Gas smell from furnace: what to do right now
Natural gas has no smell on its own. The rotten egg odor you're noticing was added to it deliberately, specifically so a leak would be impossible to miss.
Natural gas is odorless until a smell is added on purpose
Natural gas itself has no smell at all. Utility companies add a chemical called mercaptan specifically so a leak announces itself: the sharp, rotten egg or sulfur smell most people recognize the moment they've smelled it once. That odor is not a byproduct of anything your furnace is doing wrong on its own.
It is a warning built into the fuel supply itself. And it means gas is escaping somewhere along the line, whether that's at the furnace, a connection near it, or further back toward the meter.
One response, and it never changes
If you notice that smell anywhere near the furnace, or anywhere in the house, there is exactly one correct response. Leave immediately, get everyone and any pets out with you, and call your gas utility's emergency line or 911 once you are outside. Do not stop to open a window, check the thermostat, or see how strong the smell is first.
This does not change based on how faint the smell seems, or how many times something similar has happened before. A brief whiff is not a different situation than a strong, constant one. Both mean the same call, made from outside the house.
What a technician usually finds afterward
Once a technician or the utility crew has confirmed it is safe to go back in, the actual source is often something ordinary. It may be a fitting at the furnace's gas connection that was never fully tightened, or a valve that failed to seat properly. Or it may be a line that was disturbed during an earlier repair and never resealed correctly. None of that is something you are expected to identify yourself. There is no safe way to check it while gas may still be present.
This is a different hazard than carbon monoxide
A gas leak and a carbon monoxide problem share the same furnace and the same emergency instinct, but they are not the same thing. Carbon monoxide has no smell, no color, and no taste at all; a working detector is the only way to catch it. A gas leak works the opposite way: the smell is the entire warning system, deliberately added so you would notice it. See our guide to carbon monoxide and your furnace for that separate hazard.
A smoke alarm won't catch this either
A standard smoke detector, even a good one, is built to sense smoke particles, not natural gas. It will not sound for a gas leak, any more than it would for carbon monoxide. Your nose, reacting to that added mercaptan smell, is genuinely the first warning system here. That is exactly why the response has to be immediate rather than a wait and see.
A seasonal furnace tune-up is where a technician actually checks gas connections and fittings before they become a problem, the same way it catches other wear early. That inspection, not a homeowner walking around sniffing for trouble, is the real prevention here.
What not to do
- Don't try to trace the smell back to its source room by room. That is a job for the gas utility and the fire department, not something to investigate on your way out.
- Don't flip light switches, use your phone indoors, or touch the thermostat on your way out. A spark is exactly what you're trying to avoid in a room where gas may be present.
- Don't try to relight a pilot light or turn a gas valve yourself, whether the smell is present now or you think it has cleared. Let a technician confirm that first.
- Don't go back inside until the utility, fire department, or a technician tells you it's safe. A smell that seems to have faded is not the same as a leak that's been found and fixed.
After the call
Wait outside for whoever is responding, and let them do the checking. If a leak is found and fixed, a licensed technician, not you, is also the right person to confirm the repair holds before the furnace runs normally again.
Treating this as a fast phone call rather than a troubleshooting project is the entire point. Every step above exists to keep it that way.
Frequently asked questions
Is a faint gas smell near the furnace less serious than a strong one?
No. Any noticeable gas smell gets the same response: leave the house and call your gas utility or 911 from outside. A faint smell can mean a small leak that is just as real, not a false alarm to wait out.
How is this different from a furnace that smells like it's burning?
A rotten egg or sulfur smell is a gas leak and is never a furnace troubleshooting question. A dusty or hot smell on the first run of the season, or a sharp electrical smell, are furnace issues with different next steps. See our furnace burning smell guide for how to tell those apart.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Furnaces
- CDC. Carbon Monoxide Poisoning Basics