Furnace Smells Like Gas? What to Do First, and When It's Something Else

Last updated: November 2, 2026 · Published by Colton Air Conditioning, NV license 0092120 If you smell rotten eggs or sulfur, stop reading and get out. Leave and call Southwest Gas or 911 from outside. Don't flip light switches or touch appliance controls on the way out, don't use the garage door opener, and don't try to relight the furnace. Leave doors open behind you. Wait outside until emergency responders say it is safe to go back in. Short answer: Natural gas has no smell of its own. Southwest Gas adds an odorant called mercaptan so a leak smells like rotten eggs, and it says to treat that smell as a leak even when it is faint or only lasts a moment. A dusty, burning smell the first time the heat runs each fall is different and normal. Carbon monoxide is different again: it has no smell at all.

Gas or dust? Two different smells

Furnace smells like gas when turned on

A rotten-egg smell that shows up as the furnace starts is a reason to leave, not to wait and see. Southwest Gas's rule covers a smell that is "faint or momentary," so a brief whiff at start-up counts too. Don't run the heat again to check whether it comes back. When we look at a furnace after a start-up gas smell, we check the gas connections at the furnace, the gas valve, and whether the burners light cleanly and on time.

Furnace smells like gas when first turned on for the season

The first heat of the fall is when most people notice a smell, and most of the time it is dust. A summer's worth of dust settles on the burners and heat exchanger, and it burns off the first few times the heat runs. That smells like toasted dust, not rotten eggs, and it fades with use. If the first-heat smell is sulfur or rotten eggs instead, it is not the dust smell. Leave and call Southwest Gas or 911 from outside.

Furnace smells like gas when off

A gas smell with the furnace idle still means leave. Gas piping and other gas appliances can leak whether the heat is running or not, so a quiet furnace doesn't rule anything out. Get out first. Southwest Gas finds the source.

Smells like gas but no leak was found

Sometimes Southwest Gas or the fire department checks and doesn't find a leak, and the smell comes back later. Treat every return the same way: leave and call Southwest Gas or 911 from outside. Once responders clear the house, have a licensed HVAC technician inspect the furnace itself, including its gas connection, valve, burners and venting, and tell us exactly when you notice the smell.

Smells like gas outside the house

Southwest Gas lists the outdoor signs: a rotten-egg smell, hissing from the ground or from aboveground piping, dirt or water blowing into the air, plants or grass dying for no clear reason, and standing water that keeps bubbling. Move away from the area and call 911 and Southwest Gas from somewhere safe. Southwest Gas also notes that the buried gas piping between your meter and your house, including lines to outdoor gas appliances, belongs to the homeowner, not the utility. A leak on that section is a homeowner repair once Southwest Gas has made the area safe.

Gas smell is not the same as carbon monoxide

The U.S. Consumer Product Safety Commission describes carbon monoxide as colorless and odorless, so you cannot smell it. A gas smell does not tell you about CO, and the absence of a smell does not mean the air is safe. CPSC lists sooting on an appliance or vent, a furnace that runs constantly without heating the house, and moisture inside windows as signs that a fuel-burning appliance may not be working properly. For symptoms, alarms and prevention, see carbon monoxide from a furnace.

What not to do

• Don't relight the furnace or its pilot after a gas smell.
• Don't turn switches, appliance controls or the garage door opener on or off.
• Don't go back in until responders clear the house.
• Don't try to find or fix the leak yourself.

After the house is cleared

Once Southwest Gas has made the house safe, we inspect the furnace and tell you what we found and what it takes to fix it. For heating work, see heating repair. Book online or call (702) 738-6976 to schedule.

FAQ

My furnace smells like gas when it turns on. Is that normal?

No. A rotten-egg smell at start-up is not normal, even if it is faint or quick. Leave and call Southwest Gas or 911 from outside. A dusty smell on the first heat of the season is the normal one.

Why does my furnace smell like gas when it's off?

A leak doesn't need the furnace running. Gas piping and other gas appliances can leak at any time. Leave and call Southwest Gas or 911 from outside.

It smells like gas but no leak was found. What now?

Treat each return the same way: leave and call Southwest Gas or 911 from outside. Once the house is cleared, have a licensed HVAC technician inspect the furnace's gas connection, valve, burners and venting.

Is the smell the first time I turn on the heat gas?

Usually not. A toasty, dusty smell on the first heat of the fall is dust burning off and fades with use. A sulfur or rotten-egg smell is different. Leave and call Southwest Gas or 911 from outside.

Can I smell carbon monoxide?

No. The CPSC describes carbon monoxide as colorless and odorless. If a CO alarm sounds, leave and call 911 from outside.