Short definition
A pilot light is the small standing flame at the burner of a gas water heater (or older gas furnace). It heats a thermocouple that generates about 25 millivolts when hot, and that voltage is what holds the gas valve open. If the pilot goes out, the thermocouple cools, voltage drops, the valve closes, and the burner can’t fire.
What it is
Standing pilots are the legacy ignition method on gas water heaters. The pilot tube is a small gas line with an orifice that delivers a thin stream of fuel to the pilot flame. The flame’s tip touches the thermocouple — a bimetallic junction that produces a tiny voltage when heated. That voltage feeds the gas-control valve and authorizes main burner operation.
Newer gas water heaters have largely replaced standing pilots with hot-surface ignition (HSI — a ceramic igniter that glows hot and lights the burner directly) or piezo electronic ignition (a spring-loaded sparker). But many WA homes built before about 2010 still have standing pilots, and they continue to work fine for decades when maintained.
A standing pilot consumes 600–1,000 BTU/hr continuously — small but measurable. Over a year, that’s a few dollars on the gas bill, and it’s the main reason new construction has moved to electronic ignition.
Why it matters to a homeowner
If your gas water heater has stopped making hot water, a dead pilot is the most common cause. The diagnostic ladder:
- Open the access panel at the bottom of the heater. If you don’t see a flame, the pilot is out.
- Follow the relight instructions printed on a sticker on the heater. They typically say: turn the gas knob to “off,” wait 5 minutes, then to “pilot,” hold down the pilot button, light the pilot through the access port, hold the button for 60+ seconds, release.
- If the pilot stays lit and the burner fires when you turn to “on,” you’re done.
- If the pilot won’t stay lit after 60 seconds of holding, the thermocouple has failed. Replace ($10–$30 part, 30 minutes of work).
The other common pilot complaint: “it lights but the flame is yellow and wandering.” That’s almost always a spider web in the pilot tube. Disassemble, blow out the orifice with compressed air, reassemble — usually fixes it.
When you’ll encounter this term
- After a windstorm or earthquake, the pilot has gone out and you’re trying to relight it.
- Your gas heater has stopped making hot water and you’re working through the relight procedure.
- A plumber’s quote on a no-hot-water call lists “thermocouple replacement” as the fix.
- An older heater quote pitches “electronic ignition” as a more efficient upgrade.
Common variants and what a pilot light is not
- Standing pilot vs. intermittent pilot. Standing burns 24/7. Intermittent fires the pilot only when the burner is called.
- Pilot vs. HSI. HSI = hot-surface ignition; ceramic igniter glows orange and lights the main burner directly. No pilot at all.
- Pilot vs. piezo. Piezo is a spring-loaded sparker; you push the button to spark the pilot. Common on older units.
Common failure modes
- Pilot goes out repeatedly. Thermocouple worn or disconnected. Replace.
- Lights but won’t stay on after releasing the button. Thermocouple not generating enough millivolts. Replace.
- Yellow / wandering flame. Spider web in pilot tube (extremely common in WA crawlspaces). Blow out or replace pilot orifice.
- No spark from piezo. Piezo igniter failed. Replace.
- HSI cracked. Hot-surface igniter (silicon carbide) cracked. Replace.