Direct answer: A rotten egg smell near a furnace should be treated as a gas-leak warning first, not a furnace tune-up issue. Do not keep testing the heat to see if it clears.
Most likely: Most often, this smell is natural gas odorant from a leak in nearby gas piping, a shutoff connection, sediment trap fitting, flex connector, or the furnace gas train area. Less often, it is sewer gas from a floor drain or condensate drain near the furnace room.
If the smell is strong, sudden, or gets worse when the furnace starts, shut the furnace off, leave the area, and call the gas utility or a licensed HVAC or plumbing pro depending on the source. Reality check: a true rotten egg smell around fuel-burning equipment is never a wait-and-see problem. Common wrong move: homeowners often assume it is just a dirty filter or dust burn-off and keep running the furnace.
Don’t start with: Do not start by replacing furnace parts, cycling the thermostat, or sniffing around with a flame or lighter.
The odor is obvious as soon as you enter the area, and it may be strongest near the furnace gas pipe, shutoff valve, or burner compartment.
Start here: Treat this as a gas leak first. Shut the furnace off if you can do it quickly, leave the area, and call the gas utility or a licensed pro.
You notice the odor during ignition or right after the burners try to light, then it fades.
Start here: Turn the thermostat off and stop running heat. This can still be unburned gas or a combustion problem and needs prompt service.
The odor seems stronger at a floor drain, condensate pump, or drain tubing than at the gas valve side of the furnace.
Start here: Check for a dry floor drain or drain trap issue, but keep the furnace off until you are sure the smell is not gas.
The smell comes and goes, especially in a basement or mechanical room with other gas appliances or drains nearby.
Start here: Do a careful source check without operating the furnace. Separate gas-piping odor from drain odor before you assume the furnace itself is bad.
A true rotten egg or sulfur smell is the odorant added to natural gas. The leak may be at the shutoff, union, sediment trap, flex connector, or gas valve area near the furnace.
Quick check: With the furnace off, note whether the smell is strongest right at the gas pipe and gets sharper as you get closer. If yes, stop and call for service.
If the smell shows up when heat is called for, the burners may not be lighting cleanly. You may also hear repeated clicking, a whoosh, or a failed start sequence.
Quick check: Do not keep retrying. If the smell appears during startup, shut the system down and arrange HVAC service.
Basement utility rooms often have floor drains near the furnace. A dry trap can smell a lot like sulfur and gets mistaken for a furnace gas leak.
Quick check: Smell low near the drain opening, not the gas pipe. If the drain is the clear source and there is no gas smell at piping, the trap may be dry.
Water heaters, dryers, and other gas appliances often share the same room. The furnace gets blamed because it is large and noticeable.
Quick check: Without turning anything on, compare the odor near the furnace gas line, water heater gas line, and any nearby drain to narrow the source.
You need to reduce risk before you start sorting out where the smell is coming from.
Next move: The furnace is no longer trying to ignite, and you can make a safer source check if the odor is mild. If the smell remains strong or seems to be building, do not stay and investigate further.
What to conclude: A strong persistent sulfur smell is treated as an active gas-leak situation until proven otherwise.
These two smells get confused all the time, and the next move is completely different depending on which one you have.
Next move: You narrow the source to either gas piping or a drain-related odor. If you cannot clearly separate the source, treat it as gas and call for help.
What to conclude: A drain smell can often be corrected with simple maintenance, but any smell centered on gas piping or ignition needs a pro.
A dry floor drain is one of the few common sulfur-smell causes near a furnace that a homeowner can safely address.
Next move: If the odor fades after refilling the trap, the source was likely sewer gas from a dry drain. If the smell stays the same, stop assuming it is the drain and move back to the gas-leak path.
Startup-only sulfur odor points toward unburned gas, failed ignition, or delayed ignition, and that is not a safe DIY repair path.
Next move: You avoid making the condition worse and give the service tech useful clues. If you still need heat, use a safe alternate heat source only if it is approved for indoor use and kept clear of combustibles.
At this point the safe next move should be clear, and guessing past this point is where homeowners get into trouble.
A good result: You move straight to the right fix instead of chasing the wrong system.
If not: If the source changes or the smell spreads through the house, leave and escalate it as a gas emergency.
What to conclude: The job here is safe source identification and clean escalation, not forcing a furnace repair when the odor may be outside the furnace itself.
No. A dirty furnace filter can cause airflow problems and overheating, but it does not create a true rotten egg or sulfur gas smell. Do not assume this is a filter issue first.
Not always, but you should treat it that way first. A dry floor drain or condensate drain area can smell similar, especially in a basement utility room. The safe move is to shut the furnace off and separate drain odor from gas-pipe odor.
That usually points to unburned gas during ignition, delayed ignition, or another combustion problem. Stop running the furnace and call an HVAC technician rather than retrying it.
For most homeowners, no. Around a furnace, that can turn into risky guesswork fast, especially if the odor is already obvious. If you suspect gas at the furnace piping or valve area, call the gas utility or a licensed pro.
That strongly suggests sewer gas from a dry trap, not a furnace part failure. Keep watching the area. If the smell comes back quickly or you are not fully sure that was the source, call a plumber or treat it as a gas concern until checked.