Replaced thermocouple, but steam boiler pilot won't stay lit

Eric Scheidler

Yesterday at bedtime my wife observed that the heat was not coming on. I checked the boiler and found the pilot out, figured it must be a worn thermocouple, and resolved to replace it in the morning.

Replaced the thermocouple this morning, as I've done several times before, and yet the pilot would not stay lit. Tried several times to light it, and then did some research on this forum and tried clearing the orifice on the gas line to the pilot. Cleaned up as much of that unit as I could reach with a small wire brush.

Possibly these efforts made the flame bigger when pressing the pilot button, but I couldn't swear to it. In any case, it's still not staying lit. Fortunately the next few days here (Chicago area) will be in the 50s-60s during the day and above freezing at night, so I've got a little time to play with.

Here are a few photos, including a couple of the flame, which is not a steady blue but flashes yellow sporadically. The boiler is a Burnham IN8N.

pedmec

That does look like a good flame. The temporary yellowing is due to dust floating into the flame from cleaning or moving around . did you make sure that the thermocouple has bottomed out in the gas valve?

Do you have a have and know how to use a multimeter?

Eric Scheidler

pedmec said:

Did you make sure that the thermocouple has bottomed out in the gas valve?

Do you mean making sure the thermocouple is fully seated where it threads into the on/off/pilot switch unit? (My first picture). Yes, double checked that just now, still doesn't light.

pedmec said:

Do you have a have and know how to use a multimeter?

I have a multimeter, but I have not used if for a purpose like this. Here's what I've got:

Eric Scheidler

Okay, I found a video for testing a thermocouple with a multimeter. Not sure what to make of what I'm reading, though — the dial moves only slightly, I guess because this tester is designed to be used with a whole range of currents, etc. Image on left is the multimeter at rest, image on right is with contacts on thermocouple:

EdTheHeaterMan

That scale is Amps. You want to check DC volts, however your meter does not have a millivolt scale. The lowest scale is 10 Volts DC.

pedmec

Jesus, where did you find that thing? Are they still making them. I smoked so many of them my first year in the trade I switched to a digital. That's why Radio Shack went out of business because i stopped buying my analog meters from them.

Eric Scheidler

I've had that old multimeter for maybe 20 years. I just picked up a digital one, and I believe I used the right setting on the dial for it: 200m in the DC range (photo of the multimeter below). Correct? With that setting, the thermocouple hovers between 21.9 and 22.1.

pedmec

Your low. You should be close to 30Mv or more. Make sure your thermocouple hasn't slide down out of the pilot assembly. make sure you secured the thermocouple in place. You need to make sure the thermocouple is in the flame.

That's called a batwing pilot hood. it is a three way pilot hood. right and left to light burners and straight to hit the thermocouple. You might need to clean the pilot assembly again. Did you remove the orifice out of the pilot assembly and poke out the orifice holes?

Eric Scheidler

I do believe the thermocouple is seated correctly; it's positioned exactly as the presumably bad one that I replaced this morning. I've removed it and reinstalled it a couple of times now in the process of testing and cleaning.

When I try to light the pilot, the thermocouple glows nice and red.

As for the orifice, I unthreaded the gas supply line to the pilot and tried to clear out around the orifice. However, it's firmly attached to the supply line, so I haven't been able to remove it; wasn't sure it was designed to be removed from the line and didn't want to break it.

Here are some close-ups of the pilot: first unlit, and then from several angles when attempting to light.

mattmia2

It looks like you either need to clean the pilot burner or maybe turn it a bit so that the thermocouple is in the center of that middle flame. not sure if the pilot burner moved or if it is just clogged a little. you can remove the pilot line at the gas valve and pull the whole burner assembly out to work on it. This may be why you seem to have to keep replacing thermocouples because they aren't getting heated that well. some on here say that baso makes better quality thermocouples that last longer.

Eric Scheidler

I just tried a second new thermocouple. I should note that the one I replaced this morning had been in use for several years. I've probably replaced the thermocouple no more than 3 times in 17 years in this house.

The second one also does not stay lit. It tops out at 22.5 mV.

I don't know what else I can do to clean the burner, but I'll give it a shot. If that fails, how about replacing the whole assembly?

mattmia2

It looks like the flame is a little to the right of the thermocouple, maybe you need to twist the pilot burner a little counter clockwise in the bracket to get it centered on the thermocouple

Eric Scheidler

Okay, I've tried everything now: recleaning the burner assembly, blasting it with compressed air, trying to point it closer to the thermocouple. Still won't stay lit.

Tim McElwain

You need to have a professional come and take a complete set of readings. The open circuit with the t'couple disconnected should be 30 MV's or more, then you will need an adapter to take this next reading which is a closed circuit reading, it should be roughly half of the open circuit or a bout 15 mv's, then bring the burner on if you can and see if the closed circuit varies it should stay at 15 or so. Last of all a drop out it should drop out at around 1 or 2 mv.s.

I f you can't get it to stay lit then I would suggest hiring a pro to determine if the magnet assembly in the valve is gone. If so you will need a new gas valve.

retiredguy

The problem could also be with a bad gas valve. I would also reduce the pilot flame size so I envelopes only the top 1/2 of the thermocouple. If the flame heats the lower part of the thermocouple it could cause it not to work properly. I like @Tim McElwain suggest you contact a professional. A lot of the things on these boilers are not for the untrained.

Steamhead

Tim McElwain said:

You need to have a professional come and take a complete set of readings. The open circuit with the t'couple disconnected should be 30 MV's or more, then you will need an adapter to take this next reading which is a closed circuit reading, it should be roughly half of the open circuit or a bout 15 mv's, then bring the burner on if you can and see if the closed circuit varies it should stay at 15 or so. Last of all a drop out it should drop out at around 1 or 2 mv.s.

I f you can't get it to stay lit then I would suggest hiring a pro to determine if the magnet assembly in the valve is gone. If so you will need a new gas valve.

retiredguy said:

The problem could also be with a bad gas valve. I would also reduce the pilot flame size so I envelopes only the top 1/2 of the thermocouple. If the flame heats the lower part of the thermocouple it could cause it not to work properly. I like @Tim Mcelwain suggest you contact a professional. A lot of the things on these boilers are not for the untrained.

This. @Eric Scheidler, where are you located?

Eric Scheidler

As it turns out, I anticipated this most recent advise and called a local HVAC company. I first tried one listed here in Joliet, Illinois, but they don't come as far as Aurora, where I live. The tech figured out it was the gas valve, installed a new one, and we're warm again.

Thanks for all the help. Learned a lot, even if I had to hire a pro in the end.

EBEBRATT-Ed

glad you got it working