HERS Combustion Safety 3 — Questions and Answers
Question 1: When testing for ambient CO in a home, what is the BPI/RESNET action level (ppm) that requires immediate evacuation?
- 9 ppm
- 35 ppm
- 70 ppm
- 35+ ppm sustained or any reading above 70 ppm (Correct answer)
Correct answer: 35+ ppm sustained or any reading above 70 ppm
BPI protocols call for immediate evacuation when ambient CO exceeds 35 ppm sustained or any single reading exceeds 70 ppm, and HVAC systems should be shut down.
Question 2: Which type of venting system uses a fan to push or pull combustion gases through the flue, making it less susceptible to backdrafting?
- Type B natural-draft venting
- Power-vented or induced-draft venting (Correct answer)
- Gravity-fed masonry chimney
- Atmospheric draft venting
Correct answer: Power-vented or induced-draft venting
Power-vented and induced-draft appliances use a mechanical fan to move flue gases, providing positive pressure in the flue and reducing the risk of backdrafting from house depressurization.
Question 3: A natural-draft gas furnace shares a masonry chimney with a wood-burning fireplace. What combustion safety concern does this create?
- The gas furnace will overheat the chimney
- Simultaneous use can cause the furnace flue gases to backdraft into the home (Correct answer)
- The wood smoke will contaminate the gas supply
- The chimney will produce too much draft for the furnace
Correct answer: Simultaneous use can cause the furnace flue gases to backdraft into the home
When a fireplace draws combustion air from the chimney, it can depressurize the shared flue and reverse the flow of furnace exhaust gases back into the living space.
Question 4: What does the term 'orphaned water heater' mean in the context of combustion safety?
- A water heater that has never been serviced
- A water heater left alone on an oversized flue after a furnace is replaced with a high-efficiency model (Correct answer)
- A water heater installed without a permit
- A water heater with a failed pilot light
Correct answer: A water heater left alone on an oversized flue after a furnace is replaced with a high-efficiency model
When a high-efficiency furnace no longer vents through a masonry chimney, the remaining atmospheric water heater may be 'orphaned' on an oversized flue, leading to condensation and draft problems.
Question 5: Which condition would most likely cause a natural-draft appliance to backdraft even without house depressurization?
- A flue that is too short or has an obstruction (Correct answer)
- A flue that is too long
- An oversized gas orifice
- A clogged condensate drain
Correct answer: A flue that is too short or has an obstruction
A blocked or undersized flue creates resistance that prevents adequate draft, causing combustion gases to spill back into the space even when the house is at neutral pressure.
Question 6: During a combustion safety test, you measure -7 Pa in the CAZ with all exhaust fans running and the largest window open. What does this result indicate?
- The CAZ is dangerously depressurized and requires remediation (Correct answer)
- The CAZ is within acceptable limits per BPI standards (fail threshold is typically -5 Pa or worse for atmospherically vented appliances)
- The CAZ is positively pressurized
- The reading is within safe limits because it is negative
Correct answer: The CAZ is dangerously depressurized and requires remediation
A CAZ depressurization of -7 Pa exceeds the BPI failure threshold (typically -5 Pa for atmospherically vented appliances), indicating a risk of backdrafting that requires correction.
Question 7: What is the function of a draft hood on an atmospherically vented gas appliance?
- It measures flue gas temperature
- It dilutes flue gases with room air and protects the burner from downdrafts (Correct answer)
- It seals the combustion chamber from the living space
- It regulates gas pressure to the burner
Correct answer: It dilutes flue gases with room air and protects the burner from downdrafts
A draft hood allows room air to dilute flue gases and protects the burner flame from being extinguished by downdrafts or fluctuating chimney draft.
HERS Combustion Safety 3
When testing for ambient CO in a home, what is the BPI/RESNET action level (ppm) that requires immediate evacuation?