Combustion Theory: Variables


 Accounting for variations in oxygen and fuel

For any burner-boiler combination, there is an ideal “minimum excess air” level for each firing rate over the turn-down range. Usually, burners require much higher levels of excess air when operating near their minimum firing rates than they do at “high fire.”

More serious factors than dirty fan wheels and dampers inhibit air flow. Varying oxygen content in the air changes the ambient air conditions and effects the input of oxygen into the combustion process.

Hot and Cold Days

On a “standard” day of 60°F, 30 inches barometric pressure, and 45% relative humidity, seasonal temperature and pressure changes, you must take into account that a burner has to deal with additional variables.

When it seems harder for us to breathe on a hot, humid summer day, burners have a problem too. On a hot, humid day, the oxygen flow drops by almost 20% and burners that can’t adapt for this “oxygen lean” air will smoke, soot, and produce noxious emissions.

On a cold, dry winter day, the air flow would increase by 10%, and the burner must adapt.


Variations in pressure across the metering valve and fluid viscosity have the greatest effect on fuel flow.  Viscosity can vary from delivery to delivery and can be affected further by temperature changes.

Having thick oil in burner supply line can reduce the pressure at the metering valve while having thick oil in the return line can increase the pressure at the valve.

Since the burner must be set up to operate cleanly under worst case conditions, enough excess air must be provided to burn any additional fuel that the metering device at the burner may introduce as well as to ensure that the metering device at the burner may introduce as well as to ensure that there will be sufficient excess air available on a hot, humid summer day.

There is no way to prevent heat and humidity, but fuel flow can be closely controlled with the appropriate controls.

Controls are essential to the boiler-burner combination because they will reduce the amount of excess air wasted during weather and fuel changes.

Other posts in this series:

  • Understanding Local Law 87 – and laws like it
  • Combustion Theory: The Basics
  • Combustion Theory: Variables – Account for variations in oxygen and fuel
  • Combustion Theory: Efficiency – Calculate efficiency and losses
  • Combustion Theory: FGR – See how flue gas recirculation reduces NOx
  • Combustion Theory: Combustion Controls – Learn how cutting-edge tech can cut your emissions
  • Combustion Systems: Design – Basic principles to follow when designing your combustion system
  • Combustion Systems: Troubleshooting: Burner problems and their causes
  • Combustion Control: Strategies – Linkage vs. Linkageless, and why you should care

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