New Testo 310 vs. boiler tech's UEI C161

@EBEBRATT-Ed OK thanks. If I were to change nozzles from a 1.0 to a 0.75, could I omit the smoke test? (I don't have a smoke tester but could get one if needed.) Downfiring at the existing air setting should result in even higher excess air, which I would assume means I would start from an assumed zero smoke condition. Then use the Testo to dial down the air to around 10% CO2 to be on the safe side?

@HydronicMike Thanks for your thoughts. Our boiler techs are good and I doubt there's a problem with the head. I will of course check the Z dimension if/when I change nozzles. And I will take your advice and get a smoke tester. Thanks also for the correct procedure.

Not sure what you're trying to say about combustion efficiency. As long as the flue gases are sampled at the proper location (right above the boiler and before the barometric damper), the Testo and the C161 will both give correct readings of combustion efficiency, which is basically a measure of how much heat is being lost as hot (dry) flue gas and hot water vapor. I've verified this by doing the hand calculations for both dry gas and latent heat of vapor losses based on flue gas composition and net temperature, and my hand calcs agree with both the Testo and C161 results. It would probably be news to Testo and UEI that both of their combustion analyzers are wrong.

Since the combustion efficiency for fuel oil can be calculated knowing only the CO2, O2, and net temperatures, there are lookup tables already available. Here's one from TSI Incorporated's "Combustion Analysis Basics," page 12. If you look at the bottom line, you'll see those numbers for CO2 and O2 match ours closely, and for a net temp of 360 (close to our 354), the table gives a combustion efficiency of about 83%, very close to the results given by both the Testo and the C161.

I understand there are other measures of boiler thermal efficiency (such as cycle efficiency), but that's not what we're talking about here, as the Testo and C161 are only measuring combustion efficiency, and are doing so correctly. If and when I try downfiring to a 0.75 nozzle, I will of course run the Testo again and see if combustion efficiency changes, and by how much. I would point out that we're already downfired to 80% of input spec, with no measurable loss of combustion efficiency. Even if we had a spec nozzle, we would still probably be around 83%-85% combustion efficiency.

I do agree that our cycle efficiency is probably in the 60-70% range, but that's mainly because we have massively oversized boilers that have 700+ pounds of cast iron and 10+ gallons of water with a lot of residual heat after the burn, some of which drafts up the chimney. I've addressed this in other posts where I've increased the boiler run times by increasing thermostat swing, and have installed thermal post-purge aquastats that keep the circs running until SWT drops to 90F. Last season we got a measured 9% reduction in oil consumption vs. HDD's from those two changes, but ultimately we still have massively oversized boilers, and downfiring is another strategy for reducing losses by reducing gross stack temps (to within reasonable limits without condensing).

Yes, downfiring could have undesirable effects, but the whole point of having combustion analyzers is to measure those effects in stack temp, flue gas composition, and efficiency. If the effects are undesirable, the analyzer will tell the story. If the effects are positive (little or no loss of combustion efficiency, acceptable flue gas composition, reduced but still acceptable net stack temp), the analyzer will also tell the story. So I'm willing to try downfiring, and let the analyzer show whether the effects are acceptable or not. If the analyzer numbers are good, I'll run the boiler for a year, checking the HX condition for soot, condensation, etc, and measuring oil consumption vs. HDD's, and the numbers will tell the tale. But if the analyzer shows the smaller nozzle won't give acceptable numbers, I'll go back to the 1.0 nozzle.

@HydronicMike Looking at previous year printouts, we had 11%-12% CO2 the first year running the downfired 1.0 nozzles at 140 psi for a 1.2 gph input (boiler spec is 1.45 gph). Then in both of the subsequent years, CO2 dropped to around 9.5%. Nothing changed that I know of, except new nozzles of the same size, and different techs. So I will check for dirty blower wheel, leaking gaskets, etc, as you say. It could also be like @EBEBRATT-Ed said, that the two more recent techs were conservative on soot and added a bit more excess air than was needed.

Yes, we should have an F3 head now at our 1.2 gph input rate, and a 0.75 nozzle at 140 psi would drop that to 0.9 gph, still within range for the F3 head, but I will verify. Also, the 0.9 gph is not quite low enough to need a low fire baffle, but I do have the baffles on hand just in case.

Re the "true" combustion efficiency as taught by NCI, could you point me toward some technical references that explain how they define that mathematically? I'm curious, because the combustion efficiency measurements made by Testo, UEI, and other analyzers use widely-accepted standard formulas for calculating the heat lost to the flue gas and water vapor in the combustion process, and I've gone through those formulas by hand to make sure I understand them. There really isn't anything controversial about them, it's basic thermochemical math. I don't know Jim Davis but I've seen some of his posts here, and I gather he is something of a contrarian on topics like AFUE, but I haven't seen enough of his material to understand what he means by "true" combustion efficiency, and how that might differ from the widely-accepted definitions and equations used by pretty much the entire commercial and residential boiler industry. But I would like to understand what Jim defines as "true" combustion efficiency, and I would read what he has to say with an open mind if you can point me in the right direction.

@hot_rod Thanks for those references on "true" combustion efficiency. I understand the importance of taking correct readings on ambient combustion air temp, selecting the correct fuel in the combustion analyzer, and having a good/calibrated analyzer that will give correct O2/CO/CO2 readings. But as far as I can see, that simply means using the combustion analyzer correctly, as it was designed and intended to be used. I know you and other heating pros are trained on the correct use of analyzers, so by that definition, you're getting "true" efficiency readings when you use the analyzer per the mfr's instructions. And as I showed with my Testo vs. C161 example, when I followed Testo's instructions for correct use of the analyzer, I got basically the same results as my boiler tech did with his C161 (though I did note his lower CO reading which I suspect is a calibration problem, and his incorrect Tambient). So I would say that we both did get "true" combustion efficiency numbers, with the caveat that I think my Testo number is slightly "truer" than his.

Re cycle efficiency, there's nothing more I can do with these boilers to improve that without changing the firing rate. The boilers run 40 minutes at a time, then sit idle for hours because the firing rate is massively oversized for our heat loss. I've maximized run times by increasing thermostat swing, and I've done thermal post-purge to extract as much residual heat as possible. Beyond that, the only other thing I can do is reduce the firing rate, which should reduce gross stack temperatures somewhat, enough to gain maybe 1-2% combustion efficiency. Reducing firing rate should also result in fewer, longer burns, which again reduces the number of times a boiler cycle ends with residual heat in the HX that convects up the flue. So there should be some small gain from that also.

So yes, maybe I'm grasping as straws now, but since a new smaller nozzle only costs $10 and an overall efficiency gain of, say, 5% would save $350/year, I'm willing to try it. And along the way I get the benefit of ongoing education in boiler combustion and cycle efficiencies. 🙂