A question about flue gas condensation (and combustion test results)

fentonc

I always see it mentioned that the return water temperature in a non-condensing natural gas boiler needs to be above ~135F or so to prevent condensation, which can damage the boiler. When I watched the recent video from @RayWohlfarth on the 'non-condensing condensing boiler', I noticed the chart towards the end showing the relationship between 'percent oxygen' and dewpoint temperature, showing a dewpoint of 133F at 3% and all the way down to a dewpoint of 116F at 10% oxygen.

I'm always interested in better understanding my (heavily instrumented) boiler, and these were the combustion test results when it was checked on 9/30/22:

• CO2max: 11.8%
  
• O2: 10.2%
  
• CO: 4 PPM
  
• Ambient CO: 0 PPM
  
• Excess air: 117.7%
  
• CO2: 5.42%
  
• Undiluted CO: 9PPM
  

From my reading thus far, these results seem 'sub-optimal.' From my own monitoring, I know that my Weil-Mclain Gold CGa-PIDN-5 has a nominal input of 140K BTU/hr, but when clocked at the gas meter, it only uses 120K BTU/hr. Is that likely contributing to what seems to be very high excess air / O2?

Also, aside from that, Ray's chart seems to indicate that my flue gas has a condensing temperature closer to 115F or so than 135F. I think the aquastat in my boiler lets me set the high-limit as low as 135F (so it would operate 115F-135F), but I've kept it at 160F for fear of unwanted condensation - is it likely safe to lower it significantly?

EdTheHeaterMan

Safe Yes.
Advised not likely.

fentonc

Why is it not advised? The baseboards in my house are sufficiently oversized that the worst-case zone would be fine with ~115F water at the design temp, so the closer I can get it to continuous operation (and the lower temp i operate at), the less likely I am to hear the annoying expansion noises when a zone calls for heat.

EdTheHeaterMan

You have a non condensing boiler. You do not want to operate the boiler with lower return water temperature on a consistent basis. You want to reach 140° return water temperature for enough time to evaporate that condensed flue gas droplets from the boiler surface... like 5 minutes minimum. That might be a 8 to 10 minute cycle on baseboard with a average water temperature of 145° to 150°. If you make your high limit setting at anything lower than 160° your boiler will not evaporate the condensation droplets.

This might be a good thing, then you could purchase a condensing boiler after your non condensing boiler fails

RayWohlfarth

@fentonc Thanks for watching. That chart showed the condensing temperature but that is the return temperature. The supply temperature should be around 20-30 degrees F higher. That is why you want to set the boiler minimum temperature of 140 degrees F

fentonc

@RayWohlfarth - Good point that we care about return temp, not supply temp. With 160F SWT, the measured delta-T across my three zones is 12.5F, 10.5F and 8F, respectively. When the SWT dips down to 140F, the measured delta-T for the three zones is 9F, 7.4F and 4.9F, respectively.

fentonc

So according to this chart, with my measured 10.2% O2/5.42% CO2/117% excess air, my dew point is probably somewhere between 115F and 110F. So long as the RWT is kept above that range, is that (at least theoretically) sufficient to prevent any kind of sustained condensation?

RayWohlfarth

@fentonc It also depends on the combustion air temperature That affects the dew point temperature and the excess air %. Years ago, I remember there was a dew point controller that would adjust the firing rate to keep the boiler flue gases above the dew point temperature. Not sure if they are still around.

fentonc

@RayWohlfarth - It's an atmospheric boiler, so it draws air from the conditioned basement zone. When the combustion analysis was performed the air temp in the basement was 67F, otherwise it spends all winter between 62F-65F.

EBEBRATT-Ed

@fentonc your Co2 is too low this could be from your low gas input if that is accurate. I would suggest getting it tuned and combustion tested

fentonc

Those numbers are after getting it tuned and combustion tested, which raises the next question - what mechanisms in it are actually 'tunable'? Some kind of gas valve or something? The manual for it doesn't seem to have anything on this topic.

Zman

Has the boiler been properly cleaned and serviced lately? Do you know what your gas pressure is at the header? Your combustion analysis and meter clocking are both saying you are underfired.
Do not use the fact that the boiler is not running right as an excuse to turn down the water temp. Also, keep in mind that the galvanized vent can also be damaged by boiler gas condensate, particularly if it is in an unconditioned space.

EdTheHeaterMan

fentonc said:

Those numbers are after getting it tuned and combustion tested, which raises the next question - what mechanisms in it are actually 'tunable'? Some kind of gas valve or something? The manual for it doesn't seem to have anything on this topic.

I found this in the I/O manual for the WM CGa boiler.. I only added 2 sections of the "See following pages for instructions". If you want to see all of them they are here https://www.weil-mclain.com/sites/default/files/field-file/CGa Series 3 Boiler Manual 0520.pdf
I guess that after just general cleaning of the actual parts that make the air and fuel come together to make the flame, and being sure that the boiler is getting sufficient combustion air, the only thing left is to check and adjust the gas pressure. After that, the boiler just needs to operate at the designed temperature.

hot_rod

I see the Burnham X boiler manual has some guidance on flame quality.