Thermocouple vs Thermopile: What's the Difference and Which Do I Need?
What is the difference between a thermocouple and a thermopile?
Both sense a pilot flame and both generate their own electricity from heat. The difference is how much.
| Thermocouple | Thermopile | |
|---|---|---|
| Construction | A single junction of two dissimilar metals | Several thermocouples wired in series |
| Output | About 25–30 mV | About 250–750 mV |
| What it can power | Just the electromagnet holding the gas valve open | The gas valve and a control circuit or thermostat |
| Typically found on | Simpler standing-pilot appliances | Appliances with millivolt control systems |
| Typical price | From £18.99 | From £40.00 |
| Interchangeable? | No. Fitting one in place of the other will not work. | |
Why does the voltage difference matter?
A thermocouple produces just enough power to do one job: energise the electromagnet that holds the safety gas valve open. That is all. It cannot run anything else.
A thermopile produces roughly ten to twenty-five times more. That is enough to hold the valve open and power a millivolt control circuit — a thermostat, a control board, a switch. This is why a thermopile system can run a whole appliance without any mains electricity at all.
If you fit a thermocouple where a thermopile belongs, there is simply not enough power to run the control circuit, and the appliance will not work.
How do I tell which one my appliance uses?
Three quick checks:
- Count the wires. A thermocouple is a single lead with a threaded nut into the gas valve. A thermopile typically has two wires going to a terminal block or control board.
- Look at the tip. A thermopile probe is noticeably thicker — around 9mm diameter — because it houses multiple junctions. A thermocouple tip is slim.
- Does the appliance have controls that work without mains power? A millivolt thermostat or control that runs with the appliance unplugged means a thermopile is generating that power.
If you are still unsure, the safest check is the reading — see below.
How do I test a thermopile?
With a multimeter on millivolts DC, light the pilot and let it heat for at least 30 seconds, then measure across the thermopile leads.
- Open circuit (nothing connected): expect roughly 250–750 mV on a healthy unit.
- A reading well below that range means it is failing and can no longer drive the valve reliably.
- Zero means it has failed completely.
A thermopile that has weakened will often produce the classic symptom: the appliance works when cold but drops out once it is hot and busy, because the output has sagged below what the valve needs to stay held in.
What are the symptoms of a failed thermopile?
- The pilot lights and stays lit, but the main burner will not fire.
- The appliance works intermittently, and is worse when hot.
- Millivolt controls (thermostat, switches) behave erratically or stop responding.
- The burner drops out mid-service and relights after a rest.
Note the difference from a thermocouple fault: with a bad thermocouple, the pilot itself goes out. With a bad thermopile, the pilot usually stays lit but the main burner won't run. That single distinction identifies the part most of the time.
If your pilot is the thing going out, read our guide on testing and replacing a thermocouple instead.
Which thermopile do I need?
Match these:
- Probe length — must reach the pilot comfortably. 900mm is common on commercial equipment.
- Probe diameter — typically around 9mm.
- Connection — screw terminals are common; 7/16" is a frequent thread on commercial gas valves.
- Output range — should be rated in the 250–750 mV band.
We stock Robertshaw thermopiles, including the 1950 and 1951 series, which are the industry standard for gas appliance applications. Our 900mm Robertshaw thermopile has a 9mm probe, a 57mm probe length, a 250–750 mV rating and a 7/16" screw connection, at £40.00.
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Can I fit a thermopile myself?
The mechanical job is straightforward — isolate the gas, unscrew the old probe from its bracket, disconnect the leads, fit the new one so the tip sits properly in the pilot flame, and reconnect. As with a thermocouple, the tip must sit in the flame, and the connections must be clean and tight or you lose voltage across them.
But this is gas work. In the UK, work on commercial catering gas appliances must be carried out by a Gas Safe registered engineer qualified for commercial catering. Fitting the part is easy; being legally allowed to do it is the constraint.
Frequently asked questions
Can I fit a thermopile instead of a thermocouple?
No. They are not interchangeable in either direction. The voltages are an order of magnitude apart and the valves are built to expect one or the other.
Why is my appliance dropping out only when it's busy?
That is the signature of a thermopile whose output has weakened with age. When cold it just about produces enough to hold the valve; once hot and under load, the margin disappears. It will get worse, so replace it before it strands you mid-service.
Is a millivolt system safe without electricity?
Yes — that is the entire point of it. The thermopile generates its own power from the pilot flame. If the flame goes out, the voltage collapses and the gas valve closes automatically. It is a self-powered safety system, which is why it remains common on commercial gas equipment.
How fast can I get one?
Robertshaw thermopiles are held in UK stock. Order before 10am for next-day delivery. Free delivery over £25. Not sure which one? Call 073 8051 5838 with your appliance make and model.
Need help identifying the part?
Send us your make, model and a photo of the existing probe and we will tell you exactly what fits.
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