BPI Combustion Safety 5 — Questions and Answers
Question 1: What is the role of excess air in combustion, and what happens when excess air is too high?
- Excess air prevents CO; more is always better
- Excess air ensures complete combustion, but too much increases stack losses and reduces efficiency (Correct answer)
- Excess air lowers flame temperature and prevents NOx formation with no efficiency penalty
- Excess air has no effect on efficiency in residential appliances
Correct answer: Excess air ensures complete combustion, but too much increases stack losses and reduces efficiency
Some excess air is needed for complete combustion, but too much carries heat up the flue, lowering steady-state efficiency.
Question 2: In a naturally drafted gas appliance, what causes the 'chimney effect' that moves combustion gases upward?
- Positive pressure from the gas burner ignition
- Buoyancy of hot flue gases relative to cooler outdoor air (Correct answer)
- The venturi effect of air flowing past the flue cap
- Forced draft from the combustion air blower
Correct answer: Buoyancy of hot flue gases relative to cooler outdoor air
Hot flue gases are less dense than cooler outdoor air, so they rise buoyantly up the chimney — this thermal buoyancy is the mechanism of natural draft.
Question 3: What is the minimum recommended CO alarm placement height in a room per most US codes and BPI guidance?
- On the floor, since CO is heavier than air
- On the ceiling or high on the wall, since warm CO-containing air rises
- At breathing height (5 feet), since CO is the same density as air (Correct answer)
- Adjacent to each gas appliance at appliance level
Correct answer: At breathing height (5 feet), since CO is the same density as air
CO has nearly the same molecular weight as air and disperses evenly; most codes specify mounting at breathing height or follow manufacturer instructions, which typically call for a central wall location.
Question 4: A BPI analyst tests a boiler and finds O2 in the flue is 3% and CO2 is 10.5% for natural gas. What does this indicate?
- Dangerous over-firing condition
- Near-stoichiometric combustion with low excess air — efficient but should be verified for CO (Correct answer)
- Extreme excess air causing high stack losses
- Air infiltration into the combustion chamber
Correct answer: Near-stoichiometric combustion with low excess air — efficient but should be verified for CO
For natural gas, O2 near 3% and CO2 near 10-11% indicates near-stoichiometric combustion with minimal excess air, which is efficient, but CO should be confirmed low.
Question 5: What action should a BPI analyst take if an atmospheric water heater fails to draft during worst-case depressurization testing?
- Increase gas input rate and retest
- Document the spillage, classify the appliance as a safety hazard, and recommend correction before weatherization proceeds (Correct answer)
- Install a barometric damper and retest
- Reduce the flow rate on exhaust fans and retest only under normal conditions
Correct answer: Document the spillage, classify the appliance as a safety hazard, and recommend correction before weatherization proceeds
Failed draft under worst-case conditions is a confirmed combustion safety hazard; BPI requires documenting it and flagging it as a priority safety issue before any weatherization work.
Question 6: Which material is NOT acceptable for use as a connector between a gas appliance and a masonry chimney under NFPA 54?
- Type B vent pipe
- Single-wall galvanized steel
- PVC plastic pipe (Correct answer)
- Stainless steel flex connector rated for the application
Correct answer: PVC plastic pipe
PVC is not rated for the high temperatures of gas appliance flue gases in naturally drafted applications; it is only used for condensate drains or sealed-combustion exhaust on condensing equipment.
Question 7: What is the purpose of a draft hood (also called a dilution device) on a naturally drafted gas appliance?
- To increase flue gas velocity and improve chimney draft
- To protect the burner flame from chimney downdrafts and allow dilution air to enter the flue (Correct answer)
- To reduce CO emissions by mixing secondary air with flue gas
- To measure draft pressure for combustion efficiency calculations
Correct answer: To protect the burner flame from chimney downdrafts and allow dilution air to enter the flue
A draft hood decouples the burner from chimney pressure fluctuations by allowing room air to dilute flue gases, preventing downdrafts from extinguishing the flame.
BPI Combustion Safety 5
What is the role of excess air in combustion, and what happens when excess air is too high?