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Question DetailsAsked on 4/9/2016

Just replaced my termal cuopling and it stayed lite for about 20 mins then went out and wont light back up why

Ok I have replaced my termal cuopling in my water heater about a month ago now and it has went out again I tried to light it but wouldn't stay lite so bought another termal coupling it lite ran for about 20 mins went out and now won't lite again why

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1 Answer


Several possible reasons - I am assuming you mean the pilot won't stay lit. If the pilot stays lit but not the burners, whole different story and most likely NOT a thermocouple issue - could be a gas controller problem, thermostat issue, or the pilot is located so the burners are blowing it out when they kick on, which then shuts the gas off because the thermocouple cools off.

Assuming pilot is what is not staying lit, in no particular order I am sorry to say:

0) when relighting pilot, be sure to hold down the pilot light button (or the control knob on some) for 30 seconds before releasing, because the thermocouple has to heat up before it will let the pilot (and burner) get gas - some brands take longer than others which might stay lit after 5-10 seconds.

1) thermocouple is not properly located in the pilot flame - set wrong location or too short a thermocouple (they come in several lengths) - the end 1/3 to 1/2 (depending on brand) should be in the blue part of the pilot flame

2) the thermocouple is not securely clamped in the mounting bracket - in some makes that acts as the return part of the circuit.

3) thermocouple contact inside the gas control valve is poor - remove thermocouple as gas control valve, use pencil eraser to reach up in and GENTLY clean the contact inside the recess and the tip on the thermocouple, then clean any residue out of the recess with dry Q-Tip. Straighten the last couple of inches of the tubing and slide the connecting nut down the thermocouple tube out of the way, insert the contact tip of the thermocouple up into the recess being sure it goes all the way in to the contact point - not hanging up on the threads. Use Q-tip to mark the depth and compare to thermocouple tip penetration distance - common for it to hang up at the top of the threads. Then hold thermocouple tubing tightly in and bring the nut up and screw it in snug but not hard - just marginally more than finger-tight.

4) check gas control is seated fully in the PILOT or ON position as applicable - some brands (Whirlpool / Kenmore > AO Smith especially) tend to drag on the housing so can end up not firmly in the correct position and can pop out (out of ON to PILOT in mhy experience).

5) if pilot lights but not the burners, the magnetic or pile solenoid inside the gas controller is failing, so the minute amount of power from the thermocouple is no longer enough to operate the gas control valve any more. Failure of this component is pretty rare - at least in brandname controllers like Honeywell - usually last far longer than the water heater. Not something most DIY'er can troubleshoot, and cannot be repaired if bad.

6) pilot blowing out - though that would not stop you from relighting it. Or pilot flame too low so it is marginally heating the thermocouple but not enough it cannot cool off in a draft - like maybe when heating system furnace/boiler turns on.

7) could be the problem is something totally different than pilot/thermocouple - dust or rust blocked air inlet screen (some have on bottom of firebox), or burner is firing and then shutting off the gas because the firebox overtemp sensor says it is getting too hot. Some sensor types are one-use when they shut off the gas and have to be replaced.

8) I did run into at least one brand - American I think it was - where the thermocouple goes into an insulated holder, and the outer surface is the conductor - so if it touches any metal along the way it does not get the low voltage signal to the gas controller, Check your routing to be sure it is clear of all surfaces.

9) check it did not get kinked - that can break the inner element and cause failure.

10) some thermostats, especially on sealed have a TCO - a Thermal Cut Off, which is part of the thermocouple and looks somewhat like a button battery - shuts off when it get overheated by gas blowout or flaring or blowing of the burner over the thermocouple. Commonly due to draft (in room or from another appliance kicking on) or the back shield (protective finger) on the pilot is facing the wrong way so burner flame hits it. Some TCO's reset after they cool off - some are one-use and have to be replaced when once overheated. Since your new thermocouple worked for 20 minutes then died, sounds like maybe that type and it overheated and burned out.

11) if a high-efficiency water heater (with exhaust draft blower and damper) then it may not be turning on or the damper not opening, and sensor is shutting off the gas because of that (or because of sensor failure). Though in that case you would not have a standing pilot I don't think - probably a piezoelectric pilot ignitor controlled by the thermostat. And in my experience blower/damper/sensor will not prevent lighting - just shuts it back off in a short time if sensor does not detect proper pressure difference in short order.

This is a lot of possible causes - if a dedicated DIY'er and you have a volt-ohm meter which can read millivolts you can test the thermocouple (with pilot burning and cold) for proper voltage and work through the other possibilities and test automatic shutoff sensors to see if they are cutting the circuit - for most people might be time to call your plumber (or gas fitter in a few northeast states requiring they do all gas work - plumber should tell you if he is not licensed to work on gas problems).

Answered 4 years ago by LCD

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