Why are gas burners set up with more air than the exact amount needed for complete combustion?
- To raise the temperature of the burner flame as high as possible
- To increase the carbon dioxide percentage in the flue gas
- To keep the flue gas from rising too quickly in the vent
- To make sure all the fuel finds oxygen and burns completely
Show the answer
D. To make sure all the fuel finds oxygen and burns completely
Mixing in a real burner is never perfect, so some excess air is supplied as a safety margin against carbon monoxide. It comes at a cost: extra air carries heat up the vent and lowers the flame temperature, so too much of it reduces efficiency.
Reference: Combustion fundamentals; excess air
A combustion analyzer shows a higher oxygen reading and a lower carbon dioxide reading than the manufacturer's range. What does this indicate?
- Too little excess air, which raises carbon monoxide
- Too much excess air, which lowers efficiency
- Too high a gas pressure at the manifold
- A blocked vent that is holding in flue gas
Show the answer
B. Too much excess air, which lowers efficiency
Oxygen left in the flue gas is air that passed through without taking part in combustion, and it dilutes the carbon dioxide. High oxygen with low carbon dioxide therefore means excess air or underfiring. Too little air would show the reverse, with low oxygen.
Reference: Combustion analysis; oxygen and carbon dioxide
During tuning, a furnace's net stack temperature rises while its oxygen reading stays the same. What does this mean for efficiency?
- Efficiency is lower, because more heat is leaving through the vent
- Efficiency is higher, because the flame is burning hotter
- Efficiency is unchanged, because oxygen is unchanged
- Efficiency is higher, because less moisture condenses
Show the answer
A. Efficiency is lower, because more heat is leaving through the vent
Heat that goes up the vent is heat that was not transferred to the house. With the same excess air, a hotter stack means a larger share of the fuel's energy is being lost. Poor heat transfer from low airflow or a fouled heat exchanger is a common cause.
Reference: Combustion analysis; stack loss