Water Heater Thermocouple: What It Does and When It Fails

What a thermocouple is

A thermocouple is two wires of different metals joined at one end. Heat the joined end and a small voltage appears across the other ends. Resideo describes its Q390A thermocouple, made for standing-pilot gas appliances, this way: the pilot flame heats the tip, which creates a temperature difference between the tip and the base, and that difference generates the voltage.

On a water heater, it looks like a thin copper tube. One end, the tip, is clipped into a bracket next to the pilot burner. The other end has a nut that threads into the gas control valve on the front of the tank.

How it works on a water heater

When you hold the gas control knob in on PILOT, you're pushing the pilot safety valve open by hand. Once the pilot is lit and the tip of the thermocouple heats up, the voltage it makes powers the gas valve so it stays open on its own. Then you can let go.

If the pilot ever goes out, the tip cools, the voltage drops, the magnet lets go, and the valve snaps shut. That's why a dead thermocouple gives you a pilot that won't stay lit, and why you shouldn't hurry the "hold the knob" step when lighting.

The numbers that matter

Thermocouples work in millivolts (mV), thousandths of a volt. Honeywell's installation sheet for its standard thermocouples gives these figures:

The flame coverage line explains a lot of "bad thermocouple" calls. If the pilot flame only licks the side of the tip, the reading falls and the pilot drops out even though the part is fine.

Thermocouple vs thermopile vs flame sensor

Many newer gas water heaters use a thermopile instead. A thermopile is a stack of thermocouples, so it makes much more voltage. Honeywell lists its Q313 thermopile's normal output as 600 to 750 mV. That extra power can run a gas control with a status light. A.O. Smith's ProMax heaters, for example, use a thermopile with red and white wires rather than a single copper lead.

Heaters with electronic ignition use a flame sensor instead. Electronic systems check that a flame is present before letting gas flow. The job is the same, the parts are not interchangeable.

How thermocouples fail

They wear out. The tip lives in a flame every hour of every day, and over the years the metal breaks down and the output drops below what the gas valve needs. Other common reasons a pilot drops out:

• Loose connection. If the nut at the gas control isn't snug, the voltage leaks away. Honeywell's spec is finger tight plus a quarter turn with a wrench, and no more.
• Dirty or sooty tip. A coated tip doesn't heat properly.
• Tip out of the flame. A bumped bracket or a weak pilot flame leaves the tip only partly heated.
• Overheating. Honeywell notes that thermocouples work less well when the cold end gets too hot, often from radiant heat off nearby surfaces or high air temperature.
• Kinked lead. Honeywell warns not to bend the lead within 1/2 inch of either end.

A thermocouple can also be fine while something else is wrong. A.O. Smith's list of reasons a pilot won't stay lit includes drafts, poor combustion air, a clogged pilot tube and a failed control valve.

Are thermocouples universal?

Mostly, on older standing-pilot heaters. The common replacement thermocouples come in several lead lengths with adapters to fit different gas controls; Resideo sells the Q390A in a range of lead lengths, for example. Buy one at least as long as the old one so you don't stretch or kink it. On sealed-combustion heaters the thermocouple is often part of a pilot assembly or burner kit from the heater maker, and that's the part to buy.

What to do next

Test it before you buy a new one. A millivolt reading tells you in a few minutes whether the part is weak or whether the problem is the flame: see how to test a water heater thermocouple. If it's bad, replacing the thermocouple is a cheap part. Angi puts it at about $5 to $20 for DIY. HomeGuide puts a pro replacement at $100 to $325.

If a new thermocouple doesn't fix a pilot that won't stay lit, look at the gas control valve next.