Soot in boiler - and how do you spell stoichiometric

@HydronicMike interesting points. Here's how I've been thinking about the general wet kit thing as I've moved towards more dependance on it (comments/corrections from anyone welcome of course):

My understanding is that the Fyrite/Bacharach wet kit CO2 analyzer is comparable to the electronic combustion analyzers in terms of accuracy, that as far as CO2 is concerned the main difference is convenience, which is admittedly huge for pros who have hundreds of calls a year. (Obviously the CA's give instantaneous readings, which means you can fine tune the air and see instant results while I have to pump the little bladder thing 18 times and play with the dumbbell and let the liquid settle and take a reading every time I change the air, all the while hoping the boiler doesn't stop and I have to let it cool down to run again!) But I think the results are supposed to be very dependable.

The other big advantage of the CA's is that they measure CO, but I'll get to that below.

Back to CO2, you mention that "You don’t know your Excess Air or your CO, so you may get lucky with your adjustments, maybe not." True I don't know my CO directly (again, more on that below) but my understanding is that measuring CO2 is the same as measuring excess air, so when I take a measurement I do know it (sort of, see next paragraph), since the Bacharach wet kit has that slide rule thing that tells you what the excess air is given your CO2 measurement, in fact it seems the whole point of measuring the CO2 is to calculate the excess air—and the electronic CA's similarly calculate the excess air internally from their measurements (I think they measure O2 instead of CO2, but it's two sides of the same coin, in fact I think you can measure O2 if you want with the wet kit if you fill it with the blue liquid instead of the red).

But do I REALLY know my excess air? I think the following might be what @HydronicMike might be thinking about:

I was surprised to learn a couple years ago on this forum (maybe it was @HydronicMike that mentioned it, for which I'm grateful) that I can get, say, a 12.5% CO2 reading, which on the face of it seems good, yet still unknowingly have very low air and also high CO! How could that be? Most of you guys know why, but it confused the heck out of me; now I think I see that you can still get a seemingly rosy 12.5% if you are a certain distance on the bad side (the left hand side) of the stoichiometric curve (which curve I had to stare at for a few days or weeks to understand).

It seems to me now that on the bad side of the curve the reason less CO2 is produced (specifically, less than the 15% or so at optimal combustion with just the right amount of air) is because there is not enough air for full combustion, resulting in the CO byproduct which detracts from the amount of CO2 and so you get a lower CO2 percentage (and smoke, and CO) due to this incomplete combustion; whereas on the right hand side (the good side) less CO2 is produced percentagewise because there is just that much more air, more than is needed, but combustion will be relatively complete. It seems to come down to that being on the right hand side with 12.5% due to excess air (cooling the flame, true, and so creating inefficiency) is a whole lot better than 12.5% on the left due to not enough air (creating smoke and CO).

The takeaway for me is that @HydronicMike you're right, without the CO measurement I'm not 100% positive that my nice 12% reading (which I hope I can get) is on the right side of the curve. @EBEBRATT-Ed mentioned that if I have draft, whatever CO I have is probably going up the chimney; I also wonder if having decent draft almost automatically means my 12% is probably from excess air and not from not enough. But another thing that I've been relying on is this: The stoichiometric curves slopes down on either side of the center ideal point, with the result that if, when I increase the air, I see a drop in CO2 percentage, I can be pretty sure I'm on the right side of the curve and CO should not be a concern. On the other hand, if I increased the air and saw the CO2 rise, I'd probably be on the wrong side of the curve. So far I've only seen it drop with more air, making me pretty confident I'm on the right side of the curve.

True, more information is always better, and it would be nice to measure CO directly, but I would have to buy a dedicated CO detector appropriate for measuring in a boiler stack, if they make such a thing, which I would do if anyone can recommend something that doesn't cost as much as a combustion analyzer!

But my thinking is also effected by my last line of defense, which is a 'CO Experts' CO detector (for ambient room air, not something you can stick in a boiler flue), which I first found out about on this forum. The thing goes off if it measures 5 or 7ppm, I forget, which is darned sensitive, several times more sensitive than the standard Home Depot UL approved plug in detectors, of which I also have two. I keep the CO Experts one outside the bedroom, and leave it in the basement for a while after the boiler gets tuned: crude, maybe, but some reassurance.

But is anybody out there also a homeowner who invested in an electronic analyzer? I'm happy with my wet kit so far, but wouldn't mind another fancy gadget if it were affordable and not just another headache.

And @EBEBRATT-Ed thanks for your faith in the wet kit and I will try exactly what you said with the air bands.