The gas hot water thermocouple: a small sensor with a big safety job
It is a short metal probe that sits in the flame. When it fails, the gas shuts off and your hot water goes with it, and that is exactly what it is designed to do.
A thermocouple is a flame safety sensor. On gas storage hot water systems with a pilot flame, it sits in that flame and produces a tiny electrical signal while it is hot. That signal keeps the gas valve open. If the flame goes out, the probe cools, the signal drops and the gas shuts off.
This page explains the part so you can describe a fault clearly. It does not tell you how to fix it, because everything on the gas side is the work of a licensed gasfitter.
What does a thermocouple actually protect against?
Unburnt gas collecting in a home is dangerous. The thermocouple makes sure gas only flows to the burner while there is a flame to burn it. It is a simple part, and a very important one.
Many newer systems use electronic ignition and a flame sensor instead of a pilot, so you may not have one. Check your model's manual or the label on the unit.
What are the signs it is failing?
The classic sign is a pilot that lights but will not stay on once released. You may also find the system runs cold with no flame visible, or that it works for a few days and then drops out. Several different faults look alike from the outside, so none of these proves the thermocouple is the cause.
| What you notice | Possible cause | What to do |
|---|---|---|
| No hot water, pilot out | Failed thermocouple, draught, or a gas supply issue | Call a licensed gasfitter |
| Pilot goes out repeatedly | Worn or dirty sensor, or a poor flame | Call a licensed gasfitter |
| Smell of gas | A gas leak | Treat as an emergency, see below |
Why you should not attempt this yourself
Replacing a thermocouple means working on the gas appliance, and in Australia that needs a licensed gasfitter. Do not relight a pilot that keeps going out, do not bend or clean the probe, and do not bypass the safety device. Those are the shortcuts that cause gas incidents.
If you can smell gas, leave the building, avoid switches and flames, and call your gas distributor's emergency line or 000 from outside. Do not stay to investigate.
What is yours to do
- Check the basics first. Is the gas supply on elsewhere in the house, and did the power or the weather change?
- Write down what you saw. When did the hot water stop, and is the pilot visible or out?
- Find the model label. A gasfitter will want the make and model.
- Book a licensed gasfitter. An old system with repeated drop-outs is also a good moment to ask about its age, as in how long a hot water system lasts.
Servicing may be a warranty condition, so see what voids a hot water warranty before anyone else touches the unit.
What to tell the gasfitter when you call
You cannot fix this part yourself, but a clear description saves time. Have these ready.
- The make and model. It is on the label on the unit.
- When the hot water stopped. Note the day and what changed, such as the power or the weather.
- What the pilot does. Does it light and then go out when released, or is it out already?
- How often it happens. A one-off and repeated drop-outs point to different causes.
- Whether other gas appliances work. This helps rule out a gas supply issue.
- The age of the system. Repeated drop-outs on an old unit are a good moment to ask about replacement.
Pilot light problems: what each one can mean
Several faults look alike from the outside, so none of these proves the thermocouple is to blame.
| Symptom | Possible causes | What to do |
|---|---|---|
| Pilot lights but will not stay on | Failed thermocouple, draught, or a poor flame | Call a licensed gasfitter |
| Pilot goes out repeatedly | Worn or dirty sensor | Call a licensed gasfitter; do not clean or bend the probe |
| No hot water, pilot out | A gas supply issue | Check other gas appliances, then call a gasfitter |
| System runs cold, no flame visible | Control or sensor fault | Call a licensed gasfitter |
| Smell of gas | A gas leak | Leave the building, avoid switches and flames, call 000 or your gas distributor from outside |
If your system is old, read how long a hot water system lasts.
Frequently asked questions
What does a thermocouple do on a gas hot water system?
It is a flame safety sensor. It generates a small signal while heated by the pilot flame, which keeps the gas valve open. If the flame goes out, the signal stops and the gas shuts off.
How do I know if my thermocouple has failed?
A pilot that lights but will not stay lit is the classic sign, but other faults look similar. Only a licensed gasfitter can diagnose it. Report what you saw rather than trying fixes.
Can I replace a thermocouple myself?
No. It is part of the gas appliance, and work on it requires a licensed gasfitter in Australia. If you smell gas, leave the building and call the emergency line or 000 from outside.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)