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The other day we had our water heater go out and the issue was diagnosed to a faulty thermocouple. When the repair man removed the faulty thermocouple he noticed the following disk-like component in series about mid-way down the copper tubing.

This device had one of its two terminals broken off of the bottom (not shown) and was most certainly the cause for the failure. The repair man's replacement thermocouple had no such device inline and when asked what this was he replied it was a transistor. I very much doubt this device is a transistor as there are only two, not three, terminals but I didn't want to argue.

So, what exactly is this component and what is it's function? As stated, the replacement thermocouple does not have this but it appears to be functioning correctly.

enter image description here

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If I was a betting man, I'd suggest that it's a low-melting-point alloy such as Wood's Alloy designed to melt and open the circuit from the thermopile (thus closing the gas valve) in case of a house fire or fire under the water heater, but I have no evidence to support that premise!

Possibly 180°C is the melting point.

Edit: Okay, evidence gathered. See Amazon review here,

The funny thing is AO Smith replacement thermo-couples don't even come with the thermal fuse!!! But you don't really need the thermal link, and this generic thermo-couple will work just fine for your water heater (as long as it's the correct length). So measure the length of your bad one and make sure. Because the longer they are, the higher the resistance, and your gas control valve is designed for the correct length.

Finally, if you feel you just gotta have the thermal fuse (cause your'e afraid you just can't keep the area clean under your water heater...and dust or debris collecting under your water heater, may start an un-controlled fire), then for about $20 more, you can get the complete kit which includes the thermo-couple (with thermal-fuse link), the pilot tube, and a piezo-electric ignitor.

and this Amazon photo (which is the item referenced in the review).

enter image description here

You might want to discuss this with your repair service company- it appears they may have compromised the overall safety by replacing the original safety device with an inferior generic replacement. It also sounds like the guy they sent out was a dolt- "transistor"!

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  • \$\begingroup\$ Thanks so much, I in-fact do have an AO Smith water heater. Looks like I will be calling the service company 1st thing tomorrow morning. \$\endgroup\$
    – SiegeX
    Sep 19, 2014 at 23:45
  • \$\begingroup\$ Just for reference (in case someone wants to search for it), it's a "thermal cutoff fuse", called "Thermal Cut Off (TCO) switch" in the manuals of some heater brands. \$\endgroup\$
    – MV.
    Mar 31, 2018 at 4:51
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An easier way than soldering the two ends of thermocouple is to just replace with a new universal TC (~$12 HomeDepot). A fusible link soldered in series with the TC presents a weak mechanical link fusible link in series with TC.

The torque from attaching the threaded end to the burner valve control can easily tear the solder bond and create an open or even worse, an intermittent TC connection (that's what happened in my case). Either case results in a pilot light that does not stay lit, which means no main gas flow to burner.

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This Fusible link you are referring to is a high limit link. If your water heater gets to hot. This high limit will burn thru. You have to understand that at the gas valve where the thermocouple screws in. There is a valve high limit. If your thermocouple gets to hot, the valves high limit will shut down the flow of gas. There is NO such thing as a "fire fusable link" to be used incase of a fire under your water heater. I replace this style of thermocouple all the time with the standard style thermocouple. I've never had a problem with the heater. Petaluma California Plumber for over 40 years...

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I fixed my fusible link on my Maytag(Whirlpool) water heater. After inspecting the old link, which fell off when I touched it, I found it had rusted thus breaking the connection. I tried to get a new link or get advise on what to do but I had to figure this one out for myself.

Since most thermocouples do not come with a fusible link I took the two ends of wire left on the thermocouple wire and soldered them together. It worked perfectly. The fusible link only tripped in the event of a house fire or blockage. I SOLDERED it so if a fire did break out it would melt thus shutting off the gas anyway. I keep my basement and water heater clean and free of debris so hopefully no fire there. I am not recommending this as a perfect fix I found it hard to spend $90 plus to replace only a hearing aid battery size part and I believe I am as safe as I ever was. Thanks Lennie

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