# Fusible link thermocouple.



## Nailerconstruct (Jan 12, 2009)

Let me start off by saying i am not a plumber. Im a framer by trade. As most of you probably know very well being in the construction industry often leads to short or not so short temp jobs among the other trades. I've done enough plumbing to know how to fix the basic problems that arise. 



In my home i have a select state gas water heater that was made in 2003. I came home yesterday to find no hot water. So i do the basic stuff. Cleaned the vent. Vacuumed out all the crud. Blew it all out with compressed air. Checked for obstructions in the vent. Everything looks fine. I figure its probably the thermocouple. Thats when i had a oh crap moment. 


Its not a typical universal thermocouple. Its one of those fancy fusible link thermocouples. 


What i am having trouble wrapping my head around is. Why is this fancy thermocouple necessary for my water heater but not others. I've replaced several in the past that did not have this extra safety measure. I get that its purpose is to sense blow out and shut the system off and keep the water heater from burning the house down. 


But in my head un less the water heater was installed in a kitty litter box this fail safe is totally un necessary. 


I've already ordered the proper replacement part but i cant imagine why a universal one wouldn't work just fine. Someone please inform me.


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## GettingBy (Aug 19, 2010)

Nailerconstruct said:


> no hot water


Did the fusible link melt?

"There are two steps to checking thermocouples. The first is to check for a short on the terminals and the second, to make sure that voltage tracks with the temperature.

The first test can be performed with any quality multimeter. Put the meter in ohms or continuity mode; on a good thermocouple, you should see a low resistance reading. If you see more than a few ohms, you probably have a faulty thermocouple. If the reading at room temperature is close to 110 Ω, then you have an RTD on your hands—read on."



Nailerconstruct said:


> But in my head un less the water heater was installed in a kitty litter box this fail safe is totally un necessary.


It may be. 
If this thing costs 5x more than the generic, do you get 5x the safety from it?
Ask these guys
[email protected]
or
Tiffany O’Shea at [email protected]

Here's one stated reason for it in an old Robertshaw patent

"If for any'reason flue 18 should become blocked or if unusual wind conditions occur to reverse the flow of air through flue 18 during furnace operation, extremely dangerous conditions are created due to the undesirable amount of heat which will build up in the lower portion of furnace 10. One characteristic of such conditions is that the flames at main burner 14 will flare and tend to flow out of the opening where combustion air normally enters burner box 12. Fusible link assembly 42 will, therefore, be subjected to increased ambient temperatures from the main burner flames and will fuse or melt at a predetermined temperature associated with such conditions to open the thermocouple circuit and close the safety valve in the same manner as previously described with respect to pilot flame outage."

And see
http://www.google.com/search?client..."+fusible+link+thermocouple&ie=UTF-8&oe=UTF-8


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## Nailerconstruct (Jan 12, 2009)

No it didn't melt. That was a issue on a different model. Mine is suposed to reset itself after cool down. 



I don't have a meter. I came to the thermocouple conclusion just by a process of elimination. Also. It will light and stay lit. Once i turn up the heat it will shut off in about three minutes. So i pulled the thermocouple and cleaned it real good. It improved my burn time to about 15 min. My reasoning for replacing it is. For thirty bucks im willing to give it a go it before i call a pro. 






I just think they created this thing for a issue that doesn't exist. Maybe if it had a pilot that shuts off when not in use then re lights automatically, then i could see it. But to my very limited knowledge those don't even exist. 




Im not going to lie. A big part of me wants to throw a universal one on there and call it a day. It would be a waste though. As ive already ordered one.


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## Tinstaafl (Jan 6, 2008)

Nailerconstruct said:


> Mine is suposed to reset itself after cool down.


Then it's not a fusible link, it's a thermal breaker. 

Just theorizing here, because I didn't even stay at a Holiday Inn last night, but I'd say it's a kind of oddball use for a thermal breaker. 

Normally, they pop when too much current is flowing, causing them to heat up and warp a bimetallic contact arm. It'd be nearly impossible for a thermocouple to generate a significant excess of current, but under the right circumstances you could have a major buildup of unwanted heat in the combustion area. A thermal breaker would sense that condition, break the circuit and shut off the gas flow.

Thermal breakers do go bad on occasion, like anything mechanical. Excessive safety feature maybe, but I can see the reasoning.


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## Nailerconstruct (Jan 12, 2009)

If I were a betting man i put my money on the breaker being shot. I started thinking along those lines and turned the temp all the way to pilot and its been burning for about half an hour now. 




The more i read about the reasoning behind employing this extra safety measure the more i think they are only useful in a catastrophic failure type event. 


Example a tornado is touching down on your house and the first thing it hits happens to be the vent pipe. So the breaker shuts off the gas right before the tornado destroys the entire house. But even then getting the type of blow back that would start a fire would be pretty un likely. At least in my mind anyway.


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## GettingBy (Aug 19, 2010)

Someday you should get a DVM. Then you can check thermocouples if you amplify the voltage output enough to read it.
http://www.google.com/search?q="ele...0...1ac.1.34.heirloom-serp..3.0.0.izfiOK3s528

A DC voltage gain of 10x or more should be enough. This amplifier would probably be powered by a 9v battery, or two of them in series.


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