What a thermocouple is
A thermocouple is two different metals joined at a tip. When the tip is heated by the pilot flame it generates a small electrical voltage. That voltage tells the gas control valve that a flame is present. If the flame goes out the voltage drops and the valve shuts the gas off for safety.
Where it sits
- At the pilot burner assembly, with the tip positioned in the edge of the pilot flame
- The other end connects to the gas valve with a compression nut or plug-in fitting
Why it matters
- It is the primary flame safety device on many gas storage systems
- If it fails, the pilot will not stay lit and the main burner will never fire
- It prevents raw gas from flowing when there is no flame
Typical failure causes
- Metal fatigue from thousands of heat and cool cycles
- Pilot flame misaligned, too small, or lifting in a draft so the tip is not heated correctly
- Soot or dust insulating the tip
- Oxidation or poor contact at the gas valve connection that drops the millivoltage
- Mechanical damage during cleaning or from vibration
- Age. Many units fail after 5 to 10 years of service
Symptoms you will notice
- Pilot lights but goes out when the button is released
- Pilot will not relight during windy or wet weather
- Hot water works some days and fails on others
- Pilot flame looks weak, yellow, or wandering rather than steady blue
Safe checks a homeowner can do
- Look through the inspection window. The pilot should be steady and blue with a defined shape
- If it is weak, yellow, or repeatedly blowing out, note it and book a service
- Do not remove covers, do not adjust the pilot, and do not overtighten fittings. Gas work must be performed by a licensed fitter
What RJT tests on site
- Pilot flame size, stability, and alignment to the sensor tip
- Thermocouple output using a millivolt meter. Healthy readings under flame are typically in the tens of millivolts
- Condition of the tip and the seat at the gas valve
- Safety shutoff response. Gas valve must close promptly when the flame is removed
- Contributing factors such as blocked pilot orifice, drafts, or a failing gas valve or thermostat
The fix we carry out
- Isolate gas, cool the assembly, and remove the old thermocouple
- Clean the seat and threads, install a new part from stock on the truck
- Reposition the tip correctly in the pilot flame envelope
- Relight, verify millivoltage, confirm burner operation, and leak test with spray
- Record readings against manufacturer specifications
How long it takes and cost context
- Typically a short visit, part cost is modest compared to control valves or full replacements
- If the gas valve itself is failing we will quote options before any extra work proceeds
Preventing repeat failures
- Annual service to clean the pilot and check flame position
- Keep area around the heater free of dust and chemicals
- Shield from strong cross winds where practical
- If the cylinder or control train is at end of life, plan an upgrade rather than chasing repeated small faults
Safety notes
- If you smell gas, hear hissing, or see scorching, move away, turn off gas at the meter if safe, and call RJT
- Do not use open flames or electrical switches near a suspected leak
Why choose RJT
- Same day attendance across North Lakes, Brisbane Northside, and Moreton Bay
- We carry common thermocouples, pilots, and igniters, so most jobs are completed on the first visit
- Diagnosis with instruments, not guesswork. We leave the system tested to spec
Pilot will not stay lit or hot water keeps dropping out. Contact RJT Plumbing and Gas Solutions for same day diagnosis and repair.
