Estimated reading time: 8 minutes, 9 seconds
This situation caught me off guard. I was presented with a thermocouple from an AO Smith, sealed combustion chamber water heater. The symptoms included that the water heater would go off for no reason.
Eventually, he couldn’t even get the pilot lit. With careful inspection, you could see it had a little button in line. I had never seen one before, but this is what I learned.
What is a thermocouple for anyway?
Thermocouples are used in gas appliances that use a standing pilot. That means that there is a small gas flame on all the time. Without the Thermocouple doing its thing, the pilot won’t stay lit.
Other included safety devices
Flame sensors work similarly, and basically ensure that there is a flame before the gas is allowed through the main gas valve to be turned into flame for heat. If it wasn’t for a functional thermocouple, the gas valve would just allow the gas to flow into your home without being burned. This is when explosions happen, and entire buildings get blown to smithereens.
How thermocouples work
A thermocouple is an impressive little safety device that senses the heat made by a flame. Through a system of having dissimilar metals joined, thermocouples generate a millivolt charge of around 12 – just enough to tell the gas valve that the pilot light is on. I think that the coolest part is that there are no moving parts.
OOPS!Many appliances don’t have thermocouples, like gas stoves and barbecues. Because of that, they need to be watched closely and are very dangerous, especially around children and especially in windy conditions.
The thermocouple is always high on the list of possible problems in gas appliances troubleshooting. Unless you are handy and understand how to test them and or change them out, I would recommend calling a professional. The problem with this one, in particular, was that it was intermittently shutting off the pilot.
The tipping point
It wasn’t until that fateful morning when my friend’s wife couldn’t take a hot shower that hubby had to spring into action and fix it. He called on a Sunday to help make it better and being the go-to guy that I am, I accepted the challenge. It took lots of research to discover what the aforementioned button was. It had a 190 stamped on it, and this is what we finally discovered.
The typical thermocouple costs about $7.00 The fancier one is $35.00. The little button is called a Thermal Cut-Off, or TCO. Unfortunately, there is no place open on a Sunday that carries the fancier one. This reminds me of a “suspenders and belt fix” only it is a “suspenders and two belts”… a bit of overkill in my opinion. The burner assembly that includes the thermocouple is around $80
This is what A.O. Smith has to say about their fancy Thermocouple device.
COMBUSTION CHAMBER TEMPERATURE SENSOR
The C3 Technology water heaters will come equipped with a thermal cutoff (TCO) device that is
integral to the thermocouple. Temperature cutoff points range from 160°C to 200°C depending on
the model. The TCO is an automatic reset thermal switch. When activated it will open the
thermocouple circuit and shut off the main and pilot burner gas flow. The pilot would have to be relit
after the TCO has had sufficient time to cool down (the sensor is below 120 degrees F).
The TCO performs two primary functions:
• In the event of flammable vapor ignition inside the combustion chamber, it senses the
the corresponding increase in temperature and shuts off gas flow to the main and pilot burners.
• If excessive temperatures inside the combustion chamber indicate poor combustion due to
a clogged LDO screen or inadequate air for combustion, the TCO will shut off gas flow to
the main and pilot burners.
Ultimately I discovered ours had a broken weld at one side of the button that was most likely due to a manufacturing defect. I saw lots of complaints about this little button, but no good solutions or explanation of what they were on the internet. There just wasn’t any good advice about where to find them, or if they were essential.
How I fixed my problem
I replaced it with a readily available “universal type” from the hardware store. It worked just fine. There is a flimsy black plastic screen that I washed out.
On Monday, I went to the wholesale house and sure enough, I found them for $35.00 but they were out.
I just had to fix another one where the only thing I could buy was a new burner assembly! $110 This one also had a broken weld at one end too. I am going to get a few of these little gems and keep them handy for the next big water heater problem.
The mysterious LDO & Flame arrestor
Turns out there is an LDO means Lint, Dust & Oil Screen is a silly little plastic thing that you can easily take out and wash. Then there is the Flame arrestor (it is a hidden screen thing that is hard to get at). It is a 1/2″ thick sturdy ceramic air filter of sorts with thousands of very tiny holes that commonly get clogged.
NEW SOLUTION INFORMATION 2-2017
AO smith redesigned the water heater so I guess we were heard loud and clear. The temp out button still exists but it is easily accessible now and has a secret manual reset.
I was asked by a reader Joe Low, to participate in a solution for his 11-year-old water heater that would temp out upon taking a shower. Once cooled off he would have to relight the pilot. I was going to get him a new burner assembly (Which he has already replaced once) when I spoke with Lonny, the Manager of my Ferguson store. He assured me that I was making a mistake, “Temp out is a sign of a problem!” it made sense to me, so we elected to just replace the water heater for safety’s sake.
Ultimately, I used the removed water heater as a test subject. Upon disassembling it in the name of science, I found this clogged flame arrestor (Image above) and some signs of excess heat in the burner box. (His utility room housed a furnace too but it was spotless.) You will need a mirror to see the flame arrestor screen from below.
If your water heater is temping out, clean out this well-hidden part. You can get to it from the bottom with a vacuum, (you’ll need to adapt something to suck from the top side of the air intake base) or even better, use compressed air with a long flexible, or bent extension. you can also get to it from the top. It lives just under the burner assembly (You will need to remove the burner assembly to get access). I suggest that you clean it from both directions as it is partially obstructed by metal from above. When I hit this flame arrestor with compressed air the dust flew!
Naturally, there is no mention of this flame arrestor in the owner manuals. I say this sarcastically because it would be helpful if it was published for general consumption or advanced diagnostics.
Other things to check
The other thing that could be suffocating the flame is the lack of combustion air.
- Is your house too well sealed?
- Is your utility room too well sealed?
This can be easily checked with a piece of toilet tissue. (I will add a video soon)
We found one home where the air filter cover door was missing, so when the furnace kicked on, it was sucking the combustion gasses right out of the water heater top into the furnace intake (That is deadly) Always look for signs of the rollout on top of the gas water heater, and have a working CO monitor in the home.
The quick fix from readers:
This has been my most popular blog post of all time, the common theme of the comments is appreciated. The best solution I have discovered by accident is just to re-solder the button back on. I am not sure how long it will last, but I would cautiously recommend this fix to anyone. Whatever you do, remember messing with gas has risks, do not bypass safety features.
This house exploded in New Jersey as a result of a gas problem.
Mine just failed
I just had mine fail. I found it simple to remove the three parts.
- Gas line
- Pilot gas line
Then I used my soldering iron to remove the button and then filed off the back to see the button. Then I soldered the two brass parts. the straight one to the edge and the curved one to the center of the button. This was harder than it looked because all the parts are small. All better now.
Bonus Round Questions?
- Who has a war story to tell about these things?
- Who has some additional helpful information they would like to share?
Your comments are welcome. To ask questions or get more information about remodeling, click here to email me directly, or call 208-947-7261
If you or someone you know is considering remodeling or just wants to speak to a trustworthy remodeling contractor please contact me, you’ll be glad you did.