What is a pilot light?
Updated August 2026
A pilot light is a small gas flame that burns around the clock so the main burners have something to ignite from the moment the thermostat calls. Its own thin supply tube and brass orifice feed it roughly 600 to 1,000 BTU per hour, which adds a few dollars a month to the gas bill. Standing pilots were standard on furnaces, water heaters, and gas fireplaces into the mid-1990s, when hot-surface and spark igniters took over on new equipment.
Where it is
Look low, behind a panel. On a gas water heater the pilot sits behind a small removable cover at the very bottom of the tank, usually with a sight glass so you can see the flame without opening anything. On an older furnace it is inside the burner compartment at the end of the burner rack. On a fireplace it hides behind the lower louvered grille, near the gas key. Every one of these appliances carries a printed lighting label, normally inside the access door, with the exact sequence and wait times for that model. Follow that label rather than a video.
What it does
The flame does more than light burners. It heats a thermocouple, a probe that generates about 20 to 30 millivolts while hot, and that tiny current is the only thing holding the safety gas valve open. Blow the pilot out and the voltage collapses within 30 seconds, the valve snaps shut, and gas stops flowing. That is why a pilot that will not stay lit is almost always a thermocouple problem rather than a gas supply problem: a sooty or bent probe, or one sitting just outside the flame. A yellow, lazy, drifting flame instead of a crisp blue cone means the orifice is partly plugged with lint or spider web, common on water heaters in dusty garages.
Maintenance
Relighting is a do-it-yourself job wherever the label allows it, and it is worth doing each fall on a fireplace you shut down for summer. If a pilot drops out more than once in a season, stop relighting and get it serviced, because repeated outages point to a failing thermocouple, a drafty vent, or a fouled burner assembly. Fold the check into the pre-season routine in testing your furnace before winter, and keep the burners on the schedule in how often to clean furnace burners.
What it costs to replace
A replacement pilot light runs A thermocouple is $10–25 as a part; a service call to clean or rebuild a pilot assembly runs $75–200.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
Why does my pilot light keep going out?
Rank the causes by odds. A worn or sooty thermocouple explains most cases and is a $10 to $25 part. Next is a partly blocked pilot orifice, which starves the flame so it never fully heats the probe. Then drafts: a door that slams, a nearby exhaust fan, or a downdraft in the vent. Last and least likely is a failing gas valve, a $150 to $400 repair and often the point where an old appliance stops being worth saving.
Should I shut the pilot off for the summer?
On a gas fireplace, yes. It saves a few dollars a month, keeps the glass from hazing over, and removes a warm spot from the room in July. On a water heater, no, since you need hot water year round and the control stays on its pilot setting anyway. On a furnace with a summer or pilot position, shutting it down is fine, but write yourself a September note to relight it rather than discovering it during the first cold snap.
I smell gas near the appliance. What should I do?
Do not try to light anything. If the smell is strong, leave the house and call the gas utility from outside; utilities answer odor calls at no charge, day or night. If it is faint and appears only right after a failed relight attempt, air the space out for at least 5 minutes with a window open before trying again, exactly as the appliance label instructs. Never hunt for the source with a lighter, and never live with a smell that keeps coming back.
Do new furnaces still have pilot lights?
Almost none do. Equipment built since the mid-1990s uses a hot-surface igniter, a brittle silicon nitride element that glows orange for 20 to 40 seconds, or a spark igniter on many boilers and fireplaces. The gain is no standing flame burning fuel all summer. The trade-off is a wear part: hot-surface igniters typically last 4 to 7 years and run $40 to $90 for the part, plus a flame sensor that needs occasional cleaning to keep proving the flame.