Combustion Gas Analysis

Big-Al_2

I have a ten year old natural gas fired Burnham IN-6 steam boiler in my house. It is oversized and was short cycling and was hooked up to a marginal-sized chimney liner. After consulting Burnham, I down-fired it by 10% by replacing the orifices. I wanted to make sure everything was still safe, so I borrowed a Testo 330 from a service-tech friend.



Here are the results:

466 F T Stack

7.47% CO2

80.4% EFF

50.7% ExAir

7.6% Oxygen

1 PPM CO

2 PPM Undiluted CO

70 F Ambient



I just flushed the boiler very well with a pressure hose and an open 2" drain tapping, so I'm sure it's clean inside. The flueways look clean with an inspection mirror, but I didn't brush them. The boiler was running for about 15 minutes. It was steaming, but had not built any pressure. (The main vent closed right when I took the reading.)



The Stack temp seems a lot higher than the large commercial boilers at work. Is this normal?  Is the Excess Air too high?  Would I be better off with the stock orifices, even with short cycling?

Joe V_2

scroll to the right page on this document

http://www.tsi.com/uploadedFiles/Product_Information/Literature/Handbooks/CA-basic-2980175.pdf

Mike Kusiak_2

Numbers look normal

The measurements you have look normal and right in line with other Burnham atmospheric boilers. Here are some readings I took recently on a stock unmodified Burnham series 2  hot water boiler.



465.1F T Stack

2 PPM  CO

80.2 % Eff

50.7 % Excess AIR

7.6 % Oxygen

3 PPM CO Air Free



So your downfiring really has not hurt the basic combustion efficiency. Unfortunately, due to flue gas condensation concerns, these atmospheric boilers are designed for high stack temps and excess air. Its designed to be an 80% boiler and that's how it measures!

Tim McElwain

First off I do not

like derating boilers and especially steam.



Does this unit have the pressuretrol set to 2lbs off and a 1/2 lb on?



You may be getting leakage air as I read your actual calculated efficiency with the O2 and CO2 readings you gave to be less than 80% is the 466 net stack?



My calculations put you at around 75% efficiency and on that boiler derated 10% your thermal efficiency is around 60%.



Keep in mind your boiler may be short cycling on the pressuretrol and not the thermostat. It is normal for it to cycle that way on a residential application. How have your bills been for the past 10 years?



If this boiler has not been taken apart and the sections cleaned in 10 years then it is due as scale and rust affect the heat transfer.



What is the BTU input on the rating plate? What did you clock the burner at after orifice changes? What is the inlet and outlet gas pressure to the gas valve?



What is the gas valve on the unit?



What is your draft reading before and after the draft hood?

Big-Al_2

Will take more readings

Thanks for the replies! 

I put the original orifices back in last night.  The boiler was cycling off of the 2 PSI Pressuretrol again, and the radiator vents were noisy enough to wake me up this morning.  With the smaller orifices, the boiler seldom builds more than a pound of pressure, and the vents are almost silent.



The boiler is an IN-6, with a 175,000 BTU input.  The gas meter clocked right at 175,000 BTU to begin with and about 158,000 BTU with the smaller orifices.

 

The combustion readings were measured with the probe inserted through the draft diverter, well into the chamber above the boiler flueways. (The draft hood is basically built right into the boiler.) That's about as close to an undiluted reading as I could come up with.



We moved into the house four years ago.  I replaced three missing radiators since then (risers were capped) and corrected a number of system issues, so even if I had ten years of gas bills, I don't think I could draw any conclusions from them.



My HVAC tech friend plans to come over after work tonight.  We will look at it together and take some readings with the original orifices for comparison.  I'll try to gather more information and will post again.

Jim Davis_3

boiler efficiency

O2 - 6-9%  7.5% not bad, but lower is better.  Readings must be stable and not falling whatsoever once the lowest O2 is recorded.

Flue Temp should be minimum around 500 degrees, so you are a bit low

CO - no problem at this point.

Not sure why you are operating at 2# steam instead of 1/2# or 8oz.

                     Efficiency -                      100% Maximum

                         Losses                       - 14% H2O non-condensing

                                "                          - 7.5% O2

                                "                         - 13.5 Net Flue Temp

Corrected efficiency approx                      65%  Based on combustion numbers.  Does not totally include additional thermal losses

Based on past field evaluations I would say your efficiency is off around 5-7% at the most from the maximum attainable.  Another few points could be added if you lowered your maximum operating pressure.

Big-Al_2

Combustion Take Two

My service-tech friend and I spent some time with the boiler last night.  He had contacted Burnham, and they told him that the boiler was designed to operate with high stack temperatures and plenty of excess air.  In residential situations, they wanted to make sure that there wouldn't be any condensation issues with old chimneys.



With the original orifices, the combustion and efficiency numbers were similar to the downfired situation, but like I said, the radiator vents were noisy.  There isn't much that can be adjusted on the burner.  He put the smaller ofifices back in, and checked the gas manifold pressure.  It was a little high, about 3.25".  He dialed it back to the factory spec. 3" and took some new readings.  The stack temperature went down to about 390 F, and the efficiency went up to 82%.  The radiator vents are almost silent again, and the system builds pressure nice and slowly, so it won't be cycling off the Pressuretrol except perhaps after a deep-long setback.   That's how we left it.  We'll see what happens to the gas bills.



I would go out and clock the meter, but it's 40 degrees out, very windy, and pouring rain here.   Ahhhh, November in Wisconsin!  At least it isn't snow . . . yet . . .

Jim Davis_3

Burnham boilers

I sold burnham boilers for years.  Not sure the rep knew what he was talking about.  Flue temperatures of all standard boilers should be in the same range.

Steam boilers from 490 to 600 degrees depending on the steam pressure and the cleanliness of the boiler.  When talking about excess air the rep might have meant above the drafthood.  Assuming the dilution air through the drafthood is 50% the actuall flue temperture above it may only be 230 degrees and the chances of condensation are greatly increased.

You will most likely see an increase in fuel usage but then again, maybe not.  

MrDvorak

Warm-up time

Hello Al,



Do you see any significant difference in the time-to-first-steam after the burner modification?

Big-Al_2

Rep

Burnham rep said to expect 450+ stack temperatures, 50 to 60% excess air, and about 10% oxygen in the flue gas.  Heating bills? Time will tell.  The efficiency values from the stack analyzer look better, but how the system reacts as a whole might be different.

Big-Al_2

Significant?

Time to steam? Hard to tell.  I guess it depends on how hot the boiler was to begin with.  I think I'm still hearing the main vents close about twelve minutes after the boiler comes on from dead cold.  The boiler comes on at 5:30 AM, the same time my alarm goes off . . . so I'll have to listen tomorrow morning.

Jim Davis_3

Analyzer efficiency

There is not a combustion analyzer made that can even come close to calculating efficiency correctly.

First they are based on firing at full input only!!

Second they are based on testing under full load or design conditions.

Third they are based on a theoretical btu value of fuel

Fourth they rarely calculate the full latent heat loss, up to 14%.

65% is the best you can get under ideal conditions.

MikeyB

CO Training

Jim any chance of NCI CO/Combustion AnalysisTraining coming to NYC or Long Island in the future? I noticed NJ is scheduled for next week on NCI's website, Thanks Jim.