Pilot Dies When You Turn the Temp Dial

The Short Answer: The pilot is lit. You turn the temperature dial up — or you rotate the gas knob off PILOT — and the pilot goes dark. That is millivolt sag when the main operator opens, a Unitrol/Honeywell valve that dumps the magnet, or the main whoosh blowing the flame off the tip. It is not main dropout with the pilot holding. Smell gas, leave.

Search Intent & The Real Problem

You did the lighting card. The little flame held. Then you turned “the heat” up and the whole thing went out. Two knobs get blamed as one: the gas selector (OFF / PILOT / ON) and the temperature dial (Vacation / Warm / Hot). Turning either can kill a marginal magnet. On a millivolt / powerpile valve, calling for heat loads the generator and voltage collapses. On a Robertshaw Unitrol or Honeywell combination control, opening the main can starve the pilot or mechanically unseat a tired magnet. On a lazy pilot, the main whoosh simply blows it out.

If the pilot never held after a new sensor, use after replacing the thermocouple. If you never get a flame, stay lit.

What other guides get wrong

They send you to sandpaper a thermocouple that already held on PILOT. That magnet proved flame until you loaded it. They mix this up with “burner won’t stay lit,” which is pilot still there, main dumps. They call it air in the line on a tank that heated water yesterday. And they tell you to spin the dial back and forth as a test — that is how you dump unburned gas into a FVIR chamber.

Step-by-Step Troubleshooting

Step 1: Name which knob you turned

• What to look for: Two knobs vs one combination control. Photo of the valve face. Standing pilot still visible before you touch anything.
• Required tools: Flashlight, the jacket lighting card.

Many Unitrols: light on PILOT, then rotate to ON, then set temperature. Rotating the temp dial while the gas selector is still on PILOT does nothing useful. Rotating the gas selector before the magnet is hot dumps the flame — people call that “the temp dial.”

Step 2: Millivolt sag when the main operator opens

• What to look for: Thermopile / powerpile valve (thicker generator, often two leads). Open-circuit often 500–750 mV. The number that matters is with the thermostat calling: if it sags under that valve’s dropout (commonly around 180 mV — confirm the spec), the pilot solenoid and the main both dump. That is this symptom. A thermocouple-only valve (20–30 mV) does not electrically drive the main; if that pilot dies when you turn the dial, skip to pressure, whoosh, or a mechanical magnet.
• Required tools: Millivolt meter if you are qualified. Eyes on a sharp blue pilot wrapping the generator tip.

Aftermarket 30 mV thermocouple on a millivolt valve is the classic wrong part — it may limp on PILOT and collapse the instant you call for heat.

Step 3: Main-open pressure sag and the whoosh

• What to look for: Pilot holds on PILOT. You turn the dial, the main tries to light, the pilot winks out in 2–15 seconds. Other burners in the house dip at the same moment. Undersized or kinked 1/2” flex on a high-BTU tank. LP bottles low or a winter regulator — cold-weather dropout. Delayed bang then dark: that whoosh blew the pilot off the tip. Soft, yellow main then both die: starving.
• Required tools: None on the fittings. Manometer work is licensed (typically 3.5–7 in. w.c. natural gas / 10–11 in. w.c. LP at the appliance — confirm the rating plate).

Do not crack the union to “give it more gas.” Mercaptan in the room is leave, then the utility.

Step 4: Unitrol / Honeywell mechanical — the dial itself

• What to look for: Robertshaw 110-series Unitrol, Honeywell VS / combination water-heater controls — Unitrol valve diagnosis. Worn detent between PILOT and ON so the disc dumps the magnet when you rotate. Temperature rod that binds and unseats a tired pilot magnet when you crank from Vacation toward Hot. ECO / reset button on the valve that opens as soon as the main fires into an already-hot tank. Jacket too hot at the top.
• Required tools: Eyes on the valve face and the reset. Do not tap the valve with a wrench “to seat it.”

If millivolts stay above dropout with the main called and the pilot still dies when the dial moves, the valve is mechanical garbage. Replace the control — licensed, leak-tested — not the dial felt.

Step 5: Draft, FVIR, and rollout that only show under load

• What to look for: Pilot fine until the main opens, then spillage at the draft hood, soot on the door, or a rollout switch. Packed FVIR arrestor that will feed a pilot and will not feed a main. Tissue should pull in with the burner on; if it spills, shut it down. Spillage is not white smoke you ignore.
• Required tools: Tissue, flashlight. Gas off to inspect the arrestor from outside. Do not pull an FVIR door if you cannot reseal it.

When DIY Ends: Calling a Licensed Pro

Stop at gas odor, delayed-ignition bangs, spillage, millivolt numbers you cannot take, and any valve replacement. You may follow the jacket once, confirm which knob you turned, and leave the control on PILOT if that is the only position that holds. You may not hold the knob in, jumper the generator, or loosen fittings. A licensed plumber will measure millivolts under load, manifold pressure, and only then swap a Unitrol or Honeywell — then leak-test every joint.

Frequently Asked Questions

Q: Why does my water heater pilot go out when I turn the temperature dial? A: The main operator opened and the millivolt circuit sagged, the valve dumped the magnet, gas pressure collapsed when the main opened, or the main whoosh blew the pilot off the tip. A holding flame on PILOT already proved the easy half of the sensor.

Q: Is this the same as the burner lighting then going out? A: No. If the pilot stays and only the main drops, use the burner-won’t-stay-lit page. This page is the pilot dying when you turn the dial.

Q: Can I leave the gas knob on PILOT and skip the temperature dial? A: You will have a tiny flame and no hot water. That is not a workaround. It is a failed control or a starved generator. Hire a licensed plumber.