ways to screw up replacing a thermocouple? (pilot won't stay lit)

[Jim]

I found my pilot light out on my furnace recently and the thermocouple looked like it was perhaps the same age as the furnace (~15yrs). Relighting attempts failed. The furnace had been serviced recently due a home sale, and I'd hate to have an $80 service call for an $8 part.

So, off to the hardware store, standard 24V 48" thermocouple purchased (longer than the one it replaced), got it into the furnace and ... pilot still won't stay lit after being manually lighted. I manually kept it going for over a minute on each of several attempts only to watch the pilot go out after releasing the button.

Now, the thermocouple doesn't appear to sit directly in the flame on this model, but mighty close. Without bending the actual thermocouple since the tc and pilot are in fixed positions in the bracket, the way the pilot burns doesn't seem to hit the tc directly. The old thermocouple wasn't bent either.

Any clues or thoughts other than "call for service" (which of course would be the next step.).

Should the tc be bent into the flame or does that ruin them?

Thanks for any help!

Jim

[-zero]

The tip (approximately 3/8 of an inch) MUST be submerged into the flame.

The thermocouple must not be bent at all!

Your furnace has safely not allowed you to light the pilot by design. A minimum pilot must be established to safely light the main burner/s. The distance between the pilot burner and thermocouple ensures the flame is large enough. The placement of the pilot assembly onto the main burner, as well as the flame pattern of the pilot is engineered into the manufacturers design for a dependable and safe system. If the pilot flame was not large enough or mounted in the wrong position to light the main burner immediately, a delayed ignition (read BOOM) will occur.

The pilot assembly may be dirty, or the fuel tube to the pilot may be leaking gas, or a good size list of other possibilities are causing the pilot flame to be inadequate.

If it was only the thermocouple, you would be OK with replacing it. However, your attempt to correct this problem, to get the burner to start, may leave your furnace in a very unsafe state. No one over the internet can say "Yah, everything looks good ". :-)

Ask around for a better recommendation if you did not feel good about the last service company. Good luck.

-zero

[Jim]

Good to know! It is old and that wouldn't be its only issue.

If I wanted to test the old thermocouple by holding it in, say the flame of the gas range burner, could a digital multimeter be used to see if the old one was even bad?

[-zero]

Yep. Without a load (little coil inside the gas valve) you should have a minimum of 30 milavolts and probably show between 30-80 mv.

-zero