Why Won't My Gas Water Heater Stay Lit?
The Short Answer: A gas water heater that lights then dies is a weak thermocouple or thermopile, a dirty or starving pilot, a draft or clogged FVIR screen, or a gas valve that will not hold. If you smell gas, leave. Do not crack fittings to “bleed air” on a heater that ran for years. Read the lighting card, then hire a licensed plumber.
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Search Intent & The Real Problem
You held the knob on PILOT, saw a flame, released it, and it went dark. Or the main burner whooshed on and dropped after ten seconds. The internet sold you a $12 thermocouple and a sewing needle for the orifice. Sometimes that is the part. Sometimes you just bought a thermocouple for a millivolt valve that needed a thermopile, or you ignored a lint-packed FVIR screen, or you have a downdraft chimney. Brand-specific valves have LED charts — Richmond gas valve, Rheem pilot, Whirlpool pilot, flashing green light.
Electronic DSI and hot-surface tanks have no standing pilot. “Will not stay lit” on those is flame failure / lockout, not a thermocouple.
What other guides get wrong
They lead with “air in the gas line” on a tank that heated water yesterday. Air is an install/relight problem — do not purge it yourself. They also mix thermocouple (20–30 mV) with thermopile (hundreds of millivolts) and tell you to poke the orifice as step one while the gas is on. That is how you get a face full of unburned fuel.
Step-by-Step Troubleshooting
Step 1: Name the ignition system
| What you have | “Will not stay lit” usually means |
|---|---|
| Standing pilot, two-wire thermocouple | Pilot dies when you release the knob |
| Millivolt / powerpile (thermopile) | Pilot holds, main burner drops, or both drop under load |
| Electronic control with a status LED | Lockout code — count flashes, do not guess |
| Hot-surface or DSI, no pilot | Flame failure; not a thermocouple |
- Required tools: Flashlight, the jacket lighting instructions, phone photo of the gas valve.
Richmond, Ruud, and Rheem share controls. Bradford White ICON does not. Wrong part, no heat.
Step 2: Relight only per the jacket — once
- What to look for: Knob to OFF, wait five minutes with the area clear, then PILOT per the card. Pilot should wrap the sensor tip with a sharp blue flame. If it will not light at all, the supply, the orifice, or the valve is shut — not a “dirty thermocouple.”
- Required tools: The manufacturer’s lighting card. A long lighter only if that card says so. No matches in a rollout.
If it bangs, yellow-tips, or you smell gas, shut the appliance cock. That is not a second attempt.
Step 3: Draft, FVIR, and the closet — before parts
- What to look for: Pilot that dies when a bathroom fan, range hood, or dryer starts (negative pressure stealing the chimney). Wind down an uncapped vent. Lint packed in the FVIR flame-arrestor at the bottom of post-2003 natural-draft tanks. Soot on the combustion door. A heater shoved into a sealed closet with no combustion air.
- Required tools: Eyes. Tissue at the draft hood (it should pull in, not spill out) with the burner on — if it spills, shut it down.
A perfect thermocouple cannot hold a pilot that the chimney is blowing out.
Step 4: Thermocouple vs thermopile vs valve
- What to look for: Pilot dies the instant the knob comes off PILOT = thermocouple not generating, tip not in the flame, or the valve’s pilot solenoid is dead. Pilot holds, main burner opens then the whole thing drops = thermopile sagging under load, or the valve’s main operator is failing. Correct millivolts with a burner that still will not hold = valve.
- Required tools: Millivolt meter if you are qualified. A healthy thermocouple is typically about 20–30 mV open-circuit. A thermopile is hundreds of millivolts; it must still be above the valve’s dropout when the main is open.
Replacing a thermocouple is still a gas-fitting job in many cities. If you are not going to leak-test the joint, do not break the joint.
Step 5: Gas supply without opening the pipe
- What to look for: Appliance shutoff handle parallel to the pipe (open). Other gas appliances lighting. LP bottles empty. A 1/2” kinked flex on a high-BTU tank. Sediment trap missing on a recent swap. Utility red tag.
- Required tools: None on the fittings. Manometer work is licensed (typically about 3.5–7 in. w.c. natural gas / 10–11 in. w.c. LP at the appliance — confirm the rating plate).
A starving burner lights, then dies, and looks exactly like a bad thermocouple. It is not. Do not crack the union to “check for gas.”
When DIY Ends: Calling a Licensed Pro
Stop at gas odor, delayed ignition bangs, spillage at the draft hood, FVIR chamber service, millivolt diagnosis you cannot complete, and any valve replacement. You may follow the jacket relight once, look for lint at the arrestor, and confirm the shutoff position. You may not bypass the thermocouple, hold the knob in, or loosen fittings. A licensed plumber will measure millivolts and manifold pressure, clear or replace FVIR components, and leak-test the control. That is the job. A $12 sensor on a $200 valve you just contaminated is how this becomes a second visit.
Frequently Asked Questions
Q: Why won’t my gas water heater stay lit? A: Weak thermocouple or thermopile, dirty or small pilot, draft/FVIR, low gas pressure, or a gas valve that will not hold. Smell of gas means leave, not another relight.
Q: Why does the flame keep going out on my gas water heater? A: The sensor cooled, the chimney spilled, the FVIR screen choked the burner, or the valve dropped out. A flame that dies when other exhaust fans start is draft, not a random part.
Q: Can I fix a water heater that will not stay lit myself? A: You can relight once per the jacket and look for obvious draft or an empty LP bottle. Thermocouple, thermopile, and gas-valve work is licensed fuel-gas work if you cannot leak-test. Hire it.