sense rod vs. combination ignition sense rod for intermittent ignition

archibald tuttle

I am working on a Burnham 500BN that was commissioned with a Johnson G775RHA-2.
The pilot burner assembly has a separate sense rod. It sparks up like we the people but is either failing to produce sufficient amperage from the sense rod or the control is, how ironic, only responding intermittently :-).

The cheap meter I had with me last night showed no microamps. I know that can't be right as it is right on the edge of working and holds sometimes but not others.

So the Johnson guys look at me crosseyed when I called cause they haven't made these in 10 years. I'm like, you didn't expect your equipment to last 10 years (this is actually a 2000 install, so 22).

Common replacement for the module looks to be honeywell/residio S8610U. The residio 8610, like the G77x controls, seems to be able to take either single wire ignition/sense integral or two rod approach. (comes jumped from the ignition terminal to the sense terminal, I assume maybe there is a temporary cutout so the sense circuit doesn't see 10,000 volts. In any event, install instructions direct the sense jumper wire to be removed if separate sense rod is used.

I see a fair number of residio pilot burner kits with the single rod approach (e.g. although this looks like it has a second rod included but the text does not so indicate) , some with and some without wires. (the ignition connection would have to be changed to female spade to install what is there now the S8610U vs. the 'karnak the magnificent' or whatever the other standard is called). But, it might also be that I could just replace the extant sense rod and wire, if I can get it out of the pilot burner assembly. But I don't readily search up the sense rod itself as a part. Don't know how many manufacturers were making their own version and whether retaining threads, insulated rod diameters, etc. where consistent across brands.

This setup has been bomb proof for 22 years. Is there a preference to replace with the single rod style; to replace the sense rod if I can get the old one out and reference it; or a new pilot assembly with the two rod format? Maybe this is all a waste if I can get a better ammeter to show me that I make the minimum (minimum .15 microamps according to the old johnson literature).

thoughts? thanks!

brian

EdTheHeaterMan

I don't have a preference. I go with what ever works. I have been known to add a separate flame sensor on some really old stuff where the ground path of the flame rectification circuit has a lot of rust and corrosion. More often I have added a ground wire to the mounting bracket of the pilot burner and connected it directly to the control ground terminal.

I remember there was one ribbon burner, opposite to the ignitor end of a row of burners on one particular boiler. That last burner did not always ignite and would send raw gas up the heat exchanger for up to a minute or more before a large PUFF would ignite that last burner. I places a flame rod sensor at the far end of that row of burners. If that last burner did not light, the gas valve would close and try again. The second try always worked.

And before anybody says anything about cleaning the burners and the flame transfer channels, and on, and on... Already done... and air adjustments and gas pressure adjustments and spent too much time trying to get it right. Besides, that was years ago and I'm sure that customer has moved and someone else replaces the boiler by now.

archibald tuttle

@EdTheHeaterMan good point about the ground. I had thought of that but didn't add it to my novel above. that looks to be the manual practice for clean install nevermind a few years of rust down the line. I'm going to clean up pilot base and add a ground wire late this morning when i go back.

EdTheHeaterMan

archibald tuttle said:

@EdTheHeaterMan good point about the ground. I had thought of that but didn't add it to my novel above. that looks to be the manual practice for clean install nevermind a few years of rust down the line. I'm going to clean up pilot base and add a ground wire late this morning when i go back.

Make sure the wire has a green insulation cover... so it knows that it is a ground wire! and use 14 Gauge or larger. If you use 18 gauge it might think it's a fan wire, and you never know how that will turn out.

archibald tuttle

@EdTheHeaterMan you mean so the inspector knows it's a ground wire . . . :-) actually, i'm digging around to see if I can find that stove repair wire with the heat resistant jacket, not that its a problem if the ground is exposed by jacket failure; but don't need melted coating under there, even if thhn has a higher temp thermoplastic on it.

archibald tuttle

@EdTheHeaterMan

"I'm sure that customer has moved and someone else replaces the boiler by now."

that seems to be the only remedy taught these daze. pilot won't hold, replace the boiler. bad gas valve, replace the boiler. leaking pressure relief, replace the boiler . . .

I'm just too old school.

mattmia2

Bare copper would work just fine for the ground. Perhaps the ground from a scrap of romex.

EdTheHeaterMan

mattmia2 said:

Bare copper would work just fine for the ground. Perhaps the ground from a scrap of romex.

Exactly... As long as the wire knows it is supposed to be a ground. That is the important part.

archibald tuttle

CORRECTED. so back with tester and this time i put the leads in the microamp ports. Getting .3 microamps from sensor rod so tgat proves out. Min allowed is .15.

it is different than the symptoms from yesterday, when it makes the control , instead of starting the main burner it drops the pilot.

So i pulled the MVlead just to test start the boiler and read the amps on the main valve to make sure it wasnt drawing to much. Think i had the amp range set right and boiler lit rightup with the valve drawing 3.6 milliamps. At least that is what i took it to be from the range i had set.

So that didn't seem excessive. But i did notice that without the valve load on the control, the pilot didnt drop out the way it had been. So the draw to the mainvalve seems to be killing the control. Since the draw doesn't seem out of hand, i think its the control. Anyone seen a failure mode like this?.

Ill know soon enough. a new 8610 on the way for tomorrow. Meantime, tonight im an old fashioned boiler tender. Just running it manually while im sitting here for an hour and a half. shades of mike mulligan and the steam shovel.

Thanks
. Brian.

mattmia2

What is the gas pressure on both sides of the regulator in the valve? Sounds like the gas supply is too low and the pressure is dropping too far when the main valve opens.

archibald tuttle

@mattmia2
Thats a good thought for explanation but dont get a weak attempt at mains and the pilot drops instantaneously., It doesnt kind of pull back or soften. And get nothing from main.

 and its we the people if i play the control. I pulled the mv wire   let the pilot light make but then created a temporary direct 24V connection for MV and lights right  up as it should if the control were working.


So  im atill thinking its in the black . . . Eer blue . . . Box.

And, im thinking i still dont have range right measuring gas valve amps because while i. Sitting here acting as the human safety i took a flashlight to the gas valve and it should be drawing maybe 400 or 500 milliamps for the MV, (its 700 all told).  I was reading high 3s and low 4s but in single dogits  i anticipate the range is off but im going to bring autoranging tester tomorrow.  If its really drawing 3.x amps, the gas valve is way over nameplate and prob. Over the control rating. Havent gone back to the literature yet.

mattmia2

Are you measuring dc current for the flame rectification and ac current for the valve measurement?

You can measure the voltage to the control, to the mv and to the pv when it lights then shuts down to see if the control is killing the pilot or if maybe a control has bad contacts and is causing too much voltage drop.

You could test the meter's current measurement with a resistor and an ac and dc voltage source.

Steamhead

@archibald tuttle , makes no difference which sensing setup you use. I'd just install the S8610U with the present two-wire setup (making sure you set the DIP switches as directed in the cross-reference), make sure you have a good ground, clean the sensing electrode on the pilot assembly and call it a day. The S8610U has some handy built-in diagnostics that will help you out in the future, so you'll be ahead on the next call there.

archibald tuttle

thanks @Steamhead and @mattmia2 more good diagnosis/rubber hits the road info. 'film' at 11 when i get home tonite.

archibald tuttle

S8610U to the rescue. pretty close to plug and play as was suggested—with the exception of adapting the spark wire terminal.

brian