Thermocouple vs Thermopile vs Flame Sensor: How to Tell

Quick answer from a licensed plumber: It depends on the gas control. A standing-pilot heater with a plain gas knob uses a thermocouple in the pilot flame (Rheem’s standing-pilot bulletins). A heater with a status light on the gas control runs on a thermopile, which powers the control from the pilot flame (A. O. Smith FVIR, Bradford White ICON, Rheem Vesta). A power-vent heater with electronic ignition uses a flame sensor rod. They are not interchangeable, and replacing any of them is gas work for a licensed plumber.

The Three Sensors Side by Side

	Thermocouple	Thermopile	Flame sensor (flame rod)
Found on	Standing-pilot gas heaters with a conventional gas valve	Standing-pilot heaters with an electronic gas control and a status light	Power-vent heaters with electronic ignition
What it does	Sits in the pilot flame; the gas control keeps the pilot on while it is heated	Sits in the pilot flame and powers the gas control from its millivolt output	Proves to the ignition control that the burner is lit
Maker examples	Rheem standing-pilot heaters (Rheem bulletins 1410 and 1425)	A. O. Smith FVIR tanks (“PILOT/THERMOPILE ASSEMBLY KIT”); Bradford White ICON; Rheem Vesta control	A. O. Smith power vent (“Flame Sensor Rod”); Bradford White PV series; early Rheem PowerVent hot surface ignition
Sold as	A separate part that connects to the gas control	Part of a pilot assembly kit (A. O. Smith); Bradford White’s guide names the pilot assembly as the part	A part of the power-vent burner system

How to Tell Which One Your Heater Has

Is there a blower on top and a power cord? Then it is a power-vent heater; next, check how it lights. A. O. Smith’s power-vent manual covers “POWER VENTED GAS MODELS WITH HOT SURFACE IGNITION” and lists a “Flame Sensor Rod” in its parts key, and Bradford White’s PV control lights the pilot by “activating spark igniter” and proves it with a flame sensor. Not every power vent is built that way: Rheem says it made its hot surface ignition PowerVent “From April 1990 through January 1996.” If your power vent has a pilot you light at a gas knob, use steps 2 and 3.
Is there a status light on the gas control? Then it almost certainly runs on a thermopile. A. O. Smith’s FVIR manual has you click the igniter until “the Status Light starts to flash,” and its repair steps disconnect “red (+) and white (-) thermopile wires.” Bradford White’s ICON quick service guide tells a technician to “Check thermopile voltage output.”
Just a gas knob, no light, no cord? Then it is most likely a thermocouple. Rheem’s gas valve bulletin lists the connections on its control as “Thermocouple (left side),” “Pilot burner” and “Main burner.”

The model and serial number on the rating plate settle it: the parts list for your model names the part. Bradford White’s residential electric parts list, for one, says “BOTH MODEL AND SERIAL NUMBERS ARE REQUIRED FOR ORDERING REPLACEMENT PARTS.”

Thermocouple (Rheem Standing Pilot)

Rheem’s troubleshooting table lists “Defective thermocouple” and “Poor thermocouple connection” among the causes when the pilot will not light or stay lit. Its replacement bulletin shows why the fit matters: the new tip must sit so “the pilot flame envelopes the top 3/8″ to 1/2″ of the tip,” and the connection at the gas control “should be no more than one quarter (1/4) turn beyond hand tight.” The job also means disconnecting the pilot and main burner gas tubes and leak-testing them afterward with soapy water. Our pilot light guide covers the symptoms.

Thermopile (A. O. Smith, Bradford White, Rheem Vesta)

A thermopile makes enough power to run an electronic gas control. Rheem’s Vesta bulletin gives the numbers: the thermopile voltage “must be 265 mV ± 30 mV to energize the Pilot Valve solenoid,” main-valve operation waits until it “reaches 300 mV ± 40 mV,” and “Normal operating range is between 350 – 850 mV.”

