UEi Test Instruments C127OILKIT - Manual (Page 17)

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• Excess Air
- Theoretically perfect combustion would have the exact amount of O2 to combine with all of the fuel,
resulting in no excess air.
• If perfect combustion were possible, there would be 0% excess air
• Because perfect combustion is not possible, you will always have some excess air
• Too high excess air will reduce the time fuel and oxygen are together in the combustion
chamber, and lower the temperature – two of the T’s of combustion
• Too little excess air, and all of the fuel will not be burnt
- Carbon will form CO instead of CO2
• Typical Excess Air Level
O2% (measured) Excess Air %
Natural gas 3% 16.7%
Light Oil 5% 31%
Coal 8% 62%
Excess Air is also one method that can be used to determine heat exchanger integrity
• Before the blower starts the pressure is higher on the combustion side of exchange (see figure 5)
– Small amounts of CO may leak into the living space side
– Flue gas measurements will be stabilizing as the combustion chamber heats up.
• After the blower starts the pressure on the primary side will be higher (See figure 6)
– Blower forcing air through the heat system
– Fresh air may now enter the flue through the breach in the heat exchange
• Observe the flue gas levels and check the following;
- Excess Air, 02 level and CO in ppm for drastic changes.
figure 6 figure 5
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