Smell Gas When the Water Heater Lights or Turns On?

The Short Answer: If you smell rotten eggs right after lighting the pilot, or each time the burner comes on, treat it as a gas leak even if the smell fades. Unburned gas from failed lighting attempts, late ignition, or a leak at the control can all cause it. Stop, leave the building, call the gas utility or 911 from outside, and keep the heater off until a gas fitter has checked it.

Why timing matters

A gas water heater holds back gas until it is needed, so a problem may only produce an odor at certain moments:

1. Right after you lit the pilot. Most standing-pilot tank heaters are lit by holding the control knob in a pilot or reset position, which lets a small stream of gas flow to the pilot while you press the igniter. If the pilot does not catch, that gas still enters the combustion chamber and the room, and several attempts in a row add up. Gas utilities add a sulfur-smelling odorant (mercaptan) so you notice it.
2. Delayed ignition at the main burner. When you run a shower or the tank cools, the control opens and gas flows for a moment before it lights. You may hear a soft whoosh or a louder thump and smell gas briefly. This is a combustion fault, not a nuisance.
3. A leak that appears with flow. A fitting or the control can seep only when gas is moving downstream of the valve, so the room smells when the burner runs and clears when it stops. A leak at the control or its connections can also show up while you are handling the knob to light it.
4. Exhaust spilling into the room. A blocked flue, a disconnected vent, or a failed power-vent blower can push combustion products into the space. Exhaust usually smells sharp or acrid rather than of rotten eggs, and it can carry carbon monoxide.

You cannot reliably separate these by smell, so the response is the same: leave and call.

The lighting label on State and A. O. Smith-family heaters, for example, tells you to turn the control off and wait 10 minutes to clear accumulated gas before lighting, and to stop and follow the “if you smell gas” steps if the odor is still there. That instruction exists because repeated tries in a gas-filled space can end in a flash or bang. The wait is for a normal lighting attempt with no odor; once you have smelled gas, do not try again yourself.

What you noticed, and what it points to

What not to do

• Do not “air it out” and try again. Open doors on your way out if it costs you no time, but do not stay inside to ventilate.
• Do not use soapy water or a lighter to look for a leak while the space smells of gas.
• Do not loosen or tighten the union, flex connector, or control to stop the smell. Tightening a leaking fitting under pressure can make it worse.
• Do not “bleed” air from the line at the heater. Clearing air after a gas shut-off is fitter’s work; see why you should not purge the gas line yourself.
• Do not keep using hot water until the plumber comes. Each call for heat repeats the event.

Smells that are not a fuel leak

Rotten-egg odor that appears only when hot water runs, with no smell in the room, usually comes from hydrogen sulfide produced in the tank (often linked to the anode rod and the water supply). That is a water-quality problem, not this emergency. A sharp solvent smell near new pipe joints can come from pipe dope, but you cannot safely tell that from a leak by nose; let the fitter decide.

Once the utility says it is safe

The utility’s first job is to make the area safe, which may include shutting off the meter or the heater’s supply. Owner-level checks are limited, and they all happen after the area is cleared:

• Note what you heard and when (whoosh, bang, hiss), whether the smell followed lighting or burner start, and whether the heater is a standing-pilot tank, an electronic-ignition tank, a power-vent tank, or a tankless unit.
• Look from a distance for obvious problems: a vent pipe that has separated from the draft hood, a crushed flex connector, gasoline or paint stored nearby, or lint built up at the bottom air intake.
• Photograph any status light or error code before power is cycled.

Leave the gas valve, connectors, and burner door alone.

What the gas fitter will test

A static leak test checks the piping with the burner off. If your symptom appears only during firing, the fitter will also observe the heater while it runs. That visit usually includes:

• Leak-checking the supply piping, inlet union, connector, and control body with an approved detector or solution, including while gas is flowing to the burner.
• Measuring inlet and manifold gas pressure against the rating plate. Pressure that is too low or too high can cause late or unstable ignition.
• Inspecting the burner, pilot assembly, thermocouple or flame sensor, and igniter for soot, rust, or debris that delays light-off or stops the pilot from lighting or staying lit.
• Checking the combustion chamber, flame arrestor, and air-intake screen on flammable-vapor-resistant (FVIR) heaters for lint or damage that can make lighting difficult.
• Checking draft at the hood or proving the power-vent blower, and testing for carbon monoxide around the heater.

If the cause was simply several failed pilot attempts, the fix may be a new thermocouple or pilot assembly, or cleaning a blocked intake. A leaking connector or control is a part replacement. Repeated delayed ignition on an older tank with a rusted burner or a damaged combustion chamber can tip the decision toward replacement, especially past about 10 years old. For a tankless unit, the fitter will also review error history; see tankless exhaust smells like gas for that case. Either way, the fitter should relight the heater and watch it run before it goes back into service.

Frequently Asked Questions

Q: Is a faint gas smell normal right after lighting a pilot? A: No. A properly lit pilot burns the gas it receives. A rotten-egg odor in the air means unburned gas is present. A brief hot-metal smell from a new burner is different and has no sulfur note.

Q: The pilot lit on the fourth try and the smell faded. Is it fine now? A: Have it checked anyway. A pilot that takes many attempts often has a dirty pilot orifice, a weak igniter, or a thermocouple problem, and it will likely give trouble again.

Q: The smell goes away a minute after the burner lights. Can I keep using hot water? A: No. Each call for heat repeats the event. Keep the heater off, or have the utility shut it off, until it has been tested while firing.

Q: Could the smell be from the exhaust rather than a leak? A: Yes, especially if the flue is blocked or the vent has come apart. Exhaust spillage can include carbon monoxide, which has no smell at all. If a CO alarm sounds or anyone has a headache or nausea, leave and call 911.