Properly commissioning high-efficiency equipment — Part 4

I realize that the title of the article is relating to the newer gas fired high-efficiency equipment, but the reality of the matter is that there are more atmospheric appliances out there than there are high-efficiency appliances, and a person needs to understand the basics of combustion before they can move into the high-efficiency arena.

In last month's article, we ended by telling you we would look at the tales of the tape from combustion analyzer readings to analyze the analysis, and make a determination as to corrective actions. I realize that the title of the article is relating to the newer gas fired high-efficiency equipment, but the reality of the matter is that there are more atmospheric appliances out there than there are high-efficiency appliances, and a person needs to understand the basics of combustion before they can move into the high-efficiency arena.

The testing of gas fired equipment requires considerable time to be able to properly analyze, adjust and diagnose combustion problems. A person should allocate about an hour per residential/light commercial appliance for complete testing tuning and analysis on the fire side only. This does not include time, if required, for the complete removal and cleaning of the burner assembly. This time is strictly for the technical analysis side of the equation.

I was recently involved in the testing of numerous light commercial apartment complex hot water heating boilers. My job was to test the combustion efficiency, safety control operations and report my findings and any life health/safety issues. The existing appliances consisted of cast iron atmospheric boilers serving side arm tankless coil domestic hot water heaters, and copper/aluminum finned tube space heating convectors. The boilers were approximately 40 years old. The venting systems were type "B" vent, approximately 30-ft. tall. Combustion air was supplied through outside louvered vents, and the free air was more than adequate for the connected loads. Most of the boilers were within acceptable limits as it pertains to combustion parameters. However some were so far out of range that it required me to disable the appliances and request immediate emergency services to bring them into compliance.

Here are the parameters for boiler number one (pre-service conditions):

  • Oxygen: 5.8%
  • Carbon Monoxide: (too high to calculate)
  • Efficiency: 81%
  • Carbon Dioxide: 8.5%
  • Stack Temperature: 391°F
  • Air Temperature: 77° F
  • Excess Air: 34%
  • Air Free Carbon Monoxide: 2,775 ppm
  • Burner Fuel Supply Pressure: 3.5” W.C.
  • Draft: -.02" W.C.

An internal inspection of the combustion chamber determined that the side wall refractory panels had fallen on top of the burner assembly and the flames were impinging directly onto the refractory, thereby quenching/cooling the flame and causing the excess production of carbon monoxide. Having this much CO in the flue gas stream is definitely dangerous, but if it hadn’t been for the fact that there was 50 ppm CO in the mechanical room, I wouldn’t have been required to shut the boiler down. Having that much CO in the ambient air was an indication of imminent danger to the occupants of the building. Fortunately, the boiler service company was able to correct the situation quickly, so the boiler was only off for a short period of time. The CO spillage was not coming from the boilers draft relief hood, but rather was coming directly from the combustion chamber. The problem with this scenario is that the introduction of CO2 in the combustion air will foul the combustion process, and the production of CO will compound shortly thereafter. The appliance was serviced and brought back on line with the following results (post-service conditions):

  • Oxygen: 9.9%
  • Carbon Monoxide: 77 ppm
  • Efficiency: 75.9%
  • Carbon Dioxide: 6.2%
  • Stack Temperature: 449° F
  • Air Temperature: 77° F
  • Excess Air: 79.8%
  • Air Free Carbon Monoxide: 146 ppm
  • Burner Fuel Supply Pressure: 3.5” W.C.
  • Draft: -.02" W.C.

As can be seen, the tale of the tape indicates a significant reduction in the production of carbon monoxide, and the appliance stopped spilling into the room, however the numbers were still too high. Further testing and inspection determined that the appliance had never been de-rated for operation at an elevation of 6,000-ft. above sea level.

The orifices on the main burner were removed and replaced with the proper size of orifice, and the appliance retested. Post-orifice sizing results were as follows (post-orifice service conditions):

  • Oxygen: 9.3%
  • Carbon Monoxide: 36 ppm
  • Efficiency: 79.7%
  • Carbon Dioxide: 6.5%
  • Stack Temperature: 369°F
  • Air Temperature: 63°F
  • Excess Air: 71%
  • Air Free Carbon Monoxide: 65 ppm
  • Burner Fuel Supply Pressure: 3.5” W.C.
  • Draft: -.02" W.C.

As you can see, the production of carbon monoxide was brought way down without significantly affecting the appliances thermal efficiency.

Tune in next month as we continue to look at more of the actual readings obtained in the field, and what actions were taken to influence those readings. Until then, air free CO hydronicing!

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TAGS: Hydronics