Many older gas tank heaters in Orange garages use a standing pilot: a small flame that burns all the time and lights the main burner when the thermostat calls for heat. A thermocouple sits in that flame. As long as it stays hot, it holds the gas valve open. If it cools down, the valve closes and the gas stops. That design keeps unburned gas from building up, and it's also why a weak or misdirected pilot flame shuts the whole heater down.
Newer heaters may use a thermopile, an electronic igniter or a flame sensor instead, and many are sealed-combustion models with a flammable vapor sensor that locks the heater out if it detects fumes. Check which kind you have before assuming the cause.
Parts that keep a pilot going
- Thermocouple
- A metal probe in the pilot flame that produces a tiny electrical signal, holding the gas valve open while it's hot.
- Thermopile
- A cluster of thermocouples that produces more power, used on some gas controls.
- Pilot assembly
- The pilot burner, its tiny orifice, the thermocouple bracket and often a piezo igniter.
- Gas control valve
- The thermostat and valve unit on the front of the tank, with the On, Pilot and Off knob.
- Flammable vapor sensor
- A safety device on many modern heaters that shuts the burner off and may need a reset or replacement after it trips.
Symptom and likely cause
| What happens | Likely cause |
|---|---|
| Pilot lights, then goes out when the knob is released | Worn or misaligned thermocouple, weak pilot flame |
| Pilot is small, yellow or flickering | Dirty pilot orifice, draft, or a combustion air problem |
| Pilot stays lit but the burner never fires | Gas control or thermostat fault |
| Heater runs a while, then shuts down | Venting or combustion air problem, or a safety sensor tripping |
| Won't light at all, no click from the igniter | Igniter, gas supply or a locked-out safety device |
How a plumber typically diagnoses it
- Confirm gas supply.Checks that other appliances have gas and that the supply valve to the heater is open.
- Inspect the flame.Looks at the size, color and position of the pilot flame on the thermocouple or sensor.
- Test the safety circuit.Measures thermocouple or thermopile output and checks the gas control's response.
- Check venting and air.Looks for blocked combustion air openings, a disconnected flue or backdrafting that can starve or blow out the flame.
- Recommend in writing.Clean, replace a part, or, on an older heater with several failing components, discuss replacement.
Worth asking
- Is the thermocouple or sensor the cause, or a symptom of something else?
- Did you check the venting and the combustion air supply?
- Is the gas control still available as a part for this model?
- Given the heater's age, does repair still make sense?
Common questions
Can a breeze blow out a water heater pilot?
A strong draft through an open garage or a missing burner cover can disturb it. If the pilot keeps going out, have the parts and the venting checked rather than relying on shielding it.
Is replacing a thermocouple a repair or a reason to replace the heater?
It's normally a small part repair. Whether it's worth doing depends on the tank's condition and the heater's age.
My newer heater has no visible pilot. What now?
Many newer units use electronic ignition or sealed combustion. Check the status light or error code against the manual, and call a plumber if it shows a lockout.
General information for Orange, not a quote or professional advice. In an emergency, call 911.