When a pilot light will not stay lit
What a thermocouple does, why a pilot that dies when you release the knob is not fixed by lighting it again, and which drafts matter.
A standing pilot is a small flame that burns continuously so the main burner can light without a spark or a hot-surface igniter. The flame’s second job is to heat a thermocouple or a thermopile, a dissimilar-metal probe that generates a small electrical voltage when it is hot. That voltage holds an electromagnet in the gas valve open.
Let the probe cool, and the magnet lets go, the pilot gas shuts off, and the flame you just lit dies. The usual complaint is precise: the pilot lights while you hold the knob down, and it goes out the moment you release the knob, or it stays for a few minutes and dies later in the day.
Both patterns mean the valve is not being told to stay open. Lighting it a fifth time does not change the voltage.
If the heater has no standing pilot at all — no small flame to see, only a spark or a glow plug when a tap or a thermostat calls — you are on the wrong page. Use no hot water from a gas tank and the manual’s electronic codes. This page is only the standing flame.
Safety before the screwdriver
Emergency. If you smell gas before you start, or if gas odor builds while you try to light the pilot, stop. Leave the house, do not use electrical switches, and call the gas utility from outside. A pilot procedure is not a leak procedure.
Read the lighting instructions bonded to the heater.
They exist because the knob sequence differs. The common shape is: turn the control to pilot, hold the knob or button to override the magnet, light the pilot with the built-in igniter or a long lighter as the sticker allows, keep holding until the probe is hot, then turn the control to on.
The hold time on the sticker is the hold time. Thirty seconds is common and not universal. If the sticker says wait five minutes after a failed attempt so accumulated gas can clear, wait. Do not stick your face over the viewing window.
Do not use a cigarette lighter in a puddle of gasoline vapor. Flammable-vapor lockout exists because utility rooms collect the wrong cans. Move the cans out before you light anything.
The screen and the lockout are described on the gas no-hot-water page. Do not drill the screen so the pilot “gets more air.”
One careful attempt that follows the sticker is a reasonable homeowner act. A pilot that will not hold after that attempt is a parts and measurement problem.
What the flame should look like
A healthy pilot is a steady blue flame that covers the tip of the thermocouple. A yellow, lazy, or lifting flame does not heat the probe evenly, and it is also evidence of a dirty orifice or of bad combustion air. Look, decide, and do not start bending the pilot tube to “aim it better.”
The tube’s position is set so the flame impinges on the probe. A tube that has been nudged by a vacuum cleaner or by the burner during a cleaning can miss the probe by a few millimeters, which is enough to drop the voltage.
Dust, spider nests, and carbon on the pilot orifice shrink the flame to a speck. Cleaning a pilot orifice is delicate. The orifice is a precise hole. Poking it with a wire from a twist tie enlarges it and the pilot then roars, soots, or will not stay at the right height.
Technicians use the right pick or they replace the pilot assembly. If you cannot see a normal blue flame on the probe, stop after the visual check and call someone.
The combustion-air and vent reasons for a lazy flame are in venting and carbon monoxide. A pilot that blows out only on windy days, or only when the furnace starts, is a draft problem.
Downdraft from a shared flue, a bathroom fan that depressurizes a tight house, or a vent that has come apart will snuff a pilot and will also spill combustion products. That is not a new thermocouple. That is a venting repair.
The thermocouple and the magnet
A thermocouple makes a few tens of millivolts when the flame is correct. The gas valve’s electromagnet needs a minimum of that voltage to stay latched. A technician disconnects the thermocouple lead at the valve, heats the probe with the pilot or a specified method, and reads the open-circuit voltage with a meter.
A weak reading with a strong flame points to the thermocouple or its connection.
A strong reading with a valve that still will not hold points to the magnet in the valve.
The connection itself matters. The thermocouple nut at the valve should be snug and not crushed.
Over-tightening damages the seat and the contact. Under-tightening adds resistance and the magnet drops out. A bent or kinked capillary where someone folded it out of the way can break the internal conductor. Replacing a thermocouple is, on some heaters, a straightforward part.
It is still a gas-control connection. If the access requires removing the burner assembly, you are in licensed territory for practical purposes: you must reseal the gas path, you must not disturb the manifold, and you must check for leaks with an approved method.
When to call a pro puts burner removal and gas-valve replacement on that side of the line.
A homeowner who has replaced thermocouples on this exact model, with the manual open, and who stops if the burner has to come out, is the outer edge of the homeowner column. Everyone else should buy the service call. The part is cheap. The leak is not.
A thermopile looks similar and makes more voltage, because some valves use it to power a millivolt circuit, not only a magnet. The meter test is the same idea and the number you hope to see is different. The manual states the expected range. Do not compare a thermopile reading to a thermocouple number you remember from a different heater.
What else kills a pilot that used to be fine
A gas valve that is failing can produce every pilot symptom and then refuse a new thermocouple. If a new probe, correctly placed in a correct flame, does not hold, the valve is the next suspect.
Valves are replaced as a unit. They are not rebuilt on the kitchen table. Manifold pressure is set to the data plate with a manometer after the new valve is on. Skipping that step is how a heater soots up or underheats.
An overheating high-limit or a spill switch on the draft hood can shut the pilot circuit on heaters that have those switches in series. The switch opens because the flue got hot at a place that should have stayed cooler, which means products of combustion were spilling.
Resetting the switch without fixing the vent puts the heater back into the condition that opened the switch. If you find a manual-reset switch tripped, treat it as a vent problem. Do not tape it closed.
Low gas pressure from a failing regulator, or air in the line after the gas company worked on the street, can shrink the pilot until it will not heat the probe. Other appliances will often misbehave at the same time. That is a utility or a fitter call, not a hardware-store thermocouple.
| What you observe | Likely area | Next step |
|---|---|---|
| Dies the instant you release the knob | Thermocouple not hot, bad connection, or valve magnet | One lighting attempt per the sticker, then a meter test |
| Flame is yellow or tiny | Dirty orifice, poor air, or low pressure | Stop. Do not ream the orifice. Check vent and other appliances. |
| Dies only when wind or the furnace starts | Draft or depressurization | Vent inspection before any new part |
| Holds for days, then out, and the draft hood is sooty | Spill switch or backdraft | Shut the heater off and have the vent repaired |
| New thermocouple, good flame, still will not hold | Gas valve magnet | Replace the valve, set pressure, leak-check. Licensed work. |
After it stays lit
Turn the control to on, set the temperature with a thermometer at the tap rather than by the dial’s folklore, and listen for the main burner. A pilot that stays and a main burner that never comes on is a thermostat or a valve problem, which returns you to the no-hot-water page.
A pilot that stays and a burner that roars with a yellow flame is a combustion problem. Shut it down. Do not leave a heater “mostly working” with a flame that is sooting the flue. Carbon in the flue is future carbon monoxide in the room.
If the tank is old and this is the second gas control in three years, price a replacement heater against another valve. The comparison is repair or replace. A sound ten-year-old atmospheric tank with a bad thermocouple is a repair. A rusting tank with a bad valve is a replacement that happens to have revealed itself at the pilot.
Sources
Manufacturer lighting instructions printed on the appliance and in the installation manual (Bradford White, bradfordwhite.com; A. O. Smith, hotwater.com; Rheem, rheem.com). U.S. Consumer Product Safety Commission carbon monoxide guidance, applied through the venting guide. ANSI Z21.10.1 appliance standard context for residential gas water heaters, including flammable-vapor resistance on modern tanks. The expected millivolt range is whatever the valve manufacturer prints for that control, measured by a technician, not a single number borrowed from a different model.