A rotten-egg or sulfur smell near your furnace, gas lines, or vents indicates a natural gas leak. This is an emergency. Natural gas is odorless — utilities add mercaptan (which smells like rotten eggs) specifically so you can detect leaks. Follow the emergency steps below immediately. HVAC diagnostic comes second.
Most Likely Causes
1. Leaking gas valve, gas line, or fitting
Age, vibration, or corrosion can loosen gas line fittings at the furnace inlet, the gas valve, or the flexible connector. Any of these leaking releases gas into the furnace closet.
2. Cracked heat exchanger with combustion venting into the home
A cracked heat exchanger can allow combustion products — including unburned gas and carbon monoxide — to leak into the supply air stream. You may notice the smell only when the furnace is running. This one requires a flue gas analyzer to confirm and is a life-safety issue.
3. Pilot light out with gas valve stuck open (older systems)
On older standing-pilot systems, if the gas valve safety failed and the pilot has been out for a while, gas can accumulate slowly.
4. Blocked or backdrafting flue
If the flue is blocked (bird nest, ice buildup, collapsed section) or backdrafting, combustion products including unburned gas can spill into the space instead of venting up the chimney.
5. Dead animal in ductwork or furnace cabinet
Occasionally what smells like gas is actually a decomposing rodent in the ductwork or furnace cabinet. The odor is similar but not identical. Doesn't matter — evacuate and call anyway, then let us diagnose.
Try These Steps Before You Call
- 1. Leave the house immediately. Do not use light switches, phones, or anything that could produce a spark. Do not use a garage door opener. Take everyone (including pets) outside.
- 2. Do not attempt to shut off the gas yourself unless the shutoff is safely accessible from outside. Gas company technicians are trained for this and have detection equipment.
- 3. Call your gas utility from a neighbor's house or from your cell phone once outside. Atmos Energy (most of DFW): 866-322-8667. CoServ Gas: 940-321-7800. They dispatch emergency response for free.
- 4. Do not re-enter until the gas company says it's safe.
- 5. Once cleared, call us to diagnose and repair the underlying HVAC problem. The utility will make it safe; we'll fix the actual leak or component.
- If a carbon monoxide alarm has also activated, treat this as a combined CO + gas emergency — same evacuation, gas company first, then EMS if anyone feels dizzy, nauseated, or confused.
Still not working?
After the gas utility clears the immediate hazard, we handle the HVAC repair — gas valve replacement, heat exchanger inspection, flue diagnosis. Same-day priority. Call (214) 466-6465 once you're safely outside.
Call (214) 466-6465