HERS Combustion Safety Principles 3 — Questions and Answers
Question 1: What level of ambient CO in a home (in ppm) is considered immediately dangerous to life and health (IDLH) by NIOSH?
- 35 ppm
- 200 ppm
- 1,200 ppm (Correct answer)
- 400 ppm
Correct answer: 1,200 ppm
NIOSH defines the IDLH level for CO as 1,200 ppm, at which exposure can cause death or irreversible health effects within 30 minutes.
Question 2: A Category I vented appliance relies on which mechanism to exhaust combustion gases?
- Powered inducer fan forcing gases out under positive pressure
- Natural draft buoyancy created by temperature difference between flue gas and outside air (Correct answer)
- Sealed direct-vent system drawing combustion air from outside
- Balanced pressure mechanical ventilation
Correct answer: Natural draft buoyancy created by temperature difference between flue gas and outside air
Category I appliances use natural draft, relying on the buoyancy of hot flue gases rising through a vertically vented system.
Question 3: When testing a atmospherically vented furnace, the CAZ (combustion appliance zone) pressure should ideally read:
- Strongly negative (below -5 Pa) relative to outside
- Slightly negative or neutral relative to outside (Correct answer)
- Strongly positive (above +5 Pa) relative to outside
- Equal to attic pressure
Correct answer: Slightly negative or neutral relative to outside
A CAZ that is only slightly negative or neutral supports proper natural draft without creating conditions that cause backdrafting.
Question 4: Orphaned water heaters—left alone on a large flue after a furnace is removed—are at heightened risk of:
- Over-firing due to excess gas supply
- Backdrafting because the oversized flue reduces draft velocity (Correct answer)
- Short-cycling from excessive heat exchanger capacity
- Pilot outage from high combustion air volumes
Correct answer: Backdrafting because the oversized flue reduces draft velocity
An oversized flue slows flue gas velocity and allows cold outside air to cool and reverse the flow, causing backdrafting in the remaining appliance.
Question 5: What does CO ppm stand for in combustion testing?
- Carbon oxygen parts per million
- Carbon monoxide parts per million (Correct answer)
- Carbon monoxide pounds per meter
- Combustion output pressure per minute
Correct answer: Carbon monoxide parts per million
CO ppm means carbon monoxide concentration expressed as parts per million, the standard unit for measuring this toxic gas in air.
Question 6: Which type of venting system is most resistant to backdrafting caused by house depressurization?
- Category I natural draft B-vent
- Masonry chimney open fireplace
- Category IV sealed direct-vent system (Correct answer)
- Power-vented with single-wall metal pipe
Correct answer: Category IV sealed direct-vent system
Direct-vent (Category IV) systems draw combustion air from outside and exhaust directly outside through sealed concentric pipes, making them immune to indoor pressure fluctuations.
Question 7: If a combustion analyzer reads 400 ppm CO in the undiluted flue gas of a gas furnace, what action should a HERS rater take?
- Document and proceed with the rating—this is within normal range
- Recommend regular filter changes to improve efficiency
- Advise the homeowner of a potential safety hazard and recommend HVAC service immediately (Correct answer)
- Seal the flue pipe joints to reduce dilution
Correct answer: Advise the homeowner of a potential safety hazard and recommend HVAC service immediately
400 ppm CO in undiluted flue gas exceeds safe thresholds and indicates incomplete combustion requiring immediate professional HVAC inspection and repair.
HERS Combustion Safety Principles 3
What level of ambient CO in a home (in ppm) is considered immediately dangerous to life and health (IDLH) by NIOSH?