On the A. O. Smith and Bradford White heaters the thermopile is not a stand-alone part. A. O. Smith lists a “Pilot assembly kit, which contains the pilot assembly, thermopile, and retainer clip,” and its repair steps disconnect the thermopile wires, the pilot tube and the manifold tube at the gas control. Bradford White’s ICON guide lists “1) Pilot assembly 2) Gas valve” as the parts for a weak pilot signal. The flash codes are decoded in our guide to A. O. Smith status light codes.

I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.

Flame Sensor (Power Vent)

Power-vent heaters prove the flame electronically. Bradford White’s PV service manual: “Flame sensor – if flame is lost, pilot & main valves are shut down, blower runs for 15 seconds. Control attempts to re-light pilot 4 times. If unsuccessful, blower is shut down and control proceeds to 5 minute lockout.” A. O. Smith’s corrective action for its flame code is to reposition a rod “not properly seated in flame” and clean the rod “to avoid lock-out.” Rheem’s early PowerVent control combined ignition timing with “the ability to sense main burner flame.”

Why these are not swap-and-go parts: All three sit at the burner, inside the combustion chamber. Rheem notes that since July 1, 2003, 30, 40 and 50 gallon conventional gas water heaters have had to meet the flammable vapor ignition resistance (FVIR) standard. A. O. Smith sells the thermopile in a pilot assembly kit, and Bradford White’s guide names the pilot assembly as the part. Every procedure involves opening gas connections. Bradford White’s service manual is marked “To be performed ONLY by qualified service providers.”

What a Homeowner Can Safely Check

Follow the lighting instructions on the label; Bradford White’s first step for “No LED” is to “Follow lighting instructions on the water heater.”
Read and write down the status light pattern and the model and serial number before you call.
If you smell gas, do not relight anything. Follow the gas-smell safety steps printed on your heater’s lighting label.

When to Call a Licensed Plumber

Any part replacement on these three systems: the thermocouple, the pilot/thermopile assembly or the flame sensor. Each one means opening gas connections and leak-testing them, and Rheem’s thermocouple steps take the burner assembly out of the combustion chamber. Give the plumber the model number, the part name from this page and the status light pattern. If the heater is old enough that replacement is on the table, our best water heater guide compares tank models.

FAQ

What is the difference between a thermocouple and a thermopile?

Both sit in the pilot flame. A thermocouple lets the gas control hold the pilot open. A thermopile produces enough millivolts to power an electronic gas control; Rheem’s Vesta bulletin gives a normal range of 350 to 850 mV.

Does my water heater have a thermocouple or a thermopile?

If the gas control has a status light, it most likely uses a thermopile, as on A. O. Smith FVIR and Bradford White ICON heaters. A plain gas knob with no light usually means a thermocouple. The parts list for your model number confirms it.

Where is the flame sensor on a gas water heater?

Flame sensors are on power-vent models with electronic ignition. A. O. Smith lists a Flame Sensor Rod in its power-vent manual, and Bradford White’s PV service manual describes the flame sensor shutting the valves when the flame is lost.

Can I replace a thermopile by itself?

On A. O. Smith FVIR heaters it comes in a pilot assembly kit with the pilot assembly and retainer clip, and Bradford White lists the pilot assembly as the part. It is a gas-system repair for a qualified technician.

What voltage should a water heater thermopile read?

For Rheem’s Vesta control, Rheem says 265 mV plus or minus 30 to hold the pilot valve, 300 mV plus or minus 40 before the main valve operates, and a normal range of 350 to 850 mV.

Why Trust Plumbing Picks

Plumbing Picks is written by a licensed Georgia journeyman plumber. Every part number, size, rating and fit on this page comes from the manufacturer pages and documents listed below, which we opened and read, or, where the page says so, from an Amazon listing; none of it comes from hands-on testing. This page links to no products; it is here to help you name the right part before anyone orders one. Nobody pays to be included here. More on who writes this and how we pick.

Sources

Bottom Line

Plain gas knob: thermocouple. Status light: thermopile, usually sold as a pilot assembly. Power vent with electronic ignition: flame sensor. Name the part from your model’s parts list, note the light pattern, and have a licensed plumber do the gas-side repair.

Water Heater Thermocouple vs Thermopile vs Flame Sensor