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Combustion Testing

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To secure safe and efficient operation of gas or fuel oil burners it is common to test the burners for

  • carbon dioxide
  • smoke
  • excess air (O2/CO2)
  • stack temperature
  • draft
  • NOx

Carbon Dioxide - CO2

When the CO2 content of the flue gas is low (less than 8 percent) heat is lost up the chimney and the operation is inefficient.

Low carbon dioxide content may be caused by

  • to small burner nozzle
  • air leakage into the furnace or boiler
  • underfiring in the combustion chamber

When the CO2 content of the flue gas is too high it is common with excess smoke in the flue gas. High carbon dioxide content may be caused by

  • insufficient draft
  • overfired burner

Smoke (only fuel oil burners)

Smoke in flue gas indicates poor burner performance. The amount of smoke can be measured with a smoke tester where smoke particles set on a filter paper are interpreted according a Bacharach scale.

Smoky combustion can be caused by

  • soot formation on the heating surfaces
  • insufficient draft, incorrectly adjusted draft regulator, improper fan delivery
  • poor fuel supply, malfunctioning fuel pump
  • defective firebox
  • oil-burner nozzle defect, wrong size
  • excessive air leaks in boiler or furnace
  • wrong fuel-air ratio

Stack Temperature

The "net stack temperature" is the difference between the flue gas inside the chimney and the room temperature outside the burner. Net stack temperatures above 700oF (370oC) are in general to high. Typical values are between 330 - 500oF (160 - 260oC).

High stack temperatures may be caused by

  • undersized furnace
  • defective combustion chamber
  • incorrectly sized combustion chamber
  • excessive draft
  • overfired burner
  • draft regulator improperly adjusted
  • soot formation on the heating surfaces
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Related Topics


Combustion processes and their efficiency. Boiler house and chimney topics. Properties of fuels like oil, gas, coal and wood and more. Safety valves and tanks.

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