Ohms and draft

Ohms and draft are not something that has a direct cause-and-effect relationship in most cases.

The ohms measured by that control are a measurement of the amount of light that is passing over the cadmium sulfide sensor (Cad Cell) inside the burner housing. That “Cad Cell” changes resistance based on the amount of visible light that the lens is exposed to.

That reading can be changed indirectly by the amount of combustion air that is introduced to the flame, which can change the brightness of the flame. Indirectly, draft can change the ohms, but in theory the draft is supposed to remain constant over the fire, and therefore it should have little or no effect on the ohms.

If there is a flame color change that results from chimney draft changes, then your draft control adjustment may need to be addressed in order to stabilize the draft over the fire.

Your query: "So my questions are why increase in ohms and draft and is something still not right?"

There are adjustments that can change the readings, as you have already noticed. The fact that you now have a 0.75 nozzle at 140 PSI with that most recent combustion test, compared to a 0.85 nozzle at 100 PSI, tells me that your air-to-fuel mixture must have been changed in order to result in a zero smoke spot test.

The draft may be more than you need at the breach (the back exhaust connection). A draft of about -0.03 at the breech with about -0.01 over the fire would be ideal. The reading of -0.067 at the breach does seem high, and after all attempts to get a lower draft at with the existing barometric control, perhaps a second barometric draft control is one way to reduce that draft over the fire.

You see, the more draft you have pulling the very hot byproducts of combustion through the heater, the less time there is to absorb the heat from those exhaust gases. That results in a lower operating efficiency.

Oh, that's different. Your original post says the tech replaced the 0.85 with a 0.75 and that the 140lb pump was replaced and left set at 100lb instead of being adjusted to 140lbs…

If you're still getting a significant draft with your damper open, you need two dampers. It's not that common but I've seen some flues draw so much they needed a second barometric damper.

I would go with the smaller nozzle. You say the boiler comes up to temp quickly so you may e overfired for the load. Also a smaller nozzle will lower your stack temp which will increase efficiency.

Your Co2 is on the low side

You barometric damper is not installed correctly.

An .85 45°B @ 140 psi is what should be used with the L1 head and that boiler. Down firing with the LFB isn't even an option. Oxygen is high, CO2 is low. Too much excess air.

Again, forget about the ohms right now. Have the correct nozzle installed. Correct draft set (-.02 over fire. -.04 breach). 0 smoke and combustion test for better results than what you showed earlier.

73% Excess Air?? Something isn't correct. I know how to calculate nozzle/pressure relationships. Original post "…tech replaced the .85 nozzel w a .75 because that was all he had with him. The pump had been replaced 6 months prior but tech left it set at 100psi instead of setting to 140 as per the label on burner…"
That is what I commented on, and it said to me the burner was originally firing at .85 @ 140 and was left at .75 @100 psi.
I see the OP made a different comment later. Still has 73% excess air. I also doubt the air temperature was 80° in the basement.

Actually, there is a version of the WM GO-3 that can accept a lower firing rate. The proper way to do this is to use a 0.65 × 60°B nozzle with the same burner and L1 head. You can even purchase that boiler with the label that reads WGO-3R, which has the 112 MBH input information. However, it would be perfectly acceptable to make that same adjustment on a GO-3 (without the “R”), since everything on that boiler is identical except for the label.

Agree that it is still set up incorrectly.

try this approach:

If you want to get a technician out to your home without creating the impression of an emergency, you might simply tell the service company that "you occasionally hear a noise and are not sure whether it indicates a problem". Intermittent noises do occur from time to time, and explaining that you are unsure whether it is a concern keeps the call from being treated as an emergency that requires immediate attention. This can be helpful from a dispatcher’s perspective when scheduling service calls.

In larger companies with dozens of employees and a fleet of service trucks, calls are typically prioritized based on urgency, location, and workload. I’ve worked in companies with more than sixty employees where dispatching required careful prioritization. I have also been the one-person operation where the schedule was determined by practical considerations—like urgency, location, workload, and making it home in time for dinner so the Marvelous Mrs. Young could feed both me and the kids.

Once the technician is standing in front of the boiler, you can mention that "you occasionally hear a slight rumbling noise, although it may not be happening at that moment," and ask if they would "check the combustion settings while they are there." It is generally best not to bring up technical details such as ohm measurements that may not be relevant to the issue, as that can sometimes distract from the main concern.

You might also mention that the previous technician (if it was not the same person who serviced it last time) "adjusted the pump pressure and that, according to Weil-McLain’s specifications, the burner can operate with a 0.65 × 60°B nozzle at 150 PSI". Since the burner currently has a 0.75 nozzle installed, you could ask whether that change might occasionally contribute to the rumbling noise you have noticed. The technician may even have a 0.65 × 60°B nozzle on the truck and could consider replacing the current nozzle to see if that resolves the issue.

Another recommended option is a 0.85 × 45° nozzle at 140 PSI, although based on your previous experience you feel that may be more firing rate than your home really requires.