it is safe.

Why does natural gas smell?

Natural gas has no odor of its own. Utilities add an odorant that produces a sulfur-like or rotten-egg smell to help people notice a leak.

🛑 Do not assume a faint or brief odor is harmless. Smell alone cannot confirm or rule out a leak.

What to do if you suspect a gas leak

🛑 You do not need to confirm a leak before leaving. If you smell gas or suspect a leak:

1. 🛑 Get everyone out immediately. Do not delay to investigate, open windows or collect belongings.
2. 🛑 Do not turn lights, thermostats, furnace controls or appliances on or off. Do not unplug equipment, smoke, light a flame or start a vehicle nearby. Do not attempt to operate gas valves or repair the leak.
3. 🛑 Move away from the building. Use a phone only from a safe location, then call 911 and your gas utility’s emergency number.
4. 🛑 Stay outside until emergency responders or the gas utility permit re-entry. Leave repairs and restarting equipment to qualified personnel.

These steps follow PSE&G’s gas-leak guidance. Use your own utility’s emergency number.

The following background is for after the emergency has been resolved, not for troubleshooting a suspected leak.

Cracked heat exchanger

The heat exchanger in your furnace is responsible for heating the air in your home.

A gas furnace burns natural gas in its combustion chamber. The heat exchanger transfers heat to household air, which then travels through the ducts to warm your rooms.

Heat exchangers can become cracked from overheating, causing the combustion gases to mix with the heated air and flow into the rest of your home.

Cracked heat exchangers are dangerous as they can also release carbon monoxide into your home. If you suspect a cracked heat exchanger, seek help from an HVAC technician before using your furnace again.

🛑 These warning signs need professional assessment; they do not confirm a cracked heat exchanger. Do not open the furnace to look for them:

• Soot accumulating inside or on your furnace
  
• Yellow burner flames instead of blue ones
  
• Obvious cracks and corrosion
  
• Strange smells
  
• Audible sounds such as rattling, popping and banging noises
  
• Family members have flu-like symptoms from CO poisoning.

Backdrafting furnace

🛑 Backdrafting can occur in susceptible furnaces, including older models without sealed combustion. Combustion gases flow into the home instead of venting outdoors through the flue. A blocked flue or venting problem needs professional assessment. Do not stay inside to investigate a suspected gas leak or carbon monoxide exposure.

Exhaust fans can lower indoor air pressure and contribute to backdrafting in susceptible combustion equipment. Have a qualified professional assess the venting, ventilation and combustion-air supply after any emergency has been resolved.

Gas pipe or valve leak

Gas pipes, connections and valves can leak because of damage, age or installation problems. A gas odor or hissing sound can be a warning sign.

🛑 Follow the evacuation steps above. Do not search for the source, test a connection or try to shut off a valve yourself during a suspected leak.

Do not dismiss an uncertain smell

🛑 Dust odors and gas odors are different concerns. Do not dismiss a sulfur-like or rotten-egg smell as normal furnace startup, even if it goes away.

🛑 If you cannot tell what the odor is, treat it as a suspected gas leak and follow the evacuation steps above. Do not stay inside to distinguish the source.

Carbon monoxide is a separate hazard

Carbon monoxide (CO) is a separate, odorless hazard. A standard CO alarm does not detect natural gas; check which gases a combination device is designed to detect. A silent CO alarm does not mean a suspected gas leak is safe.

🛑 If a CO alarm sounds or you suspect CO poisoning, get everyone outside into fresh air immediately and call 911 from safety. Do not re-enter until responders say it is safe.

The U.S. Consumer Product Safety Commission recommends CO alarms on every level and outside sleeping areas. Follow alarm installation instructions and local requirements.

Arrange maintenance after the emergency

Routine furnace service is preventive care, not the first response to a gas odor. After emergency responders or the utility have made the situation safe, arrange necessary repairs with a qualified HVAC professional.

Have your furnace serviced annually and follow its manufacturer’s maintenance instructions.

Conclusion

🛑 Suspected gas leak: leave immediately, avoid ignition sources, call from safety and wait for clearance. Do not wait for an ordinary furnace appointment.

Safety guidance corrected October 28, 2026, using the utility and CPSC sources linked above.

After the emergency has been resolved, review PG&E’s gas-safety information and follow your own utility’s instructions. Continue with our HVAC guides for related homeowner decisions.

Scott Harding

Scott is the main author of DIY Home Comfort. Learn more about the team and our approach on the About page.