Boiler Energy efficiency tip of the day (Excess O2)

1. All combustion requires the correct measure of oxygen; too much or too little can cause undesirable effects. However* the error is almost always intentionally on the high-side (too much oxygen) because the main effect on the high side is low efficiency.
2. Too little air results in carbon monoxide formation* sooting and even explosion if accumulated soot and other non-combusted suddenly get enough oxygen to rapidly burn.
3. When boiler burners are manually tuned on a periodic basis* they are typically adjusted to about 3% excess oxygen which is about 15% excess air. This is because there are many ambient and atmospheric conditions that can affect oxygen/air supply. For example* colder air is denser and contains more oxygen than warm air; wind speed affects every chimney/flue/stack differently; and barometric pressure further affects draft.
4. Therefore* an excess oxygen/air setting at the time of tuning assumes there will still be enough oxygen available for complete combustion when conditions worsen.
5. From an efficiency standpoint* the excess O2 means there is more air in the combustion stream than there needs to be. That air also contains moisture* and it all is heated and then lost up the stack.
6. The amount of excess O2 is about directly proportional to the efficiency lost; that is* 3% excess O2 means 3% efficiency drop.
7. Although it may be possible to monitor and adjust the burner on a daily basis* it is not practical and efficient way to control.
8. Automatic O2 systems continuously monitor the flue gases and adjust the burner air supply. They are generically called ‘O2 Trim Systems’.

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