On standing-pilot Reliance gas heaters, the thermocouple senses pilot flame and signals the gas valve to keep the gas supply open. If the thermocouple fails, the safety circuit shuts the gas off — pilot won\'t stay lit.
Thermocouple vs thermopile
- Thermocouple — single junction; older units. Outputs ~25-30 mV. Used with the simplest gas valves
- Thermopile — multiple thermocouple junctions in series; newer units. Outputs ~600-800 mV. Powers the LED status display and electronic gas valve
Most modern Reliance use a thermopile. Some older Standard 6-NORT (pre-2012) used a thermocouple. Verify by part number on the rating plate.
Symptoms of failure
- Pilot lights but goes out when you release the gas button
- Pilot lights but goes out after 30 seconds
- Pilot won\'t light at all
- 2 LED flashes (thermopile voltage low)
Diagnostic test
Multimeter on mV (millivolts) setting:
- With pilot lit for 60+ seconds, measure across the thermocouple leads at the gas valve
- Healthy thermocouple: 25-30+ mV
- Healthy thermopile: 600+ mV with pilot, 350+ mV with pilot AND main burner firing
- Below threshold = replace
Replacement procedure
- Turn gas valve to "Off"; allow assembly to cool 10 minutes
- Remove the burner access cover
- Unscrew the thermocouple nut at the gas valve (7/16" or 9/16" wrench)
- Pull the burner assembly out partially to access the thermocouple bracket at the pilot
- Loosen the thermocouple bracket screw and slide out the old thermocouple
- Insert new thermocouple with tip aligned in the pilot flame envelope (about ¼" into the flame)
- Tighten bracket screw and gas valve nut hand-tight plus ¼-turn (do NOT overtighten — crushes the thermocouple)
- Reinstall burner assembly
- Light pilot per manual; hold gas button 60 seconds; release; verify pilot stays lit
Cost
- Universal thermocouple: $15-25
- OEM AO Smith thermopile: $30-50
- Plumber labor: $80-150 if hired
Common mistakes
- Overtightening the nut at the gas valve — crushes the thermocouple tip; fails immediately
- Wrong position in pilot flame — tip should be in the top ⅓ of the blue flame envelope
- Mixing up thermocouple and thermopile — they\'re different parts and not interchangeable
Bottom line
Cheapest DIY repair on a Reliance gas unit. Always inspect and clean the pilot orifice while you\'re in there. If pilot still won\'t hold after new thermocouple, suspect the gas valve.