Pilot Lit but Main Burner Never Opens

The Short Answer: The pilot is on. The main burner never opens — no whoosh, no ring of flame, tank stays cold. That is the knob left on PILOT, the temp dial on Vacation, a thermopile too weak to pull the main operator, an ECO, or a gas control that will not pass main gas. It is not lights then dies. Smell gas, leave.

Search Intent & The Real Problem

You can see the little blue flame through the port. You are waiting for the big one. It never comes. Forums sold you a thermocouple. If the pilot is still there after you let go, that thermocouple already did its job. The main needs a call from the thermostat, enough millivolts to open the operator on a powerpile valve, a gas path past the pilot tee, and an ECO that is still closed. A closed hot-water valve with a roaring main is the burner-on, no-hot-water page. This page is no main flame at all.

What other guides get wrong

They sandpaper the thermocouple. A holding pilot proved that circuit. They skip the gas selector still on PILOT and the temperature dial on Vacation — I fix more of those with a flashlight than with a millivolt meter. They confuse “never opens” with “opens then drops.” And they tell you to crack a union because “the main isn’t getting gas,” which is how closets fill.

Step-by-Step Troubleshooting

Step 1: Split never-opens from opens-then-dies from heat-not-leaving

• What to look for: A live pilot after you let go, then no main ring. A 1” pilot is not “the burner on.”
• Required tools: Flashlight, photo of the valve face and rating plate.

Step 2: Gas knob ON, temperature not on Vacation

• What to look for: Selector on ON, not left on PILOT after lighting. Temperature dial off Vacation / lowest stop — typically in the Hot band (~120°F mark, not “Very Hot” as a test). Tank already at setpoint (upper jacket hot) will not open the main until you draw water. Wait 10–15 minutes after a failed recovery before you condemn the valve.
• Required tools: Eyes on both knobs. Contact thermometer on the hot outlet nipple if you think it already heated.

I light more “dead” heaters by rotating PILOT to ON than by replacing generators. Do not crank to 140°F to “make it fire.” That scalds.

Step 3: Thermopile millivolts — enough for pilot, not enough for main

• What to look for: Powerpile valve. Pilot solenoid will often hold on a weak pile; the main operator needs more. Open-circuit often 500–750 mV. Closed-circuit with a call, still below that valve’s pull-in (commonly needs ~250 mV+ under load, dropout around 180 mV — confirm the spec) = main never opens, pilot looks perfect. Lazy yellow pilot will not drive a pile. Aftermarket 30 mV thermocouple on a millivolt valve is the classic wrong part — after a thermocouple swap.
• Required tools: Millivolt meter if you are qualified. Eyes on a sharp blue flame wrapping the generator.

Thermocouple-only valves open the main hydraulically. If that main never opens with the knob on ON and the dial up, you are past the sensor: thermostat in the control, or the valve.

Step 4: ECO, FVIR, rollout — safeties that kill the main only

• What to look for: ECO / reset on the combination control (tank overheated, dry-fire, failed limit). Flammable-vapor / FVIR lockout on electronic controls — status LED, not a standing-pilot guess. Rollout switch on the combustion door, soot, scorched paint. Packed arrestor that will feed a pilot and will not feed a main. Tissue at the draft hood should pull in if you ever get a main; if a previous attempt spilled, shut it down.
• Required tools: Eyes on the reset and the door. Do not jumper the ECO. Do not pull an FVIR door if you cannot reseal it.

A tripped ECO with a live pilot is common on Unitrol-style valves: the magnet still holds, the main stay shut. Reset once if the jacket procedure says so, find why it opened, and stop if it opens again.

Step 5: Valve, and a cock after the pilot tee

• What to look for: Knob ON, dial calling, millivolts above pull-in, ECO closed, arrestor clear — still no main gas at the burner ports. Internal main operator failed. Less common on residential tanks, more on millivolt boilers and some commercial water heaters: a manual cock in the main burner line after the pilot tap left closed from a service call — pilot has gas, main does not. Trace the pipe with your eyes. Do not open a mystery valve that dumps into a disconnected burner.
• Required tools: Eyes on the piping. Licensed manometer on the manifold (typically 3.5–7 in. w.c. NG / 10–11 in. w.c. LP — confirm the plate). No union-cracking.

Correct millivolts and a main that still will not open is the gas valve. Match millivolt vs thermocouple vs electronic, gas type, FVIR.

When DIY Ends: Calling a Licensed Pro

Stop at gas odor, bangs, a scorched door, millivolt numbers you cannot take, ECO loops, and any valve replacement. You may put the selector on ON, move the dial off Vacation, and look for a cock in the main line. You may not jumper the thermopile, hold the knob in, or loosen fittings. A licensed plumber will measure millivolts under a heat call, manifold pressure, and leak-test a control if that is the part. A $12 sensor on a main that never tried to open is the wrong invoice.

Frequently Asked Questions

Q: Why is my water heater pilot light lit but the burners will not ignite? A: The gas knob is still on PILOT, the temp dial is on Vacation, the thermopile cannot pull the main operator, an ECO or FVIR safety is open, the main operator failed, or a cock after the pilot tee is closed. A holding pilot is not a holding main.

Q: Should I replace the thermocouple if the pilot is lit and the main will not start? A: Not first. A holding pilot already proved the thermocouple path. On millivolt valves you may need a thermopile, not another 30 mV part. Prove the knobs, then millivolts.

Q: The pilot is on. Does that mean the gas valve is good? A: It means the pilot solenoid or magnet is holding. The main operator is a separate function. It can fail closed with a perfect little flame